D9311.PDF

Rev. sci. tech. Off. int. Epiz., 2000,19 (2), 463-482
Highly pathogenic avian influenza
D.E. Swayne & D.L. Suarez
Southeast Poultry Research Laboratory, Agricultural Research Service, United States Department of Agriculture,
934 College Station Road, Athens, Georgia 30605, United States of America
Summary
Highly pathogenic (HP) avian influenza (Al) (HPAI) is an extremely contagious,
multi-organ systemic disease of poultry leading t o high mortality, and caused by
some H5 and H7 subtypes of type A influenza virus, family
Orthomyxoviridae.
However, most A l virus strains are mildly pathogenic (MP) and p r o d u c e either
subclinical infections or respiratory and/or reproductive diseases in a variety of
domestic and w i l d bird species. Highly pathogenic avian influenza is a List A
disease of t h e Office International des Epizooties, w h i l e MPAI is neither a List A
nor List B disease. Eighteen outbreaks of HPAI have been d o c u m e n t e d since t h e
identification of A l virus as t h e cause of f o w l plague in 1955.
Mildly pathogenic avian influenza viruses are maintained in w i l d aquatic bird
reservoirs, occasionally crossing over to domestic poultry and causing outbreaks
of mild disease. Highly pathogenic avian influenza viruses do n o t have a
recognised w i l d bird reservoir, but c a n occasionally be isolated from w i l d birds
during outbreaks in domestic poultry. Highly pathogenic avian influenza viruses
have been d o c u m e n t e d t o arise f r o m M P A I viruses t h r o u g h mutations in t h e
haemagglutinin surface protein.
Prevention of exposure t o t h e virus and eradication are t h e a c c e p t e d methods for
dealing w i t h HPAI. Control p r o g r a m m e s , w h i c h imply allowing a l o w incidence of
infection, are not an a c c e p t a b l e method f o r managing HPAI, but have been used
during some outbreaks of M P A I . The c o m p o n e n t s of a strategy to deal w i t h M P A I
or HPAI include surveillance and diagnosis, biosecurity, e d u c a t i o n , quarantine
and depopulation. Vaccination has been used in some control and eradication
programmes for A l .
Keywords
Avian diseases - Fowl pest - Fowl plague - Highly pathogenic avian influenza Orthomyxoviruses.
Introduction
Highly pathogenic (HP) avian influenza (AI) is an extremely
infectious, systemic viral disease of poultry that produces high
mortality and necrobiotic, haemorrhagic or inflammatory
lesions in multiple visceral organs, the brain and skin ( 4 , 1 0 3 ) .
Highly pathogenic avian influenza and fowl plague are
synonymous terms, the latter being the historical designation
for the disease originally described in domestic fowl (Gallus
gallus domesticus), and the former the more accurate currently
accepted name for the disease in all bird species (12, 7 3 ) .
Fowl plague has been known by other names including fowl
pest (peste aviaire), Geflügelpest, Brunswick bird plague,
Brunswick disease, fowl disease, and fowl or bird grippe (98).
History
Fowl plague was first reported in Italy in 1 8 7 8 by Perroncito
who described a severe, rapidly spreading disease that
produced high mortality in chickens (98). In 1880, Rivolto
and Delprato differentiated fowl plague from the clinically
similar septicaemic form of fowl cholera and used the name
typhus exudatious gallinarium to describe fowl plague (98).
The disease spread throughout Europe in the late 1800s and
early 1900s via poultry exhibitions and shows, where it
became endemic in domestic poultry until the 1930s. In
1901, the cause was determined to be a filterable agent, a
virus, although only in 1955 was the virus identified and
classified as Type A influenza virus (orthomyxovirus), and
shown to be related to other influenza viruses that commonly
464
infected humans, pigs and horses ( 8 2 ) . From the 1960s
onwards, a variety of influenza viruses were isolated from
turkeys with milder disease (i.e. non-fowl plague syndromes),
and these viruses were termed mildly pathogenic (MP)-AI
viruses or AI viruses of low pathogenicity (108). The agar gel
immunodiffusion (AGID) test became the international
standard for serological diagnosis and surveillance (16). In
1972, wild waterfowl of the order Anseriformes (ducks and
geese) were demonstrated to be a principal reservoir and the
natural host for MPAI viruses (90). In 1979, the cleavability of
the haemagglutinin (HA) protein was identified as the major
determinant of virulence in HPAI viruses (21). In 1981, the
first International Symposium on avian influenza was
convened in Beltsville, Maryland, United States of America
(USA), and the term 'fowl plague' was abandoned for the more
accurate term 'highly pathogenic avian influenza' (12).
Avian influenza virus
Description of the aetiological agent
Avian influenza viruses are negative sense, segmented,
ribonucleic acid viruses of the family Orthomyxoviridae. The
Orthomyxoviridae family includes several segmented viruses
including the Type A, B and C influenza viruses. The Type A
influenza viruses, which include all AI viruses, can infect a
wide variety of animals including wild ducks, chickens,
turkeys, pigs, horses, mink, seals and humans. The type B and
C viruses primarily infect only humans and occasionally pigs.
The different types of influenza viruses can be differentiated
serologically based on antigenic differences in the conserved
internal proteins, primarily the nucleoprotein and matrix
genes, using the AGID test (16).
Type A influenza viruses have eight gene segments that
encode ten different proteins (56). The proteins can be
divided into surface proteins and internal proteins. The
surface proteins include the HA, neuraminidase (NA) and
matrix 2 proteins. The HA and NA proteins provide the most
important antigenic sites for the production of a protective
immune response, primarily in the form of neutralising
antibody. However, these proteins have large antigenic
variation, with fifteen HA and nine NA subtypes being
recognised, based on haemagglutination-inhibition (HI) and
neuraminidase-inhibition (NI) tests, respectively. The internal
proteins include the polymerase complex, including the three
polymerase proteins (PB1, PB2 and PA), the nucleoprotein,
the matrix 1 protein, and non-structural proteins 1 and 2.
Definition of pathogenicity
The pathogenicity of individual AI viruses varies and should
be determined in order to develop prevention, control and
eradication strategies. The usage of the term HP implies that
Rev. sci. tech. Off. int. Epiz., 19
the virus is highly virulent for chickens and has been
demonstrated to meet one or more of the following three
criteria (72, 115):
a) any influenza virus that is lethal for six, seven or eight of
eight ( > 7 5 % ) four- to six-week-old susceptible chickens
within ten days following intravenous inoculation with 0.2 ml
of a 1:10 dilution of a bacteria-free, infectious allantoic fluid
b) any H5 or H7 virus that does not meet the criteria in a),
but has an amino acid sequence at the HA cleavage site that is
compatible with HPAI viruses
c) any influenza virus that is not an H5 or H7 subtype and
that kills one to five of eight inoculated chickens and grows in
cell culture in the absence of trypsin.
Fulfilment of one or more of the above criteria would
categorise the virus as an HPAI virus. In contrast, AI viruses
that lack these three criteria are categorised as non-HPAI, and
include AI viruses historically designated as MP,
non-pathogenic, not pathogenic or of low pathogenicity,
based on < 7 5 % lethality in experimental studies with
chickens. 'Non-pathogenic' or 'not pathogenic' are usually
used to denote strains that cause no clinical signs or deaths in
experimentally inoculated chickens, while designations of MP
or of low pathogenicity have been used to signify AI virus
strains that typically caused some clinical signs, but mortality
of less than 7 5 % in experimental studies with chickens (one
to five of eight inoculated chickens). Non-HPAI viral strains
comprise all AI viruses of the H l - 4 , H6, and H8-15 subtypes,
and most of those of H5 and H7 subtypes. Only a small
number of AI viruses of the H5 and H7 HA subtypes have
been HPAI viruses. Historically, 'fowl plague' was associated
only with the H7 HA subtype. The outbreak of 1 9 5 9 among
chickens in Scotland was the first documentation of an HPAI
virus of the H5 subtype (Table I).
The definition of AI adopted by the European Union (EU) is
as follows: 'Avian Influenza' means an infection of poultry
caused by an influenza A virus which has an intravenous
pathogenicity index in six-week-old chickens greater than 1.2
or any infection with influenza A viruses of H5 or H7 subtype
for which nucleotide sequencing has demonstrated the
presence of multiple basic amino acids at the cleavage site of
the HA (5).
