The biggest step forward came with an outbreak of foot-and-mouth disease (FMD) in England in which there seemed to be little correlation between the direction of migrating birds or the movement of animals, people and vehicles with the overall way in which the disease was spreading which was mainly in the direction of the wind. Subsequent cooperative research between virologists and meteorologists defined the conditions under which aerosol viruses spread. Sunlight, warmth and very low humidity dried the droplets and inactivated the virus rapidly whereas the virus could survive much longer in cold, damp, dark conditions which is why many epidemics are in winter A computer programme was devised which, taking account of weather conditions, was able to predict how aerosol spread would occur in any given situation. Later, when an outbreak of FMD occurred in pig herds near the north coast of Brittany in France, the computer predicted that the virus would be blown to the south coast of England and it did. Pigs produce higher levels of aerosol FMD virus than other species. Somewhat similar observations were made on Aujeszky’s disease occurring on the Danish islands off the coast of Germany where the disease was endemic. The Danes paid for the German herds to be vaccinated and the island outbreaks stopped.
Windblown aerosol infections do not travel so readily over land as they do over water. Hills, and to a lesser extent trees and buildings cause turbulence and aerosol precipitation. Flat plains with few trees and buildings do not necessarily result in turbulence and if a farmer on a pig farm which is free from, say, EP or App has an uninterrupted view of a pig farm with EP or App, the EP-free one will almost certainly break down if weather conditions are favourable for aerosol spread. Another factor over land is that warmer regions on the ground cause the air carrying the infection to rise to cooler levels where it may be windborne variable distances before descending to ground level. Thus it has been found with outbreaks of FMD that the disease may not spread from an infected herd to a neighbouring down wind farm but to several further away.
Slow steady movement of air over relatively calm water tends to travel in layers without much turbulence. A visible analogy is a plume of smoke, for example from a steam ship, may travel in a light breeze for many kilometres without dispersing. This used to be used by submarines in the First World War to seek cargo vessels.
How can aerosol infection be prevented?
Obviously, from what has been said, a large lake provides little protection against aerosol infection. Indoor herds are often wrongly thought to be at much less risk than outdoor or semi-outdoor herds but ventilation fans, particularly inlet fans, bring virus indoors. Herds in isolated locations of very low pig density are at least risk. The risk is reduced further if a herd is situated in well wooded terrain which blocks free-flowing wind. In hilly mountainous country the wind becomes turbulent and precipitates aerosols. One side issue to such a location if it is remote is the difficulty of finding and keeping staff. There should be less difficulty with staffing farms on the sea coast where the wind is predominantly off the sea. In the author’s experience sea-side herds are usually free from respiratory disease and rarely break down with windborne infections from other pig farms.
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