656 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



filtrate, though free from all visible microorganisms, still retains the power 

 to produce hog cholera by subcutaneous injection. The disease which is 

 produced in this manner by the filtered hog cholera serum is identical in 

 all respects with the disease produced by the unfiltered serum and also 

 with the disease as it occurs in nature. The hogs which receive the filtered 

 serum present the symptoms and lesions of hog cholera. The disease 

 set up in this manner in very contagious and hogs which recover from the 

 inoculation of filtered serum are thereafter immune against hog cholera. 

 By repeated inoculation and filtration this virus may serve to infect 

 successively a large number of hogs. 



The invisible virus of hog cholera, in view of its ability to pass through 

 the Chamberland B filter, must be regarded as one of the smallest of the 

 invisible microorganisms. It has never been cultivated artificially, hence, 

 aside from its disease-producing qualities, we have little knowledge con- 

 cerning it. We do know, however, that the virus is quite resistant to such 

 common disinfectants as carbolic acid and bichloride of mercury and 

 that it is quickly destroyed by a 3 per cent solution of liquor cresolis 

 composilus (U. S. P.) as well as by a 5 per cent solution of antiformin. 

 When preserved in sealed glass bulbs in a cool dark place, the virus 

 retains its activity for six months or longer. Rabbits, guinea pigs, and 

 other small animals are entirely insusceptible to inoculations of the 

 filtered virus in amounts which would prove fatal to hogs. 



The virus of hog cholera is known to be thrown off from the body 

 through the urine, and it is probably also eliminated through the feces. 

 Therefore any agency which would serve to carry a particle of dirt from 

 infected hog yards might be the means of disseminating the virus. As 

 many sick hogs find their way to the public stock yards through shipment 

 by rail, all stock cars and stock yards are to be regarded as permanently 

 infected. It appears to be impracticable to prevent the spread of the 

 disease by methods of quarantine and disinfection, owing to the impossi- 

 bility of enforcing such measures thoroughly. It has recently been found 

 that a protective serum against hog cholera may be produced by "hyper- 

 immunization. " The process consists in giving immune hogs large 

 doses of blood taken from hogs sick of hog cholera. As a result of 

 this blood treatment their serum acquires the power to protect non-im- 

 munes. Injections of serum from hyperimmunized animals confers a 

 passive immunity, while the simultaneous injection of serum with a small 

 amount of virus produces an active immunity. 