The phrase 'highly pathogenic for chickens' does not indicate
or imply that the AI virus strain is HP for other birds species,
especially wild ducks or geese (Anseriformes). However, if a
virus is HP for chickens, the virus will usually be HP for other
birds within the order Galliformes, family Phasianidae, such as
turkeys (Meleagris gallopavo) and Japanese quail (Coturnix
japonica) (8). To date, HPAI viruses for chickens are generally
non-pathogenic for ducks and geese in experimental studies
(8, 3 5 ) . Non-HPAI or MPAI viruses have been isolated from
domestic ducks and geese in association with disease (usually
respiratory disease), and mild-to-moderate mortality.
However, these AI viruses do not produce high mortality in
465
Rev. sci. tech. Off. int. Epiz.. 19 (2)
Table I
Documented outbreaks of highly pathogenic avian influenza since discovery of avian influenza as the cause of fowl plague in 1955 (4,76)
References
(a)
Avian influenza virus
Subtype
Type and number of birds affected with high mortality or depopulated
A/chicken/Scotland/59
H5N1
A/tern/South Africa/61
H5N3
H7N3
2 flocks of chickens {Gallus gallus domesticus). Total number of birds affected not
reported
1,300 common terns (Sterna hirundo)
29,000 breeder turkeys {Meleagridis gallapavo)
77; D.J. Alexander,
personal communication
19
123
H5N9
8,100 breeder turkeys
25,000 laying chickens, 17,000 broilers and 16,000 ducks {Anas platyrhynchos)
3 commercial farms of turkeys. Total number of birds affected not reported
58
11,113
A/turkey/England/199/79
H7N7
H7N7
A/chicken/Pennsylvania/1370/83
H5N2
A/turkey/lreland/1378/83
H5N8
A/chicken/Victoria/85
H7N7
A/turkey/England/50-92/91
H5N1
17 million birds in 452 flocks; most were chickens or turkeys, a few chukar
partridges {Alectoris chukar) and guinea-fowl (Numida meleagris)
800 meat turkeys died on original farm; 8,640 turkeys, 28,020 chickens and
270,000 ducks were depopulated on original and 2 adjacent farms
24,000 broiler breeders, 27,000 laying chickens, 69,000 broilers and 118,518
chickens of unspecified type
8,000 turkeys
A/chicken/Victoria/92
A/chicken/Queensland/95
A/chicken/Puebla/8623-607/94
A/chicken/Queretaro/14588-19/95
A/chicken/Pakistan/447/95
A/chicken/Pakistan/1369-CR2/95
A/chicken/Hong Kong/220/97
H7N3
H7N3
12,700 broiler breeders and 5,700 ducks
9
84,124
22,000 laying chickens
124
H5N2
Chickens
H7N3
3.2 million broilers and broiler breeder chickens
H5N1
A/chicken/New South Wales/1651/97
H7N4
1.4 million chickens and various lesser numbers of other domestic birds in
contact with the chickens on farms and in the live-bird market system
128,000 broiler breeders, 33,000 broilers and 261 emu {Dromaius novaehollandiae)
A/chicken/ltaly/330/97
H5N2
A/turkey/ltaly/99
H7N1
A/turkey/England/63
A/turkey/Ontario/7732/66
A/chicken/Victoria/76
3,6
34,114
4,61
14,28
118
lb)
69
|c)
2,116 chickens, 1,501 turkeys, 731 guinea-fowl, 2,322 ducks, 204 quail
(species unknown), 45 pigeons {Columbia livia), 45 geese (species unknown)
and 1 pheasant (species unknown)
297 farms; 5.8 million laying chickens, 2.2 million meat and breeder turkeys,
1.6 million broiler breeders and broilers, 156,000.guinea-fowl, 168,000 quail,
577 backyard poultry and 200 ostriches
88,99
76
24
I. Capua, personal
communication
ldl
a| Most outbreaks were controlled by 'stamping out' or depopulation policies for infected and/or exposed populations of birds. Chickens, turkeys and birds of the order Galliformes had clinical signs
and mortality patterns consistent with highly pathogenic avian influenza, while ducks, geese and other birds lacked or had low mortality rates or infrequent presence of clinical signs.
b) A 'stamping-out' policy was not used for control. The outbreak of Al had concurrent circulation of mildly pathogenic avian influenza and highly pathogenic avian influenza (HPAI) virus strains.
However, HPAI virus strains were present only from late 1994 to mid-1995. Estimates of the number of birds infected with HPAI strains are unavailable, but 360 commercial chicken flocks were
'depopulated' for Al in 1995 through controlled marketing.
c) A 'stamping-out' policy was not used for control. Surveillance, quarantine, vaccination and controlled marketing were used as the control strategy. The numbers affected are crude estimates.
d) The outbreak began on 17 December 1999 and was on-going at the time of writing on 15 February 2000. A 'stamping-out' policy was in force as the control method. Estimates given include birds
present in outbreaks, either affected or depopulated.
experimental studies with chickens, ducks or geese (4). In one
report, an H5N1 AI virus was isolated from a sick domestic
goose in the People's Republic of China and the virus was HP
for chickens based on the sequence of the viral HA proteolytic
cleavage site, as listed in criterion b) above (128).
been
reported
to
be
A/finch/Germany/72
HP
for
(H7N1)
and
chickens,
namely:
A/gull/Germany/79
(H7N7) (4). The origins of the former virus remain unclear,
but the latter was possibly the result of spread from poultry, as
outbreaks
occurred
in
Eastern
Europe
at
( D J . Alexander, personal communication,
that
time
15 December
1999). The recent outbreaks of HPAI have occurred in North
Overview of highly pathogenic
avian influenza
Outbreaks since 1955
Eighteen outbreaks of HPAI have been documented in the
English-language literature since the identification of
influenza viruses as the cause of HPAI in 1 9 5 5 (4, 7 6 ) . This
includes seventeen outbreaks in domestic poultry and one in
wild common terns (Sterna hirundo) (Table I). In addition,
two AI virus strains recovered from non-domestic birds have
America, Europe, Asia and Australia (Table I). Prior to 1 9 5 5 ,
HPAI outbreaks had been reported in Africa and South
America ( 3 3 , 9 8 ) . No outbreaks have been reported in
Antarctica. Although serological evidence of infection by AI
viruses has been reported for penguins, such infections were
most probably not from HPAI viruses ( 6 7 ) .
Highly
pathogenic
commercial
avian
domestic
influenza
poultry,
is not
but
has
endemic
in
produced
local/regional epizootics in chickens and turkeys on large
commercial farms, in meat and breeder turkeys on smaller
466
commercial farms, and in poultry on farms and in retail
facilities that provide poultry for the live-bird market system
(Table I). Of the seventeen HPAI outbreaks which have
occurred in poultry since 1 9 5 5 , seven have affected over
100,000 birds each, while the remaining ten affected less than
100,000 birds each (Table I). In sixteen outbreaks, the HPAI
viruses were eliminated and in the current H7N1 outbreak in
Italy, eradication is underway. The total number of domestic
poultry affected by HPAI has been a small percentage of the
total world poultry production of 2 2 billion birds per year, or
approximately 7 5 0 billion birds since 1 9 5 5 .
Impact on trade
Assessment of pathogenicity of a newly isolated AI virus is
critical for development of appropriate control strategies and
to assess the impact on international trade. Highly pathogenic
avian influenza is a List A disease of Office International des
Epizooties (OIE) and has been used as a legitimate trade
barrier to protect countries or regions from introduction of an
exotic or foreign poultry disease, i.e. HPAI. However, reports
of isolation of MPAI viruses from domestic and non-domestic
birds, or serological evidence of AI virus infection in
non-poultry species have been used by some countries as
non-tariff trade barriers for limiting importation of poultry
and poultry products. Mildly pathogenic avian influenza is
neither a List A nor a List B disease of the OIE.
Ecology
Influenza viruses in birds and mammals
Influenza viruses can infect a wide variety of birds and
mammals, thus demonstrating an unusually wide host range.
While influenza appears to be well adapted to the natural host
(wild aquatic birds), causing little, if any disease, when
influenza viruses infect non-host species, disease can often
result. Serious disease outbreaks have been reported for
poultry species, primarily chickens and turkeys, as well as
mammalian species such as pigs, horses, mink, seals and
humans. Although influenza viruses which become better
adapted for. a particular species are considered to be
pathogens of that species only, numerous exceptions have
occurred. For example, classic H1N1 swine influenza viruses
generally only infect pigs, but this lineage of virus has been
known to infect humans and turkeys, often causing serious
disease ( 1 0 , 4 2 , 126). The genetic determinants for species
specificity of different influenza viruses are currently not
known, although multiple genes appear to play some role in
the process (92, 111).
