Incidence, epidemiology and control of bovine pestivirus infections and disease

Rev. sci. tech. Off. int. Epiz., 1990, 9 (1), 195-205.
Incidence, epidemiology and control
of bovine pestivirus infections and disease
in Australia and New Zealand
I.R. LITTLEJOHNS * and G.W. HORNER **
Summary: Pestivirus infection of cattle is widespread and common in both
Australia and New Zealand. The majority of adult animals, of the order of
60%, carry antibody. Associated disease is almost entirely that resulting from
infection in utero. This includes death of the conceptus, at any stage from
conception through pregnancy, or, in those which are born as persistently
infected carriers, mucosal disease, most commonly in a chronic form. Little
or no disease is recognised as a result of the post-natal infection of non-pregnant
animals and these appear to be of little consequence as spreaders of infection.
Transmission and enzootic maintenance depend primarily on the persistently
infected carriers that are immunotolerant after early in utero infection and range
clinically from normal, or nearly normal, to overtly mucosal diseased. The
expulsion of an infected conceptus, and associated discharges, also provides
an effective source of infection. There is generally little active control attempted.
Vaccines are not available in Australia and are not widely used in New Zealand.
However, interest in control is growing in those areas of the industry, especially
in breeding by artificial insemination and embryo transfer, where it is perceived
that the pathogenic impact of the virus may be amplified.
KEYWORDS: Australia - Bovine virus diarrhoea - Cattle diseases - Control Epidemiology - Mucosal disease - New Zealand - Persistent infection Pestivirus - Transmission.
INCIDENCE
In Australia, mucosal disease (MD) was first reported as a clinico-pathological
entity in 1957 (3). M o r e extensive disease observed in South Australia was described
in 1959 as a " m u c o s a l type disease" (17) and considered to be p a r t of a mucosal
disease-virus diarrhoea complex, but there is no evidence of it being due to a pestivirus
infection and an alternative aetiology was subsequently suggested (7). A n t i b o d y to
pestivirus, neutralising strain C24V, in bovine sera was reported in 1962 (6). Virus
was recovered from cases of M D in 1964 (7) when the disease was recognised t o occur
in acute and chronic forms. Mucosal disease, m o r e commonly chronic t h a n acute,
* New South Wales Agriculture and Fisheries, Central Veterinary Laboratory, Glenfield 2167,
Australia.
** Ministry of Agriculture and Fisheries, Wallaceville Animal Research Centre, P.O. Box 40-063,
Upper Hutt, New Zealand.
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has since been widely diagnosed in all parts of t h e country. In these cases t h e degree
of development of the various pathological facets has been quite variable (15), and
the full clinico-pathological spectrum of M D appears to range from classical disease
to virtual normality. This is consistent with the fact that some carriers, with the specific
immunotolerance a n d persistent viraemic infection that are prerequisite t o M D , may
be clinically n o r m a l over what appears t o be a normal life span. Conversely, some
lesions of chronic M D m a y be demonstrable in persistently infected (PI) carriers which
die of any other cause.
T h e picture is generally similar in New Zealand, where a severe MD-like syndrome
was reported in 1961 (25). Although cytopathogenic agents were isolated, these were
never confirmed as pestiviruses and prominent kidney lesions suggested an alternative
aetiology. In 1966 neutralising antibodies t o strain C60F were reported in cattle (5)
and the following year a cytopathogenic strain of pestivirus was isolated from a clinical
case of acute M D (11). Reports of serological evidence of bovine pestivirus infections
followed (4, 22). T h e incidence rate of clinical disease has n o t been well defined.
Between 50 a n d 100 pestivirus isolations are m a d e annually at t h e R u a k u r a Animal
Health L a b o r a t o r y and about 9 0 % of these are non-cytopathogenic strains. The
majority (58%) are recovered from beef cattle (J. Howell, personal communication).
Acute M D occurs (9) but t h e chronic form is more c o m m o n . A number of veterinary
case reports have associated the disease with foot lesions (19), polyarthritis (8), seasonal
factors (most c o m m o n in a u t u m n a n d winter), pneumonia, concurrent parasitism
and bacterial infections such as Dermatophilus and Salmonella (1).
