Commissioner of Agriculture 281 



differential diagnosis 



A mistake might possibly be made in a single patient kept by 

 itself rigidly apart from all cloven footed animals, but by any one 

 familiar with the affection this would be held to be virtually im- 

 possible where the stock is massed together in flocks or herds. In 

 a bunch of cattle allowed to move after each other in the same yard, 

 or over the same field or road, or to feed or drink from the same 

 bucket, trough, or sluggish stream, all the herd, with rarely the 

 exception of even a single animal, comes clown with the disease at 

 once, and usually within 6 days after exposure. If in a particular 

 case an outbreak is specially delayed, the conditions which led to 

 the delay (cold, etc.) will probably affect all alike, and thus the 

 eruption still appears simultaneously or nearly so in the whole. 

 A herd freely mingling on the same ground, and showing but one 

 or two diseased animals and a large number unaffected, may well 

 be suspected to be affected with something distinct from foot and 

 mouth disease. In exceptional cases where opportunities for the 

 convevance of infection from animal to animal are absent or 

 inoperative; if. for example, it is a group of calves, heifers, bulls, 

 or steers, in which there is no chance for the conveyance of infec- 

 tion from one animal to another through the hands of a common 

 milker or the mouth of the suckling ; if the beasts are kept in stalls 

 where no two can feed or drink from the same trough ; if they 

 cannot lick each other; if the same halter or cover is not used on 

 two in succession ; if they are not watered from the same bucket, 

 nor in common from the same sluggish stream, it is not impossible 

 that they may take ill at longer intervals and some may even escape 

 though in the same building. The virus of aphthous fever is the 

 most infectious known, yet it does not spread far on still air; but 

 it may cover long distances if dried up on straw, feathers, dust, or 

 other light body and blown by the wind. I have often seen, on one 

 side of a highway, a herd uniformly suffering from foot and mouth 

 disease, while on the other side of the road another herd remains 

 perfectly sound. In 1839, Guy recorded an instance in which an 

 ordinary fence proved a perfect bar to the advance of the infec- 

 tion, the herd on the opposite side remaining perfectly healthy. 



The truth remains, however, that the whole herd, exposed in 

 the near vicinity of infected animals, usually takes the disease at 



