1921. 



FoOT-AND-Moi'TH DISEASE. 



1009 



form of air pockets of negative pressure in the areas mostly 

 invaded, which could account for the suspended virus descending 

 to earth or water. These are problems which obviously should 

 be discussed with those who are now exploring the air. As 

 regards birds, it immediately suggests itself that if birds in 

 general are responsible, there should be definite periods of inva- 

 sion, given prevalence of disease in other countries, which syn- 

 chronise with those of the migration of birds inwards. There 

 are two migratory seasons, during both of which birds arrive in 

 or leave this country. In the autumn certain birds leave to 

 winter elsewhere. These can be disregarded as importers. Others 

 arrive to winter in this country. These can probably be dis- 

 regarded, as most of them come from the North where the 

 disease seldom prevails. In the Spring months birds come in 

 mainly from the South for breeding purposes, and might be 

 carriers whilst others depart for the North. 



In going back over the outbreaks in the period of '20 years, 

 however, it appears that the lowest records of invasion are 

 March — 4. April — 1. May — ; July, in which there is no migra- 

 tion, shows 8. September, October and November, when birds 

 may be expected from the north and north-east, which are not 

 the lands of prevalence as regards foot-and-mouth disease, show 

 respectively 7. 7. and 4. while December, during which there is 

 practically no migration inwards, shows 9. 



These data are against the suggestion that there is any general 

 relation between migration and invasion by foot-and-mouth 

 disease. They do not, however, exclude the agency of those 

 birds, such as ducks, geese and gulls, which may, outside the 

 migratory seasons, travel long distances for food. For purposes 

 of closer investigation it might be assumed : — (a) that such birds 

 might in their travels frequent contaminated pastures or drink- 

 ing places and afterwards deposit virus in this country from their 

 feet or plumage ; (b) That they might swallow infected material, 

 such as water and food contaminated by slobber and pieces of 

 membrane from the mouths of cattle, and afterwards excrete the 

 virus in a still active state. It is hoped that experiments which 

 are to be conducted on the viability of the virus may determine 

 the possibilities as regards (a) and that as regards Co) feeding 

 experiments with the virus, using birds, may at least show 

 whether the virus can pass through their intestines unchanged, 

 and render their excretions infective for lengthy periods. 



As the matter stands at present, however, the evidence, such 

 as it is. is most in favour of particles of virus being carried by 

 the air. 



