10 HOG CHOLERA 



at least in some instances. Just how any one of 

 the filterable viruses operates to produce disease 

 is quite unknown, but it is certain that hog cholera 

 virus has a selective action for epithelial and en- 

 dothelial cells. 



Virulence. Hog cholera virus produces speci- 

 fic disease only in swine, and very small quantities 

 of infected material are sufficient to cause death 

 in susceptible animals. According to King, sub- 

 cutaneous injections of 1/86 of a mil of virulent 

 blood produced the disease, while lesser amounts 

 produced only a mild reaction, or none at all. 

 Natural infection usually occurs by way of the 

 digestive system, but the disease is readily pro- 

 duced by subcutaneous, intravenous or intra-abdo- 

 minal injections of small quantities of virulent 

 material. 



Resistance. Most of the natural influences to 

 which hog cholera virus is subjected do not oper- 

 ate to destroy it rapidly. Drying, sunlight, and 

 low temperatures seem to have no immediate at- 

 tenuating effects, although it is a fact that most 

 infected yards which remain uninhabited from 

 three to six months do not endanger susceptible 

 pigs placed in them. There is, though, a consid- 

 erable tendency for hog cholera to recur on old 

 infected farms, and this fact indicates that there 

 are exceptional cases in which the span of life 

 of the virus is greatly prolonged. 