Mildly pathogenic avian influenza viruses in
wild birds
In contrast to the geographically restricted and limited
number of outbreaks due to HPAI viruses, MP (non-HP) AI
viruses are world-wide in distribution, predominantly
infecting a variety of wild bird species, but also some domestic
birds. Isolation of influenza from wild birds has been
Rev. sci. tech. Off. int. Epiz., 19
documented from five continents including North America,
Europe, Asia, Africa and Australia ( 1 9 , 5 2 , 6 0 , 67, 9 0 , 94,
1 0 1 , 1 1 2 ) . Serological evidence has also been reported in wild
birds from Antarctica (67).
The natural host species and reservoir for influenza viruses are
wild aquatic birds, including ducks, gulls and other
shorebirds (50, 90). Many of these birds undergo annual long
distance migrations. This wild bird reservoir is also the
original source of all viral genes for both avian and
mammalian lineages of influenza viruses. For example, all
fifteen HA and nine NA subtypes of influenza have been
found in wild birds (121). Avian species from at least nine
different orders of birds have been reported to be naturally
and asymptomatically infected with influenza viruses, but the
main reservoir is believed to be in birds belonging to the
orders Anseriformes
and Charadriiformes
( 9 3 ) . The
Anseriformes
include ducks, geese and swans, and
Charadriiformes
include gulls, terns, surfbirds
and
sandpipers. Although influenza viruses occur commonly in
both orders of birds, most epidemiological studies have been
conducted on mallard ducks (86, 87, 9 0 , 9 5 , 112). Although
MPAI viruses can be detected in ducks at most times of the
year, the highest percentage of birds shedding virus is found
in the pre-migration stage in late summer (93). Most HA and
all the NA subtypes have been isolated from wild ducks, but
certain HA and NA subtypes appear to occur much more
frequently, including the HA subtypes H3, H6 and H4, and
the NA subtypes N8, N4 and N6 (87). However, the
percentage of each subtype that is isolated can vary widely
depending on the time of year when the samples are taken,
and the geographic location.
Although influenza viruses can infect large numbers of wild
waterfowl, infection usually only produces an asymptomatic
enteric infection (26, 5 1 , 119). Infected birds can produce
large amounts of virus, often defecated directly into the water.
The contamination of the aquatic environment appears to be
an efficient method of transmission of virus to susceptible
wild birds or to domestic birds which share the same
environment. Water contaminated with AI virus in the faeces
of infected wild ducks has been a source of infection for
domestic turkeys (89).
Mildly pathogenic avian influenza in domestic
birds
Chickens and turkeys are not natural host species for AI
viruses. In two different studies of wild turkeys, no serological
evidence of infection was observed ( 2 9 , 4 5 ) . Although no
comparable studies have been performed on red jungle fowl,
the ancestor of modem chickens, this species is unlikely to
play a role in the maintenance of influenza in the
environment. Although chickens and turkeys are not natural
hosts for AI viruses, these birds can readily become infected
with the virus. The direct transmission of influenza viruses
from migrating waterfowl to range-reared turkeys has been
467
Rev. sci. tech. Off. int. Epiz., 19 (2)
implicated as a common source of infection in the USA (41),
and probably also plays a major role in transmission of AI to
chickens. However, transmission of influenza from wild
waterfowl to chickens and turkeys may also occur through
intermediates, in particular domestic ducks and geese that are
reared or marketed in close association with chickens and
turkeys. Once influenza viruses have been introduced into
commercial poultry operations, the virus can often spread
rapidly to new locations or be maintained for long periods of
time ( 2 3 , 109).
In addition to chickens and turkeys, MPAI viruses have been
isolated from domestic ducks, domestic and imported exotic
birds, ratites and occasionally quail and game birds (4). Mildly
pathogenic avian influenza virus infections in the latter two
groups of birds have been detected most frequently in
domestic poultry in live bird markets of large cities. Although
AI infection in these birds may be asymptomatic, outbreaks of
respiratory disease with mortality have been reported (7, 4 6 ,
128). Transmission of AI viruses in poultry is through
respiratory secretions and faeces.
Highly pathogenic avian influenza viruses
No wild bird reservoir has been identified for HPAI viruses.
Except for the single outbreak of HPAI in common terns in
South Africa (Table I), HPAI has been rarely isolated from
wild birds even on farms experiencing outbreaks of HPAI in
domestic poultry (70). Extensive surveys for AI viruses in wild
habitats of waterfowl in North America have identified
thousands of AI viruses, all MP viruses (4). In the outbreaks of
1983-1984 in Pennsylvania and 1 9 9 4 - 1 9 9 5 in Mexico, HPAI
viruses arose following the mutation of H5 MPAI virus which circulated widely in domestic poultry ( 4 9 , 7 5 ) . An
experimental chicken embryo model has shown that some H5
and H7 MPAI viruses can mutate to form HPAI viruses that
are similar pathobiologically and molecularly to HPAI field
virus (107). This data suggests that HPAI viruses arise in
domestic poultry populations from MPAI viruses and are not
widely maintained as HPAI in wild bird populations.
However, this does not preclude that small groups of wild
birds could serve as reservoirs of HPAI viruses derived from
poultry.
The infection and disease
Most HPAI virus strains from domestic poultry have been
isolated from turkeys and chickens, but infection, clinical
signs and high mortality have been reported in diverse genera
and species within the order Galliformes, family Phasianidae,
including Japanese quail, helmeted guinea-fowl (Numida
meleagris), chukar partridges (Alectoris chukar),
northern
bobwhite quail (Colinus virginianus) and common pheasants
(Phasianus colchicus) (24, 3 3 , 7 8 ) . Highly pathogenic avian
influenza is the result of systemic replication of the virus and
cell death in a variety of visceral organs, brain and skin.
Poultry flocks infected with HPAI virus have high morbidity
and mortality rates with birds developing severe clinical signs,
often with rapid death.
In contrast, the majority of AI virus strains are non-HP under
natural or experimental conditions, and produce either
subclinical infections or mild-to-moderate disease syndromes
affecting the respiratory, urinary and reproductive systems
( 4 , 1 0 3 ) . These viruses replicate locally, predominantly in the
respiratory and alimentary tracts. Generally, the mortality
rates due to MPAI are low compared to HPAI, but mortality
rates can be high when MPAI is accompanied by secondary
viral or bacterial pathogens (71). In this paper, the emphasis
will be placed on the epidemiological and pathobiological
features of HPAI, while MPAI will be a minor component and
used for comparative purposes with HPAI. Previous papers
have provided more detailed comparisons of lesions of
poultry infected with either MPAI or HPAI viruses (44, 6 5 ,
103).
Clinical signs
Chickens and turkeys with HPAI are typically found dead
with few clinical signs other than depression, recumbency
and a comatose state (34, 4 3 , 4 8 , 5 4 , 6 5 , 9 8 ) . Close
observation of remaining birds has revealed reduced activity,
decreased sensitivity to stimuli, reduction in 'house noise',
dehydration and decreased feed intake which rapidly
progressed to severe depression and death. The older the
birds, the greater the frequency of clinical signs appearing
before death. In breeders and laying chickens, egg production
drops precipitously to near zero after three to five days.
Occasionally, torticollis, paresis, paralysis, excitation,
convulsions and rolling or circling movements are noted in a
few birds that survive to the subacute stage of the infection.
Diarrhoea may be evident as bile- or urate-stained loose
droppings with variable amounts of intermixed mucus.
Respiratory signs such as nasal discharge, rales, coughing,
snicking, sneezing or respiratory distress have been
infrequently reported with HPAI (14), but are common with
MPAI. Difficulty in breathing has been reported in birds
affected by HPAI, with severe oedema of the upper respiratory
tract or lungs (98).
Gross observations
The appearance of gross lesions is variable depending on the
virus strain, the length of time from infection to death, and the
age and species of poultry affected ( 3 3 , 4 4 , 103). In general,
clinical signs, lesions and death have been seen with domestic
poultry of the order Galliformes, family Phasianidae, but not
for birds of the orders Anseriformes or Charadriiformes
when
infected with HPAI viruses.
