120 ANNUAL KEPORTS OF DEPARTMENT OF AGRICULTURE. 



for an indefinite time, certainly several weeks. It is a matter of great 

 importance to farmers to know when they may with safety restock 

 their farms after an outbreak of cholera. Conditions in different 

 parts of the country vary so widely that no generalization can be 

 made from the experiments. It is clear, however, that the virus of 

 hog cholera survives much longer in winter than in summer, and fur- 

 ther experiments have been planned to be carried out on a number 

 of widely separated farms to determine how long aftel- the removal 

 of all sick pigs it is safe to restock farms. 



Repeated experiments with the carcasses of hogs dead of cholera 

 show that the virus in such carcasses becomes noninfectious within a 

 comparatively short time in the summer, while it survives for long 

 periods (several months) in the winter. 



It has been observed that meat from hogs dead of cholera and which 

 contains the virus of the disease may retain the infection after passing 

 through the process of curing by means of salt and saltpeter and 

 after smoking. Observations have been made along these lines with 

 meats from pigs in different stages of the disease, and while the indi- 

 cations are that the virus is more apt to remain active when the meat 

 is taken from pigs in the late stages of cholera than when taken from 

 early cases, it has been found at times that the meat taken from early 

 cases may be infectious after curing and smoking. As there is a 

 possibility that carcasses of pigs in the early stages of cholera, before 

 the disease can be recognized, may at times get on the market, it is 

 important for hog raisers who feed hogs on garbage and table refuse, 

 in whole or in part, to see that such feed is thoroughly heated before 

 being fed to pigs. 



A series of experiments has been begun to determine the duration 

 of immunity in very young pigs following simultaneous inoculation. 

 The first series, which included 29 pigs a few days old, treated by the 

 simultaneous method, was held for 5 months and then exposed to 

 cholera. All these pigs were found to be immune at the end of that 

 period. 



During the fall of 1917 and again in the spring of 1918 much atten- 

 tion was given to the possibility of the conveyance of hog cholera by 

 insects. These studies include investigations of hog lice, flies, mos- 

 quitoes, and other insects. Much valuable information has been 

 obtained. In some cases cholera has been transmitted by means of 

 flies. It is too early to express a definite conclusion on the relation 

 of insects to the spread of hog cholera. 



DIPS AND DISINFECTANTS. 



The laboratory of dips and disinfectants received for examination 

 161 samples of stock dips, disinfectants, and miscellaneous materials. 



During the calendar year 1917 there were sent out the following 

 test outfits and supplies for making dips in the field: Seven hundred 

 and ten test outfits for arsenical dips, and supplies sufficient to make 

 447,900 tests for arsenic; 63 test outfits for lime-sulphur dips, and 

 supplies sufficient to make 11,700 tests of lime-sulphur dips; 28 test 

 outfits for nicotin dips, and supplies sufficient to make 3,375 tests of 

 nicotin di|)s. There were thus provided a total of 802 new dip- 

 testing outfits, together with supplies sufficient to make a total of 