Bovine virus diarrhoea (BVD), as a disease of high morbidity and low mortality
following post-natal infection, has never been observed in Australia, n o r have other
defined clinical signs been recognised in that context. In New Zealand, it is considered
that some animals m a y show transient diarrhoea, oral erosions, leucopaenia and
anorexia after post-natal infection.
Natural M D recognised in this region m a y not be strictly comparable, or limited,
t o the disease that is induced experimentally by the superinfection of a P I animal
with a cytopathogenic pestivirus. It has been noted that cytopathogenic strains tend
t o be associated with m o r e acute a n d non-cytopathogenic with more chronic cases
( W . A . Snowdon, personal communication) b u t there was n o strict rule t o the
relationship (7). Subsequently, no definitive association between cytopathogenic virus
and M D has been found in diagnostic experience. This m a y be due, in part at least,
t o t h e c o m m o n diagnostic practice of attempting virus isolation from blood of sick
animals in preference t o gut or other tissues which are only available post mortem.
Early experience in examining a wider range of tissues suggests that cytopathogenic
viruses a r e t h e n m o r e frequently recoverable ( P . D . K i r k l a n d , p e r s o n a l
communication). T h e New Zealand experience is that, from natural cases of M D ,
non-cytopathogenic virus is frequently isolated from the blood clot and cytopathogenic
virus from a different tissue or source, such as a nasal swab, of the same animal.
W h a t is recognised as chronic M D may be comparable t o what has elsewhere been
termed 'chronic B V D ' . However, we see no clear line of demarcation between the
acute and chronic forms of M D . F o r this reason, a n d also because n o acute BVD
is recognised, the option of referring t o disease, due t o persistent infection by noncytopathogenic virus only, as 'chronic B V D ' is not apposite. Both acute and chronic
forms of M D have early in utero infection as an essential prerequisite.
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Other consequences of early in utero infection have been observed as field cases
(2, 21) a n d / o r demonstrated after controlled natural infection with local strains of
virus (18). These include early embryonic death, mummification, abortion, stillbirth
and perinatal mortality, congenital abnormalities of the eye and central nervous
system, and illthrift. The last two syndromes are seen with or without classical clinical
signs and lesions of M D . However, in the south coastal regions of Western Australia,
congenital abnormalities, stillbirth and illthrift are seen as the most c o m m o n
manifestations of disease while chronic M D is u n c o m m o n , perhaps because young
stock, particularly steers, are turned off for slaughter before clinical signs develop
(R. Reuter, personal communication).
The incidence of post-natal infection, as reflected by the prevalence of animals
with serum antibody, is high and comparable to that described in others parts of the
world where cattle industries are well developed. In Australia, n o study o n a national
scale has been made since that published by St. George et al. in 1967 (23), when
previous data (6, 7) were confirmed and neutralising activity against the C24V strain
was found in 6 0 % of individual sera and 9 0 % of herds overall. Higher rates were
found in the more northern areas and where herds are larger, although lesser figures
(average within-herd prevalence of a p p r o x . 4 0 % ) for antibody prevalence in n o r t h
Queensland have been found in a recent sample of 1,632 animals o n ten properties
(A. J a n m a a t , personal communication). M o r e recent casual experience and
observations would suggest that the 1967 figures would still be about right in the 1980's
and, in testing of Western Australian cattle in 1985 for export purposes, approximately
60% were found to have antibody ( T . M . Ellis, personal communication). In South
Australia, from specimens submitted over approximately two years, including all of
1988, for the diagnosis of herd problems (mostly abortion), antibody was found in
7 6 % (of 70) herds and 4 5 % (of 777) individual animals ( A . M . P o i n t o n , personal
communication). In the larger of the New Zealand serological studies (4), 3 4 % of
cattle were found to have antibody neutralising the local Bovax strain (11). However,
an analysis of serological testing on diagnostic sera over ten years ( G . W . H o r n e r ,
unpublished data) showed 5 8 % of sera to have neutralising antibody, and this is
probably a more accurate indication of the current status of the New Zealand cattle
population.
