D8576.PDF

Rev. sci. lech. Off. int. Epiz.,
1986, 5 (2), 315-321.
Aerobiology of foot-and-mouth disease (FMD):
an outline and recent advances
A.I. DONALDSON*
Summary : This paper describes how information obtained from epidemiolo­
gical and meteorological analyses of outbreaks of FMD, together with experi­
mental investigations on the excretion, survival and infection by airborne
FMD virus, have been used to develop mathematical models which can be
used to predict the probability of airborne FMD spread in the field.
KEYWORDS: Airborne infection - Aphthovirus - Cattle diseases - Epidemio­
logy - Model - Statistics.
INTRODUCTION
In the era before the adoption of mass annual vaccination by countries on conti­
nental E u r o p e in the mid-1960's, there were many occasions when F M D spread to
the United Kingdom and Scandinavia. Frequently, such spread was preceded by the
appearance of outbreaks on the nearby continent and often there was n o history of
contact linking outbreaks separated by seaways. Over the years, several authorities
have attributed these episodes of unexplained distant spread to the carriage of virus
on the wind (12, 13, 24, 25).
Although vaccination and other control measures have dramatically reduced the
number of outbreaks of F M D in E u r o p e during the last 20 years, there have still
been occasions when the circumstances have suggested that airborne spread has
taken place during epidemics. This has occurred both within and between countries
(17, 18, 22).
Experimental studies on the aerobiology of F M D at the Animal Virus Research
Institute have been in progress since the 1930's (14, J . T . E d w a r d s , unpublished
papers) but they were given a considerable impetus by the 1967-68 United Kingdom
( " O s w e s t r y " ) epidemic. In this epidemic m o r e than 300 farms were affected during
the first three weeks. This explosive start, attributed to airborne spread (21, 30),
undoubtedly contributed to the subsequent enormous size of the epidemic which in
total involved m o r e t h a n 2,300 farms. This experience identified a need to establish
data on the quantitative aspects of airborne spread and to determine if such infor­
mation could be used to develop the means of predicting airborne spread in future
outbreaks so that improved control measures could be used.
As a consequence of experimental and epidemiological investigations and colla­
boration with the Meteorological Office, Bracknell, it has, in fact, been possible to
» Animal Virus Research Institute, Pirbright, Woking, Surrey, G U 2 4 0NF, United Kingdom.
develop mathematical models for predicting the airborne spread of F M D . The sta­
ges in the evolution of these models are outlined in this paper under the following
headings: (i) sources of airborne virus; (ii) survival of virus in the air; (iii) airborne
infection of susceptible animals; and (iv) operational use of mathematical models.
SOURCES OF AIRBORNE VIRUS
The duration and quantity of airborne F M D virus excreted by different species
of domesticated and wild animals was determined by infecting groups of animals
with a series of different strains of virus and then sampling the air in the rooms in
which they were housed, using a range of air sampling devices. It was found that
the quantity of airborne virus recovered was dependent on the species of animal,
the strain of virus and the stage of disease. Infected pigs excreted by far the largest
quantity of infectivity — the maximum amount recovered per pig per day being
logl0
T C I D 5 0 (7). The maximum a m o u n t of virus excreted during the same
period by ruminants, domesticated or wild, was around 1,000-3,000 times less (10,
28). The duration of excretion with all species was generally 4-5 days. Inoculated
sheep excreted most virus when only primary lesions were evident whereas cattle
and pigs excreted maximally when vesicular lesions were at the early acute stage of
generalisation. The strains which resulted in the highest yields of airborne virus
were several O strains and C Noville (10).
8 . 6
These results demonstrated that infected animals, particularly pigs, are a p r o b a ­
ble source of airborne F M D virus during outbreaks. Other possible sources may
include: the splashing of contaminated milk or faecal slurry; the spray disposal of
infected slurry; rain falling onto contaminated ground; and the burning of carcas­
ses on which vesicles are present. The quantities of airborne virus which these pro­
cedures could potentially generate have not, however, been determined.
Another possible source of airborne virus dissemination is from bulk milk tan­
kers containing contaminated milk. During the filling of milk tankers it has been
observed that the displaced air results in the production of milk aerosols near the
air-outlet vent. However, the experimental seeding of milk tankers with spores as
tracers and air sampling during venting suggested that the quantities of airborne
F M D virus likely to be emitted by this source during outbreaks are not likely to
constitute a serious hazard ( G . J . Harper, unpublished results).
