BUREAU OF ANIMAL INDUSTRY. 377 



to hog cholera. We therefore carried out during the past year 

 tests of immunity in pigs from actively immunized sows, the pigs 

 at the time of the test being from 3 days to 3 weeks of age. It 

 irhould be noted that Craig (Bulletin 140, Purdue Agricultural Ex- 

 periment Station, February, 1910) has reported that pigs from two 

 immune sows were apparently resistant to the disease up to the time 

 they were 4 weeks old, but that they later succumbed. Reynolds 

 (American Veterinary Review, vol. 38, November, 1910) has also 

 tested the immunity of pigs from immune sows, and reports that they 

 show a high degree of resistance to hog cholera until they are from 2 

 to 16 weeks of age. 



In these experiments we tested the immunity of pigs 3 days old 

 from 3 immunized sows, from 1 hyperimmunized sow, and from 



2 nonimmime sows. We have also tested the immunity in pigs 



3 weeks old from 5 immunized sows, making a total of 11 litters 

 tested, 8 from immunized sows, 1 from a hyperimmunized sow, and 2 

 from nonimmune sows. In testing the immunity of these pigs, all 

 were injected subcutaneously with approximately 0.5 cubic centi- 

 meters of disease-producing blood, to which phenol had been added. 

 The virulence of the bloocl injected was proven in all cases by the 

 injection of nonimmune pigs. The results of these experiments show 

 that not a single pig from an immune or hyperimmune sow died 

 from the injections or showed any ill effects therefrom, whereas all 

 of the inoculated pigs from the two nonimmune sows died of hog 

 cholera as a result of the injection. Furthermore, the mothers of the 

 two nonimmune litters" contracted hog cholera from their pigs and 

 both died of the disease. These experiments seem to prove quite 

 definitely that pigs from immune or hyperimmune sows are them- 

 selves immune against hog cholera at birth, and that this immunity 

 lasts for at least three weeks. The exact time at which this passive 

 immunity transmitted b}'^ the mother becomes reduced to such a point 

 that the hogs are no longer able to resist infection must be determined 

 by future experiments. It likewise remains to be determined whether 

 the injection of virus during the period of passive immunity will 

 result in the production of a permanent active immusJty similar to 

 that secured by the simultaneous method of vaccination. 



ATTENUATION OF HOG-CHOLEEA VIRUS BY HEAT. 



A number of years ago, immediately after the discovery that the 

 virus of hog cholera is filterable and exists in the blood of ^^ick hogs, 

 a long series of attempts were made to attenuate the virus by heat and 

 by other agents. None of these experiments resulted satisfactorily, 

 for it was found that the virus did not react uniformly to the attenu- 

 ating agents. In one case it might be apparently attenuated so as 

 safely to produce an active immunity; in other instances, under pre- 

 cisely the same conditions, the virus would not be attenuated at all 

 and would cause the death of the injected animals; while in still other 

 instances it appeared that the virus was killed by the same process 

 and while not injuring the inoculated animals it at the same time had 

 no effect in conferring an immunity upon them. In the summer of 

 1911 is was claimed by an investigator that the virus of hog cholera 

 could be satisfactorily attenuated by heating at a temperature of 