In herds in which pestivirus infection is enzootic, antibody prevalence, excluding
PI carriers, often reaches 100% in groups which include carriers. Antibody-free groups
in such herds are usually those that have, for one reason or another, been raised in
isolation from carriers. Within-herd and between-herd antibody prevalence estimates
therefore depend largely on the definition of what constitues a ' h e r d ' , whether by
management, property or ownership.
EPIDEMIOLOGY
Most of the understanding of the epidemiology of pestivirus infections and disease
relates to cattle held under extensive pastoral conditions, usually beef animals which
are subject to minimal handling under relatively simple management regimes. Relevant
information is derived from diagnostic and subsequent investigations, and is therefore
liable to be biased t o those circumstances that allow overt disease t o develop and
be recognised. Quiescent enzootic infections, and epidemiological parameters that
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favour that state, are therefore likely to be under-represented. More attention is now
being directed towards the situation in dairy cattle (12) with the expectation that it
will be more complex because of a n u m b e r of factors. These include more handling,
non-seasonal breeding and in some areas, disposal of male calves and the hand-rearing
of replacement heifer calves and, particularly, the feeding of pooled milk to calves.
The pooled milk might be expected often to contain b o t h virus from a P I cow and
neutralising antibody from her herdmates, while many of the calves to which it is
fed will have some, decaying, maternal antibody. Under these circumstances, sources
of infection and patterns of spread are likely to be different from those understood
under management for beef production. P o t e n t exposure of calves to infection while
they carry marginal amounts of maternal protection is likely to compromise the
sensitivity of serological tests subsequently used.
Maintenance of pestiviruses ultimately depends o n transplacental infection of
fetuses within the first few m o n t h s of pregnancy, to produce immunotolerant PI
carriers. Fetal infection can follow either P I (congenital) or transient (post-natal)
infection of the d a m , but the resulting epidemiological patterns of disease m a y be
rather different. The P I cow m a y produce a number of P I animals over a number
of years whereas, particularly in seasonally bred herds, effective horizontal infections
tend to occur in clusters, producing clusters of PI animals which later become clusters
of cases of M D . Horizontal transmission from a transient infection is most unlikely
t o occur under grazing conditions or even with the levels of close housing and
husbandry that occur in Australia. Consequently, serial horizontal transfers are of
little importance in maintaining the virus and it is likely that enzootic infection in,
and status of, a herd will usually depend on the presence within that herd of one
or more P I animals. Even then, transmission rates under grazing conditions m a y be
quite low, of the order of 1% per day (14). This is illustrated by observations made
o n animals found to be in contact with a P I carrier (10) when, after at least three
m o n t h s , only four of nine previously susceptible animals had developed antibody.
However, the transmission rate can be greatly increased by closer contact, such as
yarding for short periods. For example, 8 of 13 (62%) susceptible animals were
transiently infected after they had been yarded overnight with one P I carrier (14).
A similar result was achieved using similar conditions to establish natural infections
for experimental purposes (18). Valid generalisations about the importance of the
P I carrier as the major source of infection do not preclude that less c o m m o n and
less effective mechanisms m a y obtain occasionally. For example, in one herd which
was otherwise serologically free of pestivirus activity over two years, two calves of
one cow each developed active antibody. T h e cow h a d circulating precipitating
antibody and n o virus was recovered from a wide variety of secretions, including
milk, excretions or swabs from mucosal surfaces. Nevertheless, the conclusion that
this systemically immune cow could, somehow through the intimacy of nursing, infect
its own calves but not others, is difficult to refute (LR. Littlejohns, unpublished data).
The prevalence of persistent infections has not been established over large numbers
and is likely to vary considerably over the diverse conditions under which cattle are
managed. A n overall estimate, again based on casual experience and observations,
of the order of 1 % is suggested. Prevalences of 2 % to 5 % , of P I animals which m a y
be clinically normal over periods of years, have been found in individual herds. Higher
figures, usually recognised retrospectively after an incidence of M D , m a y be reached
when there has been previous epidemic exposure of pregnant susceptible animals.
Preliminary results have been reported for a survey of maiden dairy heifers in
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43 herds in the H u n t e r Valley of New South Wales (12). Further results from this
work ( D . J . Kennedy, personal communication) indicated an overall prevalence of
0.8% (4/514). Testing of healthy cattle for export certification at the Central Animal
Health L a b o r a t o r y , Wallaceville, has shown t h a t 1.8% (16/876) were viraemic. It
is probable t h a t most of these were P I animals.