In most cases of peracute infections with death (days one to
two of infection), poultry have lacked visible gross lesions
(44). However, some strains, such as the A/chicken/Hong
Kong/220/97 (H5N1) and A/chicken/Italy/330/97 (H5N2),
have caused severe lung lesions of congestion, haemorrhage
and oedema in chickens, such that the excised tissue exudes
468
serous fluid and blood (Fig. l a ) ( 2 4 , 9 9 ) . Oedema of the brain
has also been reported (99).
During the acute stages of infection with death (days three to
five of infection), chickens have ruffled feathers, congestion
and/or cyanosis of the comb and wattles, and swollen heads,
especially prominent from periorbital and intramandibular
subcutaneous oedema (Fig. l b ) ( 1 , 4 4 , 5 4 , 1 0 3 ) . Some viruses
produced hyperaemia and oedema of the eyelids, conjunctiva
and trachea (14). In birds which die, generalised congestion
and haemorrhage may occur (44). Lesions are common in the
combs and wattles, especially in adult chickens, and include
petechial-to-ecchymotic
haemorrhages,
swelling
from
oedema and eventually depressed dark red-to-blue areas of
ischaemic necrosis as the result of vascular infarction (Figs l c
and ld). Subcutaneous haemorrhages and oedema may be
present around the hock, on the shanks and feet (Fig. l e ) and
occasionally on feathered skin all over the body. Some HPAI
viruses, such
as A/Queretaro/14588-19/95
(H5N2),
commonly cause thickening of the skin over the distal legs
with gelatinous oedema (Fig. lf) (103). Petechialto-ecchymotic haemorrhages may be present in multiple
visceral organs or on the serosal surface, such as the
epicardium of the heart (Fig. l g ) , serosa of small intestine,
abdominal fat, serosa of sternum, caecal tonsils, Meckel's
diverticulum, Peyer's lymphoid patches of the small intestine
(Fig. l h ) , proventriculus around the glandular ducts or
between glands (Fig. l i ) , under the cuticle of the ventriculus
and interfascicular regions of skeletal muscle. Primary
lymphoid organs such as cloacal bursae and thymuses are
severely atrophic, while the spleen may be normal in size or
enlarged. Occasionally, spleens have white foci of necrosis.
The pancreas may have red to light orange to brown mottling
(44). Ruptured ova with 'yolk peritonitis' have been reported
in layers and broilers and turkey breeders (1, 5 8 ) .
Histopathology and immunohistochemical
localisation of the virus
Histological lesions usually vary in severity and location, but
typically include necrosis, haemorrhage and/or inflammation
within multiple visceral organs; especially the heart (Fig. l j ) ,
brain (Fig. lk), adrenal and pancreas (Fig. 11); and skin ( 1 , 5 4 ,
64, 103, 117). Variation in the distribution and severity of
lesions is the result of differences between strains of HPAI
virus and species of bird.
In peracute deaths caused by HPAI viruses, necrosis and
inflammation is generally lacking in parenchymal cells of
most visceral organs, brain and skin. If present, such lesions
are mild and multifocal in distribution. Most consistently, the
HPAI virus replicates in the vascular endothelial cells
(Fig. l m ) and cardiac myocytes (Fig. ln), causing generalised
cell death (99).
In acute infections with HPAI virus, the predominant lesions
are necrosis, and to a lesser extent, apoptotic cell death with
Rev. sci. tech. Off. int. Epiz.. 19
associated inflammation, haemorrhage and oedema (44). The
longer the birds survive, the more prominent the
inflammation and the less prominent the necrosis or
apoptosis.
Lesions are most prominent in the brain, heart, pancreas,
lung, adrenal and skin, but similar lesions have been reported
in most
visceral organs. Typical lesions include
lymphohistiocytic meningoencephalitis with vasculitis and
focal rarefication, widespread caseous necrosis of the
pancreas, dermal vasculitis with thrombosis and infarction,
lymphohistiocytic myocarditis with hyalin necrosis of
myofibres and severe lymphocytic apoptosis of primary and
secondary lymphoid tissues. The cells that are necrotic have
associated AI viral replication (Fig. l o ) , while the apoptotic
lymphocytes do not have demonstrable AI viral antigen.
Epidemiology and diagnostic
tests
A disease outbreak is usually the first indication of an infection
with AI virus, and a combination of virus isolation, serological
tests, and direct antigen detection is often used to detect
infected flocks (108). Isolation of influenza viruses has been
achieved by direct inoculation of nine- to eleven-day-old
embryonating chicken eggs with homogenates from the lung,
trachea, faeces and internal organs. Alternative methods of
virus isolation have been used, primarily cell culture.
Influenza viruses have been routinely isolated from clinical
samples, however failures have occurred due to bacterial or
viral contamination, incorrect storage of samples, inadequate
samples or sample numbers, or because the infected birds
were sampled after viral shedding had ceased. Once a virus
has been isolated, the HA and NA subtypes are determined
typically using the HI and NI tests (108). These tests require a
panel of reagents specific for each HA and NA subtype. The
virulence of the virus isolates can be tested using the
previously described standardised intravenous pathogenicity
test (115).
Following diagnosis of an outbreak of influenza, efforts to
control the outbreak rely upon epidemiological tracking to
determine the source of the outbreak and the extent to which
the virus has been disseminated. Since influenza viruses can
vary greatly in pathogenicity and more than one subtype of
influenza may be circulating at the same time, the use of HA
and NA subtyping of viruses responsible for an outbreak has
become an important tool to track viruses. To thoroughly
evaluate an outbreak, virus isolation and serology of infected
birds must both be analysed.
The serological response of potentially infected birds remains
the other important prerequisite tool in an eradication effort.
Flocks are usually tested as a group, rather than testing all the
individual birds. Often ten to thirty birds are randomly
Bev.sdtech.Off.int.Epiz., 19 (2)
469
m\
a) Severe pulmonary oedema and haemorrhage in the lung, A/Hong Kong/156/97 (H5N1). (Bar = 1 cm) (99)
b) Severe swelling of the head, comb and wattle from subcutaneous oedema, A/chicken/Queretaro/14589-660/94 (H5N2|. < Bar =
= Sm) (33)
cl Severe oedema, necrosis and haemorrhage of comb and wattles, highiy pathogenic embryo derivative,
MMen^™W
). (Bar = 2 cm)
d) Serous conjunctivitis with oedema of comb, wattles and periorbital area and necrosis of tips of comb, A
e) Severe subcutaneous haemorrhage and oedema of feet and leg shanks, A/Hong Kong/156/97 (H5N1 ). (Bar - 2 cm) (99)
f) Thickened dermis from oedema of distal leg, A/chicken/Queretaro/14589-660/94 (H5M2). (Bar = 2 cm)
gl Petechial haernorrhages in epicardial fat, A/chicken/NJ/12508/8B (H5N2). (Bar = 3 cm) |33)
h) Haemorrhage in lymphoid tissue of Peyer's patches and Meckel's diverticulum of the jéjunum, A/Hong Kong/220/97 (H5N1 (Bar = 1 cm|
il Submucosal haemorrhage surrounding ducts of glands in proventriculus, A/chicken/Hong Kong/156/97IH5N1) Bar = l cm) («ai
il Non-suppurative myocarditis with necrosis of individual myocytes, A/chicken/Queretaro/14589-660/94 (H5N2). (Bar - bO um|
k) Focus of neuron necrosis in cerebrum, A/Hong Kong/156/97 (H5N1). (Bar = 50 um)
D Severe acute necrosis of pancreatic acinar glands. A/Hong Kong/156/97 (H5N1 HBar = 50 (jm)
,„,-.,. W R a r , n M r r , H q q l
ml Avian influenza viral antigen in the cytoplasm and nuclei of endothelial cells in the cerebellum, A/Hong Kong/156/97 (H5N1 ). (Bar = 30 um) (Mai
n) Avian influenza viral antigen in the cytoplasm and nuclei of cardiac myocytes, A/Hong Kong/156/97 (H5N1I. (Bar- a um|
o) Avian influenza viral antigen in the cytoplasm and nuclei of Purkinje neurons, Bergmann's glial cells and granular cells of the cerebellum, A/Hong Kong/156/97 |H5N1). (Bar - 50 pm)
Grasl (a-i), histological (j-l) and immunohistochemical (m-o) changes in chickens following expérimental infection with highiy pathogenic avian
influenza viruses
Rev. sci. tech. Off. int. Epiz., 19 (
470
selected from a suspect flock and the birds are tested with a
allows for these naïve birds to be monitored by virus isolation
type-specific influenza detection test. Two different tests are
or seroconversion.
primarily used for the detection of type-specific antibodies,
including the AGID and the enzyme-linked immunosorbent
An alternative test which is used in conjunction with the
assay (ELISA).
serological
tests
is
the
antigen
capture
ELISA.
