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Potential for wind-borne spread of FMD in Australia
Summary:
Foot-and-mouth disease (FMD) is one of the most contagious of animal diseases. Animals may be infected by inhalation or ingestion. Ruminants are especially sensitive to infection via the respiratory tract.
Movement of infected animals is the most important method of spread of FMD from one property to another. However, on occasions movement of airborne virus particles by wind has been responsible for infecting properties some distance downwind. Under favourable climatic conditions wind-borne spread can be an important factor in FMD epidemics.
For wind-borne spread to occur, virus must be able to survive long enough and in sufficiently high concentrations to infect livestock downwind. Our analysis of weather data shows that for much of Australia, conditions, particularly at night, are suitable for survival of FMD virus in aerosols. Long-distance spread of virus particles requires stable atmospheric conditions and low wind speeds. Such conditions are common in Australia. Thus, for much of Australia, for much of the year, weather conditions will not be a limiting factor for wind-borne spread.
For infection to occur downwind, animals must be exposed to sufficient virus particles. This depends on the amount of virus produced and the volume of air breathed by exposed animals. The risk of wind-borne spread is proportional to the strength of the virus source. As infected pigs excrete 1000-3000 times as much virus as cattle or sheep, they pose the greatest threat.
The risk of spread is proportional to the density of livestock downwind, with large concentrations of animals such as saleyards and feedlots being particularly vulnerable. Cattle are more likely to be infected than are sheep or pigs because of their higher respiratory volume sheep have one quarter, and pigs one twelfth, the risk of cattle. Hence, the typical pattern of wind-borne spread is from pigs to cattle. Once one animal in a herd has become infected, the disease will spread rapidly through the herd by close contact.
These findings have been confirmed in simulated outbreak studies. Except in close proximity to an infected property, there is minimal risk of wind-borne spread from typical beef, dairy and sheep properties in Australia. Cattle feedlots, because of their size pose a greater risk, especially if slaughtering of infected animals is delayed. However, infected piggeries represent the greatest threat, with spread greater than 10 km likely. Even a small number of infected pigs pose a significant risk of wind-borne spread.
The weather conditions at the time of the outbreak will determine the survival of airborne virus and how far it spreads. These cannot be predicted in advance and must be analysed at the time to determine premises at risk. Surveillance effort can then be targeted accordingly.
Several countries have developed tactical models and decision aids to evaluate the risks of wind-borne spread during FMD outbreaks. It is recommended that a tactical FMD wind-borne spread model suitable for use in Australia be developed and made available to disease control authorities.
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However, because many of the figures in the original printing were generated directly onto paper, only a few are included in the pdf version of the document.
The remaining figures have been combined into six files:.
| Download | Figure | Caption | Page |
|---|---|---|---|
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A. |
4 |
Number of days per year that sites meet the criteria for survival of FMD |
28 |
|
5 |
Comparison between night and day survival of FMD virus |
29 |
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|
6 |
Sensitivity to virus survival criteria |
31 |
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|
7 |
Sensitivity to less stringent virus survival criteria |
31 |
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|
8 |
Sensitivity to more stringent virus survival criteria |
31 |
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B. |
9 |
Comparison between seasons for survival of FMD virus |
32 |
|
10a, 10b |
Conditions suitable for survival of FMD virus in aerosol - 16 year charts |
33 |
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|
11a, 1b |
Conditions suitable for survival of FMD virus in aerosol - 1 year charts |
35 |
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C. |
13 |
Number of days that are conducive to survival of FMD virus in aerosol |
38 |
|
14, 15, 16 |
Cattle, sheep and pig density maps |
41 |
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17, 18 |
Potential virus production maps |
43 |
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D. |
19, 20, 21, 22, 23, 24 |
Receptiveness to airborne infection and transmission potential maps |
45 |
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E. |
39 |
Examples of weather data used to derive virus concentrations downwind |
67 |
|
40 a, b, c, d, e, f |
Virus concentrations at 10 km: yearly variability for 12 sites |
68 |
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F. |
43, 44, 45, 46, 47, 48, 49, 50, |
Rosette diagrams of simulated outbreaks |
83 |