It is suggested that the prevalence of the P I state may be sex-related. Of 14 P I
animals detected without selection bias in three herds, 12 were male (exact p < 0.01
compared to 1:1 ratio). This would imply an interesting possibility of there being
a higher in utero mortality rate a m o n g female t h a n male fetuses. Impressions from
diagnostic experience over the years would suggest that the incidence of M D m a y
also be sex-related, with the majority of M D cases being steers. However, a recent
dissection of 159 diagnostic virus isolations, for which the sex of the case animal was
known, at the Central Veterinary L a b o r a t o r y , Glenfield, found only a modest bias
with a male/female ratio of 92:67 (exact p = 0.03 compared t o 1:1 ratio). T h e sex
ratios of b o t h persistent infection and disease deserve to be investigated further.
It is recognised that the P I prevalence will decrease with age a n d , hence, is at
its highest in fetuses. As several hundred or m o r e fetuses contribute to each b a t c h
of fetal calf serum produced commercially, it is statistically unlikely that any will
be free of pestivirus. It is also likely that any pool which includes serum of a
pestiviraemic fetus will also include homologous antibody, from a h e r d m a t e which
was infected after achieving immunocompetence, thereby increasing the difficulty
of virus detection. In studies on samples representing different stages of processing,
pestiviruses were recovered more frequently from those taken after m o r e handling,
presumably indicating carry-over contamination from reused equipment (A. J a n m a a t ,
personal communication). Results from these studies also give strong indirect evidence
of virus presence being extensively camouflaged by the concurrent presence of
antibody. The hazards of using fetal calf serum and, to a lesser extent, any bovine
serum, in work with pestiviruses is now generally appreciated. A n even broader scope
to the problem is suggested by the recovery of pestivirus from horse serum used in
cell culture medium, which was presumed to be due also to contamination from shared
processing equipment ( G . W . H o r n e r , unpublished data).
The frequency with which overt M D occurs in P I carriers is not k n o w n . In New
South Wales approximately 100 diagnostic pestivirus isolations are m a d e annually,
mainly from cases of M D . They represent only about 0 . 5 % of an estimated 20,000
PI animals b o r n annually (estimated as 1% of a total of a p p r o x . 2 . 1 0 calves b o r n ) .
This probably reflects a low rate of disease-reporting more than it does a low incidence
of disease a m o n g P I animals.
6
Although the incidence of post-natal infections, based o n antibody prevalence,
must be high at some stage of life, there is very little detail available o n the
circumstances under which these usually occur. The extent to which they may enhance
the effects of other disease under conditions of intensive management a n d / o r stress
is not k n o w n . Only a very small proportion of the cattle populations of Australia
and New Zealand are raised under intensive conditions.
Related pestiviruses occur in sheep and goats and, on serological evidence, in pigs,
buffalo and deer, including large feral populations of all of these species except sheep.
The prevalence of C24V neutralising antibody in sheep in Australia in 1967-68 was
approximately 10% of adult animals and 1 3 % of flocks (24), implying a within-flock
prevalence of the order of 7 5 % . In contrast to the geographic distribution of reacting
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cattle, the prevalence of antibody in sheep increased from n o r t h to south. However,
lower prevalences were found in a survey conducted in the south-west of Western
Australia in 1985, when pestivirus group precipitating antibody was found in 31/1226
(2.5%) sheep, and C24V neutralising antibody in 10/679 (1.5%) (T.M. Ellis, personal
communication). The comparison of antibody prevalences in sheep and cattle (approx.
60%) in this area of Western Australia, where the two species would have considerable
contact, would imply that transmission between species would not commonly occur
under field conditions. In New Zealand neutralising antibody is not u n c o m m o n in
sheep (19% of 1013 sera tested at R u a k u r a ) but these titres are probably due mainly
to infections with ovine strains of pestivirus since clinical hairy shaker disease (Border
disease) is c o m m o n in some, expecially southern, areas. Similarly, pestivirus antibody
has been detected in deer ( 9 % of 907 sera) and goats ( 4 % of 539 sera) (G.W. Horner,
unpublished data).