SURVIVAL OF VIRUS IN T H E AIR
The concentration of airborne virus may be reduced by diffusion and sedimen­
tation, i.e. by physical loss, and also by inactivation, i.e. by biological loss.
The effect of environmental factors on the viability of different strains of F M D
virus has been investigated and it was found that the factor which had the greatest
influence was the relative humidity (RH) (1, 4). In moist air (above 5 5 % R H ) , air­
borne virus had a low rate of inactivation but in dry air (below 5 5 % R H ) , the inac­
tivation rate was high. For example, strain O , BFS sprayed from tissue culture fluid
decayed at a rate of l o g l 0 per hour at 6 0 % R H whereas at 4 0 % R H the rate was
logl04-2per hr (1).
0 . 6
The fluid from which virus was aerosolised was found to influence subsequent
decay rates. Physiological fluids generally were protective. For example, very low
decay rates were obtained with virus aerosols from nasal fluid, milk or faecal
slurry. An exception was bovine salivary fluid from which high decay resulted. A
small dialysable, heat-resistant molecule was identified as being responsible for
virus inactivation in bovine saliva (2, 5).
The respiratory tract was determined as the main source of airborne F M D virus
from infected animals so the biological decay rates obtained with virus aerosolised
from nasal fluid most probably approximate those which occur in nature.
Other environmental factors examined for their influence on the viability of air­
borne virus were daylight and atmospheric pollutants. Neither was found to be a
strong inactivant under the test conditions used. In the case of daylight, virus held
as droplets on microthreads and exposed for 30 min. was found to be photoresistant. The ambient R H was kept high during the tests to avoid the inactivation of
virus by dehydration (6).
Airborne virus excreted by infected animals has been found to be associated
with a range of particle sizes (8, 28). Between 6 5 - 7 1 % of the total infectivity was
associated with particles greater t h a n 6 µm in diameter, 19-24% with particles 3-6
µm in diameter. It is considered that the physical decay rate of 6 µm and smaller
particles will be small compared to the larger scale movements in the atmosphere
which spread and dilute the particles (16).
Once airborne virus has been emitted from a source, a plume will be formed
which will be subjected to dispersion in both horizontal and vertical planes. Prevail­
ing climatic conditions, particularly windspeed and the vertical temperature struc­
ture in the lower atmosphere, will be the major determinants of physical decay. The
roughness of the surface over which the air passes will influence the a m o u n t of tur­
bulent mixing and topographical features will determine the direction the plume
travels. Virus concentration is likely to be maintained for longer during travel over
the sea than over the land.
The conditions under which the physical decay of airborne virus are likely to be
minimal and, therefore, the potential for distant spread greatest are: in light winds;
after a sea passage; when the air temperature over the sea exceeds the sea tempera­
ture i.e. little vertical dispersion; and when the humidity is high (17).
These theoretical considerations have been borne out by epidemiological and
meteorological investigations of epidemics of F M D in the UK in which airborne
spread is believed to have taken place. The determinant climatic factors in airborne
spread have been identified as: wind direction (21, 26); wind speed (26, 30); wind
veer (30) and high humidity (26). The furthest distance over which airborne spread
is believed to have occurred is a r o u n d 250 km for spread over the sea (9) and 60 km
for spread over the land (21).
AIRBORNE INFECTION OF SUSCEPTIBLE ANIMALS
The amounts of virus which can initiate infection in cattle, sheep and pigs by the
respiratory route are considerably less than those by the oral route. Henderson and
Brooksby (19), for example, failed to infect cattle by feeding them l o g 1 0
ID of
virus in glass capillary tubes which the animals chewed, though Burrows (unpublish6.5
5 0
—
318
—
5 . 8 - 6 . 8
ed results) infected some cattle which were given l o g 1 0
ID
of virus by
m o u t h . In contrast, Eskildsen (11) found that an aerosol dose containing l o g 1 0
mouse I D was sufficient to infect a heifer, and in recent work cattle were infected
following exposure to a dose of 25 T C I D o f airborne virus (A.I. Donaldson, C . F .
Gibson, R. Oliver, C. H a m b l i n and R . P . Kitching, unpublished results). T h e mini­
m u m dose reported to infect sheep by the respiratory route is 10 T C I D (15) but
the minimum dose to infect this species by the oral route has not been reported.
The lowest reported dose to infect a pig by the respiratory route is l o g 1 0 I D (29)
and by the oral route logl03.9 I D (23).
