102 DISEASES OF SWINE 



reaches its greatest height. In areas which have been free from 

 the disease for a considerable period the outbreak is most hkely to 

 occur about midsummer, or early in the fall, and from the middle 

 of July to the appearance of frost in the autumn the disease usually 

 shows its greatest ravages in the great hog-feeding districts of the 

 Central West. 



With the coming of frost there is usually to be noted a dropping 

 off in the number of cases, and this decrease progressively continues 

 until midwinter, when the disease is usually at its lowest ebb. 



This order of things, however, does not always obtain, and often 

 some of the most severe outbreaks of the disease occur during the 

 middle of winter. Outbreaks of this type are not generally widely 

 distributed, and the epidemic is usually located in a small district, 

 and does not have a tendency to assume the widespread proportions 

 that characterize the midsummer outbreaks. 



The reasons for this order of things are many. In the winter 

 months a large number of the predisposing factors for spreading 

 of cholera are absent. 



For example, crows and buzzards, which play such a large part 

 in the transmission of the infection to new herds at a considerable 

 distance from the seat of the original outbreak, are not so numerous 

 in the winter months, and especially not so in the northern section 

 of the hog-raising district. 



Running streams, which carry the infection down their banks 

 from infected herds higher up their course, and cause the appear- 

 ance of outbreaks in pastures through which they wend their way, 

 are usually frozen during the greater part of the winter season, and 

 hence the virus of cholera is shut off from another frequent avenue 

 of dissemination. 



Filthy hog wallows, which often prove to be hot-beds of cholera 

 infection in the summer months, are also frozen and in disuse dur- 

 ing the winter months. 



Animals also are kept more confined for the greater part of 

 the winter season, and hence do not have as great opportunity to 

 come in contact with animals from infected herds. Close con- 

 finement, however, often means poor hygiene, bad air, and very 

 frequently results in an outbreak of lung fever or pneumonia in a 

 herd, which may be very severe in character and often mistaken 



