144 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



MODES OF SPREAD OF HOG CHOLEBA. 



The investifjntion of the role of insects in the transmission of hog 

 cholenv was continued. The experiments dealt principally with 

 stable flies, house flies, and gnats. 



In the work with gnats, the Simulium (buffalo gnat) was the in- 

 sect used, as that is the most common gnat in central Iowa, where 

 the exi)eriments were carried out. Valuable cooperation was given 

 by the Bureau of Entomology. The gnats were allowed to bite -pigs 

 affected with cholera, and later a number were allowed to bite non- 

 infected pigs. In no case was it possible to transmit cholera through 

 the bite of the gnats. 



Considerable work was done to determine the length of time the 

 virus of hog cholera survived in the body of house flies after they 

 were infected by feeding on hog-cholera blood. The infectiousness 

 of the flies was tested at intervals after feeding by injecting the 

 macerated flies subcutaneously into susceptible pigs. It was found 

 that the time during which the virus remained alive in the body of 

 the house fly depended very largely upon the temperature at which 

 the flies were kept. At low temperatures the virus survived for some 

 days, while at summer temperature of the air its virulence was lost 

 after 24 to 48 hours. In other experiments, house flies which had 

 been infected with cholera and killed were placed in pens with sus- 

 ceptible pigs. Also live house flies, infected by feeding on diseased 

 blood, were liberated in screened pens containing susceptible pigs. 

 In still other experiments, flies hatched from eggs deposited by flies 

 which had fed on hog-cholera virus were ground up in salt solu- 

 tion and injected into susceptible pigs. In none of these experi- 

 ments was it possible to transmit hog cholera except when flies which 

 had fed on cholera blood were ground up and injected subcutane- 

 ously into the susceptible pigs. 



Attempts w^ere made to transmit hog cholera by causing stable 

 flies, which had previously bitten pigs affected with cholera, to bite 

 susceptible pigs. In some cases the same pig was bitten by a number 

 of different infected flies. Cholera was transmitted only in excep- 

 tional cases, and this when the susceptible pig was bitten by a con- 

 siderable number of flies within a very short time after the flies had 

 fed on the sick pig. A repetition of the work previously done by 

 allowing a large number of stable flies to feed on sick pigs and 

 then liberating them in clean, screened pens with susceptible pigs 

 always gave negative results. Stable flies hatched from eggs de- 

 posited by infected flies w-ere found to be free of the virus of hog 

 cholera by macerating them and injecting them into susceptible pigs. 



Considering these experiments in connection with those of pre- 

 vious years, we are led to the conclusion that neither stable flies nor 

 house flies are important factors in the dissemination of hog cholera. 

 The experiments with gnats have been much less extensive, but 

 there is at least no indication that the buffalo gnat is capable of 

 transmitting hog cholera. 



CAUSE OF HOG CHOLERA AND BELATED DISEASES. 



A scientific worker in a commercial institution having reported 

 that he had cultivated the virus of hog cholera in pure culture, a 