This
commercially available diagnostic test can detect Type A
The AGID test uses influenza antigens derived from the
influenza viral nucleoprotein directly from clinical samples,
chorioallantoic
usually from a cloacal or tracheal swab. The test provides a
membranes
from
infected
embryonated
chicken eggs, and antibodies to both the nucleoprotein and
rapid and simple test to provide field identification of infected
matrix 1 AI viral proteins are detected (16). The main
birds (30). However, the test will not differentiate AI viruses
advantages of the AGID test is that the test is relatively simple
by HA or NA subtype and the cost per test is relatively high
to
compared to other diagnostic tests.
perform
and
requires
no
sophisticated
equipment.
However, the AGID test is not as sensitive as the ELISA and
typically requires an incubation step of two days before the
Current evaluations of influenza disease outbreaks often rely
results can be determined (63, 9 1 ) .
upon sequencing of multiple genes of the virus. This sequence
information provides several important pieces of information,
Several different
types of ELISA have been
developed
some of which cannot be determined by any other method.
including two commercial indirect ELISAs for chickens, and
The
competitive ELISAs for all bird species (18, 130). Both tests
determination of the relationships of the virus to previous or
sequence data has two
primary
uses, firstly
the
rely upon the detection of antibody against nucleoprotein.
concurrent outbreaks can be determined. Although the HA
The indirect ELISA uses purified nucleoprotein as the antigen
and NA subtyping test can provide important clues about the
bound to the ELISA plate, the test serum is applied and any
parentage of a virus, outbreaks with no direct classic
anti-nucleoprotein antibody present will bind to the antigen
epidemiological links cannot be unambiguously determined.
and will not be removed during the washing steps. A
For example, three separate H5N2 outbreaks have occurred
secondary antibody specific for the animal being tested is
in North America in the 1980s and 1990s, and phylogenetic
attached to an enzyme that allows for quantitative measure of
analysis of multiple genes demonstrates that these outbreaks
the bound antibody (2, 63). The competitive ELISA is similar
were the result of separate introductions of virus (Fig. 2) (37).
to the indirect assay in that the nucleoprotein antigen is
Phylogenetic
bound to the ELISA plate, but the test serum competes for
comparisons between isolates to identify unique sequence
analysis and
the
use
of direct
sequence
binding to the nucleoprotein with an anti-nucleoprotein
markers provide the most detailed examination of influenza
monoclonal
secondary
viruses. It is also important to evaluate multiple genes, since
antibody is specific for the monoclonal antibody, and the
influenza viruses are segmented and reassort commonly. Two
results of the test are inversely related to the strength of the
viruses with the same HA gene may have different internal
colorimetric reaction ( 8 5 , 1 3 0 ) . The primary advantage of this
genes ( 3 7 , 1 0 0 ) .
antibody.
The
enzyme-linked
test is that any antibody subtype can be detected and sera
from almost any bird or mammal species can be tested.
The second principal reason to sequence influenza isolates is
to determine virulence traits from nucleotide sequences,
If positive type A influenza serum samples are found, the
which is the basis of criterion b, in the section 'Pathogenicity
samples are further tested to determine the HA and NA
determination', for defining HPAI viruses. Currently, only
subtype specificity of the antibody response with the HI and
the HA cleavage site can be evaluated in this way, but
NI tests using a panel of reagents to differentiate the fifteen HA
additional virulence genotypes should eventually become
and nine NA subtypes (108). The determination of the HA
available. The sequence of the HA cleavage is most important
and NA subtypes of the infecting virus provides additional
for the evaluation of influenza viruses of the H5 and H7
confirmation of the outbreak virus and provides a convenient
subtypes, the only subtypes associated with the HP form of
tracking tool. In some cases, only the HA subtyping test is
the virus. If multiple basic amino acids are present at the HA
necessary if an epizootic of a known subtype is occurring.
cleavage site, this indicates the potential for the virus to be HP
Serological tests on a flock basis are considered a useful tool
or to become HP, and necessitates a more vigorous control
for determining whether a flock has been infected with
programme.
influenza, but serological tests may miss a recent flock
infection, because a measurable antibody titre does not
Differentiation from closely related agents
develop until after approximately one week (108). An
The previously described clinical features, and gross and
alternative method to determine if a flock of birds is still
histological lesions are not diagnostic of HPAI ( 1 0 8 ) . Similar
actively shedding virus is to introduce sentinel birds into the
features and lesions have been reported for highly virulent
flock. Sentinel birds are specific-pathogen-free (SPF) birds
forms of Newcastle disease (velogenic Newcastle disease) and
susceptible to influenza or another pathogenic organism that
occasional acute cases of bacterial septicaemia, especially
Rev. sci. tech. Off. int. Epiz., 19 (2)
471
Ck/Scotland/59
Fig. 2
Phylogenetic analysis of the nucleotide sequence of the haemagglutinin HA1 subunit of representative H5 isolates from North America
The tree was generated using the phylogenetic analysis using parsimony (PAUP) 3.1 computer program using a heuristic search. The tree is midpoint rooted, and
Ck/Scotland/59 is included as a representative Eurasian H5 haemagglutinin sequence. Three clusters of H5N2 viruses are presented in the tree including the
Pennsylvania/83-89 lineage, the live bird market associated/93 lineage viruses and the Mexican/93-95 lineage. All three groups, although all H5N2 viruses, are
phylogenetically distinct and are the result of separate introductions of virus into the poultry population
Pasteurella multocida. Definitive diagnosis of an HPAI virus
requires the following:
a) virus isolation and identification, serological identification,
or demonstration of AI viral antigens or nucleic acids, and
b) determination of pathogenicity.
Virus isolates must be distinguished from all other
haemagglutinating
agents,
including
some
avian
adenoviruses, Newcastle disease virus and other avian
paramyxoviruses by use of the AGID test with known positive
monospecific influenza A antiserum. Dual isolations of
influenza virus and Newcastle disease virus are not unusual.
472
Diagnosis can be complicated by the presence of other
micro-organisms, such as mycoplasmas. Standard laboratory
techniques using specific antisera to neutralise other viruses
or antibiotics to control bacterial growth are helpful in such
situations. Classification of AI viruses as HPAI viruses requires
inoculation of susceptible chickens and production of > 7 5 %
mortality, sequencing of the HA cleavage site to identify
multiple basic amino acids, or production of cytopathic effect
in tissue culture without exogenous trypsin (115).
Rev. sci. tech. Off. int. Epiz. 19
conjunctivitis caused by an MPAI virus (H7N7), in a
forty-three-year-old woman ( 1 3 , 5 5 ) . The woman tended
ducks that shared a lake with feral waterfowl. The illness was
mild and the woman recovered completely. The third
incident involved infection and hospitalisation of eighteen
human patients following infections by HPAI viruses (H5N1)
in Hong Kong during 1 9 9 7 ( 2 0 , 3 8 , 9 9 , 1 2 9 ) . The patients
ranged in age from one- to sixty-years-old and six patients
died during hospitalisation. Serological evidence of infection
in asymptomatic poultry farmers was reported, but the
Public health implications
infection rate was low. The live poultry markets and farms in
Hong Kong were depopulated of 1.4 million chickens and
other poultry at the end of 1 9 9 7 (76). This action prevented
Influenza viruses are adapted to a host species with
transmission occurring most frequently and with ease
between individuals of the same species, such as transmission
from human to human, pig to pig and chicken to chicken
(105). Interspecies transmission of influenza occurs, most
easily and frequently between two closely related host species
where the adaptation process to a new host can occur rapidly,
e.g. transmission from chicken to turkey or chicken to quail
(105). Interspecies transmission between mammals and birds
has occurred rarely, examples are transmission from chicken
to human or wild waterfowl to pig. The majority of such
transfers have produced single or isolated cases of infection
with elimination from the population before adaptation or
reassortment with host adapted influenza viruses can occur in
the new host population. One exception to the adaptation
rule has been the ease and frequency of transfer of swine
H1N1 viruses to turkey breeder hens ( 6 6 ) , but these are
sporadic occurrences and have involved only MPAI viruses.
