IN 1865, a "virulent Distemper" laid waste the farming stock of England, and caused Queen Victoria to compose a prayer for use in every church: "Stay, we pray Thee, this plague by Thy word of power; defend us from the pestilence with which many foreign lands have been smitten, and keep it far from our borders."
One might be tempted to renew this entreaty against the current outbreak of Newcastle Disease in Northern Ireland, were it not for the awkward fact that that previous plague continued into 1866. Indeed its persistence prompted the then Archbishop of Canterbury to ask the Home Secretary to appoint a Day of National Humiliation, on the assumption, I suppose, that this might improve the chances of divine intervention. In more recent times, however, the authorities have tended to look instead to meteorologists, inter alia, for help in coping with such epidemics.
The fowl pest virus can be released into the air from an infected bird on miniscule solid or liquid particles. Once airborne, the distance travelled by these particles depends firstly on their size: the larger ones fall to earth by force of gravity a short distance from the source, while smaller infected particles float with the wind to drift to ground elsewhere. It is mainly these latter which contribute to the airborne spread of the disease.
In dry weather, the risk of infection even from these small windborne particles falls off rapidly within a short distance of the source, because the "plume" fans out both horizontally and vertically, dramatically lowering local concentration. The dispersion process is particularly effective when the atmosphere is what we call unstable - when its thermal structure is such as to encourage vertical currents to carry surface air up to great heights.
Wet deposition as it is called, however, is effective at a much greater distance from the source. A shower of rain falling through a virus-laden atmosphere can scavenge the full depth of the infective plume, and deposit its lethal charge on a small area of ground beneath, perhaps some considerable distance from the originally infected poultry flock.
There are, of course, other ways in which the Newcastle virus can be spread. But the risk of secondary outbreaks from airborne infection can be worked out with reasonable confidence using a computer model whose inputs are the strength and direction of the expected wind, the thermal characteristics of the atmosphere, and the topography of the local ground. The relative humidity also enters the equation, since it is known that high humidity favours the lengthy survival of the virus.