304 VETERINARY HYGIENE 



populated with pigs and in such districts it may be considered to 

 be enzootic. There does not appear to be any particular seasonal 

 incidence in the United Kingdom, possibly on account of the more 

 equable temperature. In the middle-west of the United States of 

 America however, the disease shows a marked seasonal incidence, 

 reaching its greatest height in October and November and then dying 

 down rapidly, particularly after snow falls, and reaching its lowest 

 point in February. In the southern States, as in the United 

 Kingdom, possibly owing to the more equable temperature, severe 

 outbreaks occur at any season of the year.* 



With regard to age incidence, in most outbreaks the young stores 

 are the first to become affected, but this is not invariably so and pigs 

 of any age are liable to attack. 



The period of incubation is at least 4 days, often 10 to 20 days. 

 Hutyra and Kovesf found that in 5 lots of pigs the first symptoms 

 were observed during the second half of the first week, in 18 lots 

 in the course of the second week, and in 3 lots in the course of the 

 third week, and this variability was considered to be due to 

 individual susceptibility and to the degree of virulence of the virus. 

 It is doubtful if the virus remains active outside the body for a 

 longer period than 14 days, though the author has known 30 clear 

 days elapse between the slaughtering out of the entire animals in 

 one piggery and the first visible signs of disease in the adjoining 

 piggery which was separated by a brick wall (infection was pos- 

 sibly carried by wagtails). 



The mortality varies greatly with the character of the outbreak, 

 depending principally upon the very varying virulence of the virus. 

 In very acute outbreaks from 70 to 90 per cent, of deaths may be 

 expected, the disease being most fatal in young pigs. It is seldom 

 fatal in less than a week and it often lasts 2 or 3 weeks. The 

 average mortality for all outbreaks is placed at about 30 per cent. 



The virus is present in the blood and also in the faeces and the 

 urine, and it is through the agency of these excretions that the 

 disease is spread throughout the piggery and from one place 

 to another. Infection occurs by ingestion, and, the disease being 

 strictly contagious, the source of infection is always the diseased 

 pig. Once the disease is introduced into a piggery, it is readily 

 disseminated by attendants carrying infection on their boots, 

 brushes, and the like. The disease may in the same way be spread 

 by castrators, dealers and butchers, rats and birds. Infected part- 

 icles of manure, &c., may be transported by the wind. Pigs are 

 * Farmers' Bulletin, 834, U.S. Depart, of Agric., August, 1917. 

 . Comp. Path., 1917, Vol. XXX., p. 176, Abs. 