Other factors, such as geographic restriction, intermixing of
species, age and density of birds, weather and temperature
also affect the ability of the AI virus to move within and
between host species and affect the overall incidence of
infections (105).
Three pandemics of influenza have occurred in the human
population during this century, in 1 9 1 8 (H1N1), 1 9 5 7
(H2N2) and 1 9 6 8 (H3N2) (80). The immediate source of the
influenza viruses is unknown, but molecular data suggests
that some of the influenza genes originated from the AI
genetic
reservoir
and
recombined
with
existing
human-adapted influenza viruses ( 8 0 , 121). These AI genes
reassorted with genes from existing human-adapted influenza
viruses to create new influenza viruses that were adapted
rapidly to the human host and exposed to an
immunologically naïve population.
Four incidences have been documented of single cases or
small clusters of cases demonstrating direct transmission of
influenza viruses from birds to humans. The first was a fowl
plague-like virus (H7N?) isolated in 1 9 5 9 from a
forty-six-year-old man with hepatitis, following his return
from a two month trip overseas. The virus was HP "for
chickens, but was only moderately pathogenic for the man,
who recovered ( 3 1 ) . The second case was reported as
further cases of AI in humans and probably averted a
recombination event between the avian and human influenza
strains or adaptation of the H5N1 AI virus to humans. In the
case-controlled study ( 6 8 ) , exposure to live poultry a week
before illness was associated with the H5N1 influenza, but
eating or preparing poultry products was not associated with
H5N1 influenza. The fourth incident of direct transmission of
AI viruses from birds to humans involved infection of seven
individuals with MPAI (H9N2); two children in Hong Kong
and five patients in the People's Republic of China were
affected ( 2 5 , 7 4 ) . All patients recovered. In these four
incidences of transmission of AI to humans, the influenza
viruses that infected the humans had all eight genes of avian
origin, but transmission was limited because of inefficient
spread from human to human. Transmission was probably
from aerosol exposure to high levels of virus in the respiratory
secretions or faeces of the poultry and not from preparing or
consuming poultry meat or products.
These occurrences emphasise the interspecies transmission
potential of some AI viruses. However, the interchange or
spread of influenza viruses between humans and other
species, including birds, has been an exceedingly rare
occurrence in nature ( 9 9 ) . Some evidence suggests that AI
viruses have different potential for infecting and causing
disease in humans in such rare instances. During the
seventeen HPAI outbreaks (Table I), human infections were
identified in association with only the Hong Kong H5N1
outbreak in 1997. While the largest number of birds involved
in a single outbreak of HPAI was in 1 9 8 3 - 1 9 8 4 in the USA
(Table I), attempts to demonstrate primary replication and
recovery of the H5N2 AI virus from workers in depopulation
crews were unsuccessful (15). Using an experimental
laboratory mouse model to predict the replication and
virulence potential of HPAI viruses for humans, three AI
viruses from Hong Kong (A/human/HK/156/97, A/chicken/
HK/220/97 and A/chicken/HK/728/97) were shown to
replicate to high titres in the lungs and to be HP for mice
without prior passage or adaptation in mice ( 3 2 ) . The
strains A/chicken/Scotland/1959 (H5N1), A/chicken/Italy/97
(H5N2), and A/chicken/Queretaro/7653-20/95 (H5N2) had
much lower levels of pulmonary replication in the mouse
model and caused neither illness nor death in the mice.
473
Rev. sci. tech. Off. int. Epiz., 19 (2)
A/Turkey/England/91 (H5N1) caused mild illness and death in
one of eight inoculated mice. These mouse studies suggest
that only certain AI viruses have the potential to cross from
birds to mammals. Furthermore, no association has been
found between the human cases of AI and the ability of the AI
viruses to be HP or MP for chickens. Other workers have
suggested a greater potential of influenza viruses to cross from
pigs to humans (53), and pigs may be the intermediate mixing
vessel of avian and mammal influenza viral genes (83).
The probability of an avian virus entering the human
population, reassorting and establishing a new lineage of
human influenza virus capable of causing a pandemic is
extremely low, but this is consistent with the time span
between emergence of the three human pandemic influenza
viruses of the 20th Century (13). The main threat of AI viruses
is not the direct spread of AI virus to the human host and in
toto adaptation to the new human host, but the recombination
of the AI virus with human-adapted influenza viruses that
would lead to an antigenic shift and emergence of an influenza
virus which could cause a pandemic in humans (13).
Prevention, control and
eradication
Various strategies have been used for prevention, control and
eradication of AI throughout the world. Strategies utilised
cover the spectrum from no action on some MPAI viruses to
the extreme of implementing stamping-out or depopulation
programmes for eradication of HPAI ( 3 9 , 1 0 6 ) . Historically,
individual countries have chosen control or eradication
requirements to meet domestic expectations ( 5 7 ) . However,
the implementation of international trade agreements and
standardisation
of sanitary health requirements
has
challenged this approach. Measures taken to prevent, control
or eradicate AI will vary depending on the pathogenicity of
the AI viruses, types of birds infected, geographic distribution
of infected birds, requirements of domestic and international
markets and economic status of the nation ( 3 9 , 102, 1 0 6 ) .
With regards to HPAI, prevention should be limited t o .
strategies to preclude introduction and exposure of poultry to
the virus. However, if HPAI does occur, eradication is the only
viable option. In contrast, control implies reduction in
incidence to a low or economically manageable level. A
control programme may be an acceptable strategy for some
strains of MPAI, but is not an acceptable option for an OIE List
A disease such as HPAI or a highly virulent form of Newcastle
disease. For MPAI or HPAI, an effective control or eradication
programme has the following essential elements:
a) comprehensive,
integrated
national
surveillance
and
diagnostic programmes
b) enhanced biosecurity practised at all levels of production
and processing by all employees of companies, diagnostic
laboratories and government agencies that have contact with
poultry or equipment from poultry operations
c) education of poultry farmers and other workers about AI
control and sharing of information on surveillance and
control strategies at all levels in the production process
d) quarantine or controlled movement of Al-infected poultry
e) stamping-out or slaughter programme for all HPAI and
some H5 and H7 MPAI outbreaks
f) vaccine use as one element of a comprehensive control
programme and under specific situations with national
government control.
Surveillance and diagnostics
A comprehensive, integrated surveillance and diagnostic
programme is essential to determine the magnitude of AI virus
infections in commercial and live-bird market domestic
poultry, and migratory and non-migratory wild birds. Such
information is essential to establish the national or regional
prevalence of MPAI and HPAI within a country, which in turn
affects the import and export potential of poultry, poultry
products and other related avian commodities. Transparency
of surveillance data .between countries is essential in
supporting international trade in poultry and poultry
products. A national or regional veterinary medical diagnostic
infrastructure is essential to make rapid and accurate
diagnosis of AI and to characterise the virus isolates as MP or'
HP. The rapid diagnosis of the index case of HPAI is crucial, as
this will allow a rapid response to eliminate the disease before
additional birds are infected and the disease becomes
endemic. An example of such an infrastructure for
surveillance and diagnostics exists in Australia, where in
1 9 7 6 , 1 9 8 5 , 1 9 9 2 , 1 9 9 5 and 1997, limited outbreaks of HPAI
occurred (Table I) (124). The presence of comprehensive
veterinary diagnostic services in provincial and national
government laboratories in addition to specially trained
private and government veterinarians resulted in rapid
diagnosis of HPAI and swift action by the national
government to depopulate infected and exposed poultry
resulting in the eradication of the disease early in the
outbreak.
Biosecurity
Enhanced biosecurity is an important part of any ÄI
prevention and control programme, and serves two purposes,
as follows:
a) containment of
(biocontainment)
the
AI
virus
on
infected
farms
b) prevention of introduction of the virus to naive farms
(bioexclusion).
The practice of biosecurity must be implemented at all levels
within the poultry industry and allied groups, and b y all
employees involved. This includes personnel within the
production
companies,
diagnostic
laboratories
and
government agencies that have contact with poultry, poultry
474
products, poultry waste, equipment or other resources used
on poultry farms. Proper disinfection and decontamination
are essential for all equipment used on more than one farm,
and common personnel should take the correct precautions
to avoid carrying virus from faecal and respiratory secretions
on boots, shoes and clothing. Ideally, workers should be
restricted to one farming operation, and workers should not
visit or work on other poultry farms and not own or tend
poultry at their homes. The strength of a biosecurity
programme does not lie in the written policy, but in the
practice of biosecurity, from the simplest detail, by every
individual concerned, and on all farms. When workers must
be shared between farms, such as vaccination crews, in
addition to depopulation and other crews, the highest level of
biosecurity must be practised. Failure to do so presents a real
danger as reported in eradication efforts against an outbreak
of virulent Newcastle disease during the 1970s in California.
