446 BIOLOGY OF THE LABORATORY MOUSE 



cent hydrogen peroxide in 2 hours at 37°C., and by desiccation in the frozen 

 state at — i6°C. 



The virus is Umited in its host pathogenicity. Mice are susceptible, but 

 guinea pigs, rabbits, and rhesus monkeys are resistant. Theiler and Gard 

 (270) have recently reported that one of their strains of encephalomyelitis 

 virus is pathogenic for the cotton rat. This work is of considerable interest, 

 since the cotton rat has been reported by Armstrong (12, 13) to be suscepti- 

 ble to the Lansing strain of human poliomyelitis virus, and the virus recov- 

 ered from the inoculated animals was then found to be pathogenic for mice 

 by the intracerebral route. An immunological relationship apparently 

 exists between these two viruses, since Theiler and Gard (270) found "... 

 that mice which had been infected with the virus of mouse encephalomyelitis 

 were resistant to a subsequent intracerebral inoculation of Armstrong's 

 Lansing strain of human poliomyelitis virus ..." Jungeblut and Sanders 

 (114) have also isolated a virus from a cotton rat injected with the SK 

 (New Haven) strain of human poliomyelitis virus. The animal died a week 

 after inoculation without presenting signs of disease, but subsequent passage 

 of the agent to cotton rats resulted in flaccid paralysis of the hind legs and 

 death. Inoculation of mice produced an illness clinically like mouse 

 encephalomyelitis. Mice from a colony immune to the spontaneous murine 

 encephalomyelitic agent, however, were susceptible to infection with their 

 virus. All later attempts to produce infection in rats or mice with the 

 original material were unsuccessful. Further investigation is necessary to 

 clarify the relationship between the above strains of virus. 



Immunologically, the murine strains of this virus thus far isolated are 

 antigenically related (270). Recovered but paralyzed animals are resistant 

 to a second inoculation regardless of the route used to infect them. Mice 

 infected intracerebrally with a relatively avirulent strain of virus are immune 

 to subsequent inoculation with a highly virulent strain (GD VII) . Although 

 the interference phenomenon might be responsible for erroneous conclusions 

 in experiments of this type, the high degree of resistance to a second inocula- 

 tion is more suggestive of true active immunity. The two more virulent 

 strains (FA and GD VII) are not immunologically identical, however, since a 

 greater resistance is produced by immunization with the homologous than 

 with the heterologous strain. The greater resistance of older mice is prob- 

 ably due to previous contact with the virus (271, 189), but the same phe- 

 nomenon is seen with other infectious agents and may be the resultant of 

 anatomical and physiological (228), as well as immunological factors. 



