BUREAU OF ANIMAL INDUSTRY. 125 



jaclvet of which contains a liquid which boils at a temperature ex- 

 actly sufficient to maintain the desired degree of heat in the serum 

 itself. 



Some progress has been made in the study of the duration of im- 

 munity in suckling pigs. One hundred and thirty-nine pigs given 

 the simultaneous inoculation when very young (less than 3 weeks, 

 some of them but 1 week old) were exposed to hog cholera by 

 the injection of virus six months after simultaneous inoculation. 

 Three of them died, but it was not possible to establish <Bhat even 

 they died of hog cholera. 



Experiments by the division have shown that the condition of 

 the immune hogs which serve as serum producers is of prime 

 importance in the production of potent serum. The influence which 

 the time elapsing between immunization and hyperimmunization 

 might exert upon the potency of the serum produced was studied 

 and at the same time studies were made of diluted and laked virus 

 blood as hyperimmunizing agents. The experiments show that hogs 

 which are hyperimmunized within a short interval after a simul- 

 taneous inoculatioift do not produce serum of satisfactory potency; 

 also that as a rule the ability of hyperimmunized hogs to yield 

 potent serums increases within certain limits as the interval 

 between immunization and hyperimmunization increases. The 

 conclusion was therefore reached that an interval of not less than 

 seven weeks should be allowed to elapse between immuniza- 

 tion and hyperimmunization, and that the most uniformly satis- 

 factory results will be obtained by allowing at least three months. 

 The ability of immune hogs to respond to hyperimmunization 

 to the desired degree, once acquired, remains unimpaired for at 

 least a year. It was found also that simultaneous inocula- 

 tion of nonimmune pigs with serum and virus is followed almost 

 immediately by a remarkably firm immunity, so that they are able 

 to withstand enormous doses of virus blood within a day or two 

 after simultaneous inoculation; hence there is no evidence of a state 

 of a hypersusceptibility to hog cholera following simultaneous inocu- 

 lation. Diluted and laked virus blood w^as found not to possess any 

 advantages over the undiluted blood. A full report of this work has 

 been published in the Journal of the American Veterinary Medical 

 Association (vol. 8, n. s., p. 259). 



MODES OF SPREAD OF HOG CHOLERA. 



Studies of the relation of insects to the spread of hog cholera have 

 been continued. Studies of the hog louse {Heniatopinus suis) have 

 indicated that this parasite is incapable, under ordinary conditions, 

 of transmitting the virus of hog cholera. Most of the work during 

 the year related to the spread of hog cholera by means of flies, the 

 common hous& fly {Musca domestica) being used as a representative 

 of the nonbiting fly, and the stable fly {Stomoxys calcitrans) as a 

 representative of the biting fly. Both species are commonly found 

 about hog pens, and both are capable of traveling considerable dis- 

 tances. It was learned that individual house flies which feed upon 

 the eye secretions or blood of sick pigs may harbor the virus of hog 

 cliolera for some daj^s at least, and may convey the disease to healthy 

 pigs by feeding on their eyes or on fresh wounds on their skins ; yet 



