306 AGRICULTURAL BACTERIOLOGY 



The disease is like other acute communicable diseases in 

 that it has an ebb and flow. These waves occur at intervals 

 of several years. The reason for this variation in severity 

 is not known. In Germany, after a period in which the 

 disease was not especially important, it began its ravages, 

 and in 1911, 3,000,000 cattle were affected, 1,000,000 sheep, 

 and 2,500,000 hogs. The loss from death of animals is not 

 great, only about 1 per cent. The economic losses are due 

 to the loss of flesh, to diminished milk production, or to loss 

 of reproductive power. It has been estimated that for 

 cattle this loss ranges from seven to twenty dollars a head, 

 and proportionately less for smaller animals. It is thus 

 clear that the disease imposes a great economic burden upon 

 stockmen, and if by the expenditure of reasonable sums 

 the country can be protected from it, it is certainly wise 

 to spend the money. 



The disease presents an excellent example of the influence 

 of modern commercial conditions on the rate of spread. 

 The infection of the great shipping-yards is an especially 

 important factor in the distribution. The outbreak of foot- 

 and-mouth disease of 1908-09 was due to the importation 

 from Japan of vaccine virus that was used in one of the 

 vaccine establishments in Pennsylvania. Some of the virus 

 was sent to a Detroit establishment that rented calves from 

 a dealer for the manufacture of smallpox vaccine. After 

 the animals had been used for this purpose they were re- 

 turned to the dealer and resold by him to farmers. 



Bovine animals inoculated with the mixed virus of cow- 

 pox and foot-and-mouth disease develop symptoms of cow- 

 pox alone, but when brought in contact with healthy ani- 

 mals the virus of foot-and-mouth disease may spread from 

 the animals that show no symptoms. The calves in ques- 

 tion were placed in pens in the Detroit stockyards, and 

 from there distributed as shown in the diagram. Four days 