The vaccination crews were epidemiologically linked to
prolonging the outbreak b y spreading the field virus between
farms (116).
Education
Education and dissemination of information to poultry
farmers and other workers, veterinarians, government
regulators and the media, concerning AI status and required
biosecurity practices are essential for the acceptance,
implementation and successful outcome of a control
programme. Failure to communicate critical information will
encourage complacency and failure to comply or participate
in the control efforts. Failure to provide information or the
presentation of inaccurate information to the media will
reduce public trust in the safety and wholesomeness of all
poultry products for human consumption. Especially
important is communication of the low potential for
transmission of AI to people through consumption of poultry
products.
Quarantine
Containment of the infection in individual farms or regions
(zones), through controlled movement of poultry, equipment
and personnel is essential in preventing spread of AI to
non-infected locations. During outbreaks of HPAI, birds
should be depopulated and disposed of within the quarantine
zone. Disposal should be by incineration, composting,
alkaline hydrolysis, fermentation or burial in accordance with
environmental standards and laws. All equipment should be
cleaned and disinfected before being removed from the
infected farm. In some situations, controlled movement of
poultry infected with MPAI may be necessary to minimise
financial losses to farmers. Under such circumstances, the
birds should be moved only after the infection has subsided
and virus excretion titres are low, usually a minimum of three
weeks after the first appearance of clinical signs of AI. Birds
should be moved and processed at the end of the day.
Transportation should be along routes that avoid poultry
farms as much as possible.
Rev. sci. tech. Off. int. Epiz., 19
Depopulation
In the case of outbreaks of HPAI, a stamping-out or slaughter
programme is the best first line of defence for halting spread
and eliminating the disease. The EU, USA, Australia and other
countries, as well as the OIE, have policies that dictate
depopulation as the principal control method for use in
outbreaks of HPAI and other List A diseases of the OIE ( 5 , 4 7 ,
73, 1 2 4 ) . In addition, a depopulation programme may be
necessary in outbreaks of low virulence H5 and H7 AI
depending upon the impact on export of poultry products or
the potential for MPAI viruses to mutate and become HPAI
viruses (131). However, stamping-out programmes must be
initiated early in an outbreak to be effective and economical,
and these programmes tend to be expensive and subject to
institutional inertia (106). The larger the geographic region
involved and the greater the density of poultry, the greater the
expense and difficulty in using a depopulation programme for
eradication. In 1 9 8 3 - 1 9 8 5 , the H5N2 HPAI outbreak cost the
government of the USA US$63 million to eradicate, including
the depopulation of 17 million birds (Table I) (59), while the
eradication of H5N1 HPAI in Hong Kong during 1 9 9 7 cost
the government of Hong Kong H K $ 1 0 0 million including
depopulation of 1.4 million birds (US$12 million) (L. Sims,
personal communication, 12 September 1999).
Vaccines
Vaccination should be used only as one element of a
comprehensive
eradication
programme
for
HPAI.
Historically, AI vaccines have been used mostly in the USA as
part of a control programme against sporadic outbreaks of
MPAI in meat turkeys in the upper Midwest ( 3 9 ) . Between
1979 and 1997, 2 2 , 3 8 5 , 0 0 0 doses of inactivated AI vaccines
were used, but generally vaccines against H5 and H7 subtypes
have been discouraged by the United States Department of
Agriculture ( 4 0 ) . A common use of vaccines has been to
protect turkey breeders against sporadic outbreaks of MP
swine H1N1 influenza viruses (79).
Although vaccines have been demonstrated to be protective
against fowl plague in experiments, vaccination was not used
in control and eradication programmes against HPAI until
1995, in Mexico and Pakistan (Table I) ( 3 6 , 6 9 ) . Since June
and August 1995, HPAI has not been identified in Mexico or
Pakistan, respectively. However, in Mexico, vaccines have
continued to be used to control MPAI (H5N2) in broiler
chickens, with 1.2 billion doses of inactivated AI vaccine and
0.3 billion doses of recombinant fowl pox vaccine with AI H5
gene insert being used between 1 9 9 5 and 1 9 9 9 ( 3 6 ) . The
government of the USA has provisions for usage of vaccines in
future eradication efforts should HPAI appear in the USA
(47). However, unrestricted use of AI vaccines will be
counterproductive to elimination efforts unless the other five
components listed earlier are integrated into the eradication
programme (106). In general, vaccines should be used only if
the number of HPAI cases is increasing and the disease is not
475
Rev. sci. tech. Off. int. Epiz., 19 (2)
contained within a single quarantine zone. Vaccine
application should be limited to poultry within quarantine
zones, barrier or 'ring' vaccination around the periphery of the
quarantine zone, or vaccination to protect poultry pedigree
stock within a region during an expanding HPAI outbreak.
The difficulty with vaccine usage is deciding when to stop
vaccination in order to conduct the final steps of the
stamping-out programme.
Two vaccine technologies have been used in the control or
eradication of HPAI, as follows:
a) inactivated whole virus vaccines
b) a recombinant fowl pox vaccine with an H5 AI HA gene
insert.
Inactivated whole virus vaccines were used extensively in
Mexico and Pakistan during the recent outbreaks involving
H5 and H7 HPAI viruses, respectively ( 3 6 , 6 9 ) . The
recombinant fowl pox vaccine has been used in Mexico in the
control programme
against
MPAI (H5N2).
Other
technologies hold promise for future use, including
deoxyribonucleic acid (81) and subunit HA protein vaccines
(27).
The HA protein elicits the major protective immunity in
poultry, but the NA also contributes to protection ( 3 3 , 6 2 ) .
Vaccines provide protection in chickens and turkeys against
illness and death following challenge by homologous HA
subtypes of HPAI viruses; i.e. H5 vaccine protects against an
H5 challenge virus, but not against an H7 challenge virus ( 2 2 ,
96, 9 7 , 1 2 5 , 127). This HA subtype protection is consistent
between vaccine and field virus with at least 8 7 % similarity in
HA amino
acid
sequence
(110).
Haemagglutinin
subtype-specific vaccines have been shown to lower infection
rates and reduce quantity of challenge virus shed from cloaca
and oropharynx of vaccinated chickens, but prevention of
replication or 'sterilising immunity' has not been achieved
( 1 0 6 , 1 0 9 , 1 2 2 , 1 2 7 ) . Vaccines can provide protection against
HPAI following a single dose for over twenty weeks ( 1 0 6 ,
109). Immunity induced by vaccines can provide protection
even against challenge with high doses of HPAI virus. For
recombinant fowl pox H5 AI viral gene insert vaccine,
chickens three weeks post-vaccination were protected against
illness and death when exposed to an HPAI virus from Mexico
(A/chicken/Queretaro/14588-19/95
[H5N2]),
even
to
challenge doses reaching 1 0 mean chicken embryo lethal
doses ( E L D ) per chicken (104).
7 . 5
50
Usage of inactivated AI vaccine can interfere with serological
surveillance. Inactivated AI vaccines produce high antibody
titres in poultry that are indistinguishable from field virus
challenge as determined by the HI and agar gel precipitin
(AGP) tests. To prevent interference with surveillance,
unvaccinated sentinel birds should be placed within
vaccinated flocks (39), or vaccines that do not elicit antibody
responses to internal virus proteins should be utilised. Such
vaccines include recombinant fowlpox with an AI HA gene
insert or HA subunit protein vaccines that lack the
nucleoprotein and matrix components and thus do not give
an AGP response ( 2 7 , 1 0 6 ) .
Vaccine usage for AI control and eradication has definable
limits. Experimental studies have shown good or excellent
protection against MPAI and HPAI in SPF chickens, but in the
complexity of the field situation, vaccine use and protection
will not reach maximum potential. In the field, improper
vaccination technique, infections by immunosuppressive
viruses, incorrect storage and handling of vaccines and other
factors make field vaccinated chickens and turkeys less well
protected than those in laboratory studies. In control or
eradication of AI, emphasis should be placed on vaccination
as only one of the elements required in a comprehensive
strategy. Otherwise, any gains achieved through vaccine
protection from illness, death or reductions in shedding rates
will be nullified. Control or eradication of AI can be achieved
only if all aspects of the intervention strategy are operational.