The question of pestivirus infection of Australian marsupials needs to be examined
further. Sera from 2 of 44 Bennett's wallabies neutralised C24V virus but those of
29 other marsupials, including 7 forester kangaroos, 10 pademelons, 2 p o t o r o o s , 3
brush possums, one b r o w n bandicoot and 6 T a s m a n i a n devils did not (20).
The PI animal, as a source of infection, includes the infected conceptus of a normal
cow, now systemically i m m u n e . Its expulsion, together with associated discharges,
at any stage u p t o and including the birth of a viable P I calf, presents a potent source
of virus. A cluster of new infections has been observed to occur in heifers after
herdmates had been infected following successful joining, and some returned to service
at 21 days. This suggests that virus excretion, presumably from the reproductive tract,
was associated with early embryonic death and consequent vaginal discharges (M.R.
M c G o w a n , P . D . Kirkland and I.R. Littlejohns, unpublished data). This has not been
regarded as transmission from the transiently infected parent. Discharges at and after
parturition of a P I calf are almost certainly highly infectious but the period over which
discharges might remain so is not k n o w n .
Transmission rates are also greatly increased by handling, particularly when
procedures such as the use of nosegrips or shared needles for bleeding or inoculations
are involved. H u m a n operatives, including veterinarians, m a y be effective vectors
of infection. The importance of close contact and handling in facilitating pestivirus
transmission cannot be overstated. For such generalisation, however, the nature or
'quality' of the contact may be as important as the apparent 'quantity'. On occasions,
even when there has been apparent close contact, transmission rates have been
inexplicably low (A. J a n m a a t , personal communication).
W h e n infection of the fetus is derived from a PI d a m , a familial incidence is
established which may extend over at least several generations (13, 14). This may
be responsible for a low incidence of disease within a herd over a number of years.
However, because a proportion of the individuals within viraemic families will develop
disease and die, and those which survive for a time are generally not very successful
breeders, the families are at a survival disadvantage compared to the rest of the herd
and tend to die out. Survival of the virus in a population therefore depends o n the
alternative mechanism whereby horizontal transmission results in transient
asymptomatic infection of a pregnant female followed by transplacental infection
of its fetus. It is believed that self-cure of individual herds m a y occur if horizontal
transmission to pregnant females is precluded.
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Infection of pregnant beef females most commonly occurs when heifers have been
raised in isolation from the adult herd, within which there is one or m o r e carriers,
and are then mixed with these older animals at or after joining. W h e n heifers are
mated separately, for example when bulls of particular conformation are preferred,
then their isolation and susceptibility m a y be maintained for a further year and their
exposure to effective infection m a y be deferred by one year. Other circumstances
which m a y allow susceptible cows of any age to become infected are those which
necessitate emergency movement and agistment of stock, typically occurrences of
drought, flood or fire. Production of P I animals may then occur in numbers, resulting
in a high incidence or ‘outbreak’ of M D some 12-30 m o n t h s after the emergency,
that is, allowing 5-9 m o n t h s in utero plus 6-24 m o n t h s of age.
M a n y pestiviruses readily infect alternative hosts (including sheep, goats, deer,
buffalo and pigs) experimentally, so that cross-infections, either of cattle by a virus
which is normally maintained in another host species, or of cattle by a virus of bovine
origin which has infected an alternative host, can potentially play a p a r t in the
epidemiology of bovine pestivirus infections. Thirty three of 41 sheep were infected
by a yearling carrier heifer over 147 days when they were grazed together in a small
paddock with shared trough watering and supplementary feeding ( T . M . Jessep and
I.R. Littlejohns, unpublished data). This intimacy of contact between species m a y
be u n c o m m o n in the field and n o information is available o n the extent t o which
pestiviruses are exchanged between host species under natural conditions. Despite
the differences, previously mentioned, in the geographic distributions of higher
prevalences of antibody in sheep and cattle, a weak association (p = 0.12) was found
between the presence of cattle o n a property and the detection of antibody in sheep
(24).