5 0
2 . 7
5 0
5 0
5 0
2 . 6
5 0
5 0
It is evident, therefore, that sheep and cattle are especially susceptible to infec­
tion by the respiratory route and that the lowest dose which can initiate infection is
similar for both species. Under experimental conditions when sheep and cattle were
exposed to an environment containing airborne virus, the cattle became infected
before the sheep (3). This was probably because cattle have a much higher respira­
tory tidal volume than sheep and thus sampled m o r e air.
In epidemiological analyses of epidemics, cattle are the animals most frequently
affected downwind during airborne spread of F M D and infected pigs are the ani­
mals most frequently identified as the source of airborne virus emission (9, 17, 18,
2 1 , 26, 27). There is agreement, therefore, between the results obtained under labo­
ratory conditions and in the field.
OPERATIONAL USE OF MATHEMATICAL MODELS
The mathematical models which provide an objective estimate of the area most
at risk from airborne F M D spread in the event of an outbreak have been developed
by a blending of the aerobiological characteristics of F M D and its causal virus, as
described in the previous sections, with the mathematical and physical parameters
governing the diffusion of particles in the atmosphere. T w o separate models are
available; one for short-range and the other for long-range prediction. The shortrange model is computer-based and can be used to forecast the extent of dispersion
of airborne virus over land within a 10 km radius of a known source. The longrange model is operated manually and is for analysing the dispersion of airborne
virus across the sea over long distances.
The following input data is required for their operation: (i) an estimate of the
daily airborne virus output from the infected animals; (ii) hourly or three hourly
observations of windspeed, wind direction, R H , cloud cover and precipitation in
the region of the outbreak; (iii) latitude and (iv) topographical features of the area.
The daily airborne virus output is estimated by determining the total n u m b e r of
infected animals at the source of virus release. This may be done crudely in the first
instance but should be more accurate once an epidemiological team (9) or experien­
ced clinician has been deployed to the infected premises. The start of the period of
airborne virus excretion delineated for analysis can also be crudely estimated at first
but should be more accurately defined by an experienced clinician once the age of
the oldest vesicular lesions present on the affected animals at the premises has been
established. W h e n this has been done, the start of the airborne virus excretion
period on the premises can be estimated. The period of excretion and total daily
output having been determined, mathematical analysis and generation of plume
— 319 —
profiles can be d o n e at the Meteorological Office*. Details of the equipment and
programmes used have been given elsewhere (16).
Finally, plume profiles can be laid over animal distribution maps to identify the
premises and herds which are potentially at risk. Epidemiological analyses have
shown that cattle downwind, and especially those in large herds, are the species
most likely to be infected during episodes of airborne spread (20).
The models have been tested on past outbreaks of F M D where there has been
strong circumstantial evidence for airborne spread a n d a good agreement has been
found between the spread of disease predicted by the models and that which
actually took place (16, 17). The models were used operationally in M a r c h 1981
when a risk of spread from Brittany, France, to the United Kingdom, was successfully forecast (9).
*
* *
AÉROBIOLOGIE DE LA FIÈVRE APHTEUSE: PRÉSENTATION ET PROGRÈS
RÉCENTS. — A.I. Donaldson.
Résumé : L'auteur décrit la méthode adoptée pour obtenir des informations
sur les foyers de fièvre aphteuse à partir d'analyses épidémiologiques et météorologiques. De même, grâce à des recherches expérimentales sur l'excrétion, la
survie et le pouvoir infectieux du virus aphteux propagé par voie aérienne, il a
été mis au point des modèles mathématiques utilisables pour prédire le risque
de transmission aérienne de la maladie sur le terrain.
MOTS-CLÉS : Aphthovirus - Epidemiologie - Infection aérienne - Maladies
des bovins - Modèle - Statistiques.
*
**
AEROBIOLOGÍA DE LA FIEBRE AFTOSA : PRESENTACIÓN Y RECIENTES PROGRESOS. — A.I. Donaldson.
Resumen : Describe el autor el método adoptado para lograr informaciones
sobre los focos de fiebre aftosa en base a análisis epidemiológicos y meteorológicos. Asimismo, merced a las investigaciones experimentales de la excreción, supervivencia y poder infeccioso del virus aftoso propagado por vía
aérea, se han elaborado modelos matemáticos que se pueden utilizar para anticipar el riesgo de transmisión aérea de la enfermedad en el campo.
PALABRAS CLAVE : Aftovirus - Enfermedades de bovinos - Epidemiología
Estadísticas - Infección aérea - Modelo.
*
* *
* Meteorological Office, London Road, Bracknell, Berkshire, RG12 2SZ, England.
— 320 —
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