This includes strict biosecurity practices on infected and
non-infected farms involving all personnel and equipment
moved between farms, limiting human access to farms,
providing adequate surveillance and diagnostics, imposing
quarantine on infected flocks, and providing a low-risk
method of elimination and disposal of infected birds.
As an alternative to vaccines, antiviral drugs have been
proposed for treatment of avian influenza viral infections in
poultry (120). However, in experimental studies simulating
field conditions with HPAI, resistance to amantadine
developed rapidly, calling into question the value of antiviral
drugs for treatment of influenza in poultry (17). Furthermore,
the cost of the drugs would be prohibitive, and in the USA, no
antiviral drugs are approved by federal regulatory agencies for
treatment of influenza in poultry. Poultry treated with
antiviral therapy cannot be used for human consumption.
Measures to prevent importation of avian
influenza
Based upon accepted OIE sanitary standards for List. A
diseases, the presence of HPAI typically prevents an affected
country from exporting poultry and poultry products ( 7 3 ) .
Such movement restrictions are instituted by importing
countries as legitimate measures to prevent the importation of
HPAI. However, any restriction imposed by an importing
country should be justified by a science-based risk analysis
shared between the trading partners,. For example, continued
trade with an HPAI-free region of an exporting country could
be accepted, despite the presence of HPAI in another region of
that country, if adequate risk reduction movement controls
are in place within the country.
Demonstration of freedom from HPAI can be accomplished
through serological surveillance. Negative results in
serological tests such as the AGID test or ELISA provide
476
Rev. sci. tech. Off. int. Epiz., 19 (2)
reliable evidence of HPAI-free status. However, positive
results in these serological tests can result from infections by
either MPAI or HPAI viruses. To avoid trade restrictions, an
exporting country must verify through virus isolation and in
vivo and/or in vitro pathotyping tests that any positive
serological results are from infections with MPAI viruses,
especially if the viruses are H5 or H7 subtypes. The risk of
importing MPAI in meat products is negligible because MPAI
viruses replicate in the digestive and respiratory tracts, but not
in meat tissue. In contrast, HPAI is a systemic disease and the
virus can be present in most tissues, including meat.
Acknowledgements
The assistance of Joan R. Beck in production of the colour
plate is gratefully acknowledged. Permission was granted to
reprint colour Figures la, e, i and m by the Journal of Virology
(99), Figures l c and l g by Iowa State University Press (33),
and Figure l b by Veterinary Pathology (103). Thomas J .
Myers is thanked for critical review of the manuscript.
•
Influenza aviaire hautement pathogène
D.E. Swayne & D.L. Suarez
Résumé
L'influenza aviaire hautement pathogène est une maladie extrêmement
contagieuse d e la volaille, à t r o p i s m e multiple e t systémique, qui s'accompagne
d'une mortalité élevée. Elle est due à certains sous-types H5 et H7 du virus
influenza de type A , de la famille des Orthomyxoviridae.
Toutefois, la plupart des
souches virales de l'influenza aviaire sont modérément pathogènes et provoquent
soit des infections infracliniques, soit des maladies respiratoires et/ou de la
reproduction chez plusieurs espèces d'oiseaux domestiques et sauvages.
L'influenza aviaire hautement pathogène fait partie de la Liste A de l'Office
international des épizooties (OIE), alors que l'influenza aviaire modérément
pathogène ne f i g u r e n i s u r la Liste A ni sur la Liste B d e l'OIE. Dix-huit épisodes
d'influenza aviaire hautement pathogène ont été décrits depuis que l'origine de la
peste aviaire de 1955 a été imputée au virus de l'influenza aviaire.
Les virus de l'influenza aviaire modérément pathogène sont présents chez les
oiseaux migrateurs aquatiques, qui servent de r é s e r v o i r ; ces virus atteignent
o c c a s i o n n e l l e m e n t des volailles domestiques, entraînant l'apparition de maladies
peu sévères. Les virus de l'influenza aviaire hautement pathogène n'ont pas de
réservoir reconnu chez les oiseaux sauvages, mais ils peuvent parfois être isolés
chez c e s derniers lors d'épidémies affectant les volailles domestiques. D'après la
documentation existante, les virus de l'influenza aviaire hautement pathogène
proviendraient des virus de l'influenza aviaire modérément pathogène suite à des
mutations de la protéine de surface de l'hémagglutinine.
La stratégie r e c o m m a n d é e en cas d'influenza aviaire hautement pathogène
consiste à éviter toute exposition au virus et à éradiquer la maladie. Les
programmes de prophylaxie qui tolèrent une faible incidence de l'infection ne
constituent pas une méthode acceptable pour faire f a c e à des c a s d'influenza
aviaire hautement pathogène, mais ils ont été utilisés lors de certaines épidémies
d'influenza aviaire modérément pathogène. Les stratégies de lutte contre
l'influenza
aviaire
hautement
et modérément
pathogène,
reposent
essentiellement sur le diagnostic, l'hygiène, l'éducation, la quarantaine et la
réduction de la taille des élevages. La vaccination a été utilisée dans certains
programmes de prophylaxie et d'éradication de l'influenza aviaire.
Mots-clés
Influenza aviaire hautement pathogène - Maladies aviaires - Orthomyxovirus - Peste
aviaire.
477
Rev. sci. tech. Off. int. Epiz., 19 (Z)
Influenza aviar altamente patógena
D.E. Swayne & D.L. Suarez
Resumen
La influenza aviar altamente patógena, causada por algunos subtipos H5 y H7 del
tipo A del virus de la influenza (familia Orthomyxoviridae),
es una enfermedad de
c a r á c t e r sistémico y extremadamente contagioso que afecta a las aves de corral,
atacando diversos órganos y provocando una elevada tasa de mortalidad. La
mayoría de las cepas víricas de la influenza aviar, sin e m b a r g o , son
m o d e r a d a m e n t e patógenas (influenza aviar de tipo leve) y provocan infecciones
subclínicas o enfermedades del aparato respiratorio y/o reproductivo en diversas
especies de aves salvajes o domésticas. La influenza aviar altamente patógena
es una enfermedad de la Lista A de la Oficina Internacional de Epizootias (OIE),
mientras que la influenza aviar m o d e r a d a m e n t e patógena no figura ni en la Lista A
ni en la Lista B de la OIE. Desde que en 1995 se identificó el virus de la influenza
aviar como causante de la peste aviar, se han registrado dieciocho brotes de
influenza aviar altamente patógena.
Las aves a c u á t i c a s salvajes constituyen el reservorio de los virus de la influenza
aviar m o d e r a d a m e n t e patógena, que de manera ocasional pueden afectar a las
aves de corral domésticas y dar lugar a brotes infecciosos de c a r á c t e r leve. En
cuanto a los virus de la influenza aviar altamente patógena, no se les c o n o c e
reservorio alguno entre la fauna salvaje, pese a que de vez en c u a n d o , en el curso
de brotes entre las aves de corral domésticas, han aparecido en muestras
t o m a d a s de aves salvajes. Está c o m p r o b a d o que los virus altamente patógenos
provienen de sus congéneres m o d e r a d a m e n t e patógenos, mediando entre ambos
una serie de mutaciones de una proteína de superficie llamada hemaglutinina.
La prevención de la exposición al virus y la erradicación son los dos métodos
aceptados para hacer frente a la influenza aviar altamente patógena. La
aplicación de programas de control, cuya lógica admite la presencia de la
infección a niveles bajos de incidencia, no constituye un método de gestión
aceptable en este caso, aunque ello no ha impedido recurrir a programas de ese
tipo ante algunos brotes de influenza aviar altamente patógena. Una estrategia
adecuada para hacer frente a la influenza aviar (en cualquiera de ambas formas)
debe c o m p r e n d e r medidas de vigilancia y diagnóstico, seguridad biológica,
f o r m a c i ó n , cuarentena y sacrificio sanitario. Para algunos programas de control y
e r r a d i c a c i ó n de la influenza aviar se ha utilizado la v a c u n a c i ó n .
Palabras clave
Enfermedades aviares - Influenza aviar altamente patógena - Orthomixovirus - Peste
aviar.
•
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