Artificial breeding techniques are also capable of contributing to the spread of
pestiviruses. At least two occurrences of multiple cases of infection and disease have
been diagnosed following embryo transfers (13, details unpublished; P . D . Kirkland,
K.G. H a r t , A . Moyle and E. R o g a n , unpublished data). In the first case, pestivirus
was recovered from a stored sample of the fetal calf serum which had been used in
the flushing fluid. However, the management of recipient cows would also have
allowed for infection t o be passed between animals at the critical time of pregnancy.
Either or b o t h of those factors m a y have contributed to the final outcome. Artificial
insemination has not been found to be responsible for problems but the potential
for transmission via semen exists. Other factors attendant on the act of insemination,
including close yarding and intimate handling by inseminators, are also relevant at
this critical time. At present, few precautions are taken with regard t o pestivirus
infection, either transient or persistent, in collecting centres in Australia. T h e
introduction of control measures is planned in New South Wales, and in screening
stock already held, a young P I bull has recently been detected in one centre. Its semen
appeared to be of normal quality but contained more t h a n 10 T C I D of virus per
straw. This material h a d been distributed to approximately 100 herds for progeny
appraisal and the consequences of its use are now to be assessed ( P . D . Kirkland,
personal communication). In New Zealand there are no formal controls applied to
artificial breeding establishments but it is c o m m o n practice for semen collections to
be checked for freedom from pestiviruses at regular intervals.
7
5 0
Vaccinations have not been widely recognised as causing pestivirus transmission,
perhaps largely because live virus vaccines have been little used on cattle. However,
the risks implicit in the use of vaccines of animal or cell culture origin, which have
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not been treated to inactivate pestiviruses, are now appreciated. General handling
procedures which are part of any vaccination procedure, including the multiple use
of hypodermic equipment, may facilitate pestivirus transmission if PI and susceptible
animals are handled together.
CONTROL
Mucosal disease is notifiable in several States of Australia, viz. Queensland,
Victoria, Western Australia and the Northern Territory. This appears to be primarily
for awareness, especially for health certification purposes and overview of its
differential diagnosis from exotic disease. Pestivirus infection per se is not notifiable
and no State of Australia has a control policy in regard t o it. National quarantine
measures relating t o animals and animal products have in the past been incompletely
effective, as M D has been diagnosed in a n imported bull ( P . D . Kirkland, personal
communication). It can be expected that more attention will be given t o this risk in
the future. N o pestivirus vaccines are available or registered for use in Australia.
Control of disease is therefore based on management, and the detail is at the discretion
of individual herd managers a n d their veterinary advisers. Various options in this
direction have been suggested (16) and include measures to eliminate carriers from
herds, to maximise opportunities for breeding females to contact PI carriers, identified
or not, outside of the critical period just prior to and during early pregnancy and,
conversely, to minimise possibilities of contact within that period. In the south coastal
region of Western Australia, separation of heifers from cow herds until mating is
seen as a major factor which allows the critical persistent infections to occur.
Management which allows young stock to be exposed to infection before pregnancy
has been successful in the control of individual farm problems (R. Reuter, personal
communication). In one government-owned herd in New South Wales, use has been
made, over several years, of controlled exposure of heifers. At least one m o n t h before
joining, neat serum from a P I herdmate is inoculated intranasally. Alternatively, serum
diluted u p to 1/1000 can be inoculated subcutaneously (L.G. Cook, T . M . Jessep and
I.R. Littlejohns, unpublished data). This procedure has been successful in inducing
seroconversion and in interrupting the sequence of P I births and disease in the herd.
In South Australia, cows t o be used as recipients in one embryo transfer operation
are inoculated intramuscularly with cytopathogenic virus at least four weeks prior
to use. The results obtained in this programme are the subject of ongoing study (A.M.
P o i n t o n , personal communication).
In New Zealand, M D was notifiable in the 1960's due to its resemblance to serious
exotic diseases. Once its aetiology was defined and it was established that the
responsible virus was widespread, it was removed from the list of notifiable diseases.
A live attenuated vaccine (Bovax), developed from a local cytopathogenic isolate (11),
was available for a number of years until economic reasons led t o its withdrawal.
It produced good levels of neutralising antibody and n o undesirable side effects were
documented. Several imported dead vaccines have recently become available.
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ACKNOWLEDGMENTS
A p a r t from colleagues acknowledged in the text, the authors are grateful to M .
Flanagan for his careful and constructive criticism of the draft of this contribution.
*
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INCIDENCE, ÉPIDÉMIOLOGIE ET PROPHYLAXIE DES INFECTIONS ET MALADIES
À PESTIVIRUS BOVIN EN AUSTRALIE ET EN NOUVELLE-ZÉLANDE. I.R. Littlejohns et G.W. Horner.
Résumé : L'infection bovine à pestivirus est fréquente et largement répandue
en Australie comme en Nouvelle-Zélande. Les animaux adultes sont porteurs
d'anticorps dans leur majorité (60 % environ). La maladie correspondante
résulte presque toujours d'une contamination in utero. Elle se traduit soit par
la mort de l'embryon ou du fœtus, à n'importe quel stade de la gestation à
partir de la conception, soit, chez ceux qui naissent et sont des porteurs infectés
de manière persistante, par la maladie des muqueuses, le plus souvent sous forme
chronique. On observe peu ou pas de cas de maladie à la suite d'une
contamination post-natale de vaches non gestantes ; leur rôle dans la propagation
de l'infection semble limité. La transmission et le maintien du virus sous forme
enzootique reposent essentiellement sur les porteurs infectés de manière
persistante ; ceux-ci, après une contamination précoce in utero, sont
immunotolérants ; leur état clinique peut varier d'un état normal, ou presque,
à la maladie des muqueuses manifeste. L'expulsion d'un fœtus infecté et des
liquides associés représente aussi une source de contamination importante. En
général, peu de mesures de prophylaxie active sont entreprises. Il n'y a pas de
vaccins disponibles en Australie ; leur utilisation en Nouvelle-Zélande est limitée.
Toutefois, la prophylaxie suscite un intérêt grandissant parmi les professionnels
de l'élevage, notamment dans le domaine de la reproduction par insémination
artificielle ou par transfert d'embryons, où l'on pense que l'impact pathogène
du virus pourrait être amplifié.
MOTS-CLÉS : Australie - Diarrhée virale bovine - Epidémiologie - Infection
persistante - Maladie des muqueuses - Maladies des bovins - Nouvelle-Zélande Pestivirus - Prophylaxie - Transmission.
*
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INCIDENCIA, EPIDEMIOLOGÍA Y PROFILAXIS DE LAS INFECCIONES Y
ENFERMEDADES POR PESTIVIRUS BOVINO EN AUSTRALIA Y NUEVA ZELANDA.
- I.R Littlejohns y G.W. Horner.
Resumen: La infección bovina por pestivirus es frecuente y muy difundida tanto
en Australia como en Nueva Zelanda. Los animales adultos son portadores de
anticuerpos en su mayoría (aproximadamente 60%). La enfermedad
correspondiente es casi siempre resultado de una contaminación in utero y da
lugar, o bien a la muerte del embrión o del feto, cualquiera que sea el estadio
de la gestación a partir de la concepción, o bien, en los animales que nacen
y son portadores infectados de manera persistente, a la enfermedad mucosa,
muy a menudo en forma crónica. Se observan pocos o ningún caso de
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enfermedad como consecuencia de una contaminación posnatal de vacas no
gestantes; su papel en la propagación de la infección parece limitado. La
transmisión y mantenimiento del virus en forma enzoótica se basan esencialmente
en los portadores infectados de manera persistente. Tras una contaminación
precoz in utero, éstos son inmunotolerantes y su estado clínico puede variar
entre uno normal, o casi, y la enfermedad mucosa manifiesta. La expulsión
de un feto infectado y de los líquidos asociados representa también una fuente
de contaminación importante. En general se han tomado pocas medidas de
profilaxis activa. No hay vacunas disponibles en Australia y su uso en Nueva
Zelanda es limitado. Sin embargo, la profilaxis suscita un interés creciente entre
los profesionales de la cría, en particular entre quienes trabajan en la
reproducción por inseminación artificial o por transferencia de embriones,
situaciones en que se piensa que el impacto patógeno del virus podría crecer.
P A L A B R A S CLAVE: Australia - Diarrea viral bovina - Enfermedad mucosa Enfermedades de los bovinos - Epidemiología - Infección persistente - Nueva
Zelanda - Pestivirus - Profilaxis - Transmisión.
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