INFECTIOUS DISEASES OF MICE 



447 



Neutralization of the virus by the serum of convalescent mice has not 

 been satisfactorily demonstrated by the methods so far employed. The 

 results suggest that relative protection can be conferred, but the degree of 

 protection is insufhcient to permit an immunological comparison of the 

 various strains of virus by this method. The murine virus is not neutralized 

 by antiserum for the virus of human poliomyelitis. 



Differential diagnosis. — The clinical course of this disease following 

 intracerebral inoculation is sufficiently distinctive to difTerentiate it from 

 other encephalitis -agents such as lymphocytic choriomeningitis, equine 

 encephalomyelitis, etc., whereas the diameter of the virus, pathological find- 

 ings, and host specificity distinguish it from acute meningo-encephalitis (8i). 



Final diagnosis is made on the basis of size, host range, pathology, and 

 cross-protection. Parasitic meningo-encephalitis may be distinguished 

 pathologically, and bacterial infections by cultural methods. 



Epidemiology. — ^Considerable interest is centered in epidemiological 

 studies (271, 189) of this disease because of its similarity to human polio- 

 myelitis. The significant features thus far demonstrated are the widespread 

 distribution of the virus as evidenced by its almost constant presence in the 

 intestines of young mice, the low incidence of spontaneous disease, the 

 prolonged period of excretion in the feces, and the gradual development of 

 resistance with increasing age. The exact route of natural infection is not 

 known, but in all probability is either nasal or oral since fecal excretion must 

 keep the environment almost constantly infected. That such excretion by 

 an infected mouse is not dependent on continuous infection, however, is 

 shown by isolation experiments in which the opportunity for self-infection 

 was minimal. Under such conditions, the intestinal wall is apparently the 

 site of elaboration of the virus, and invasion of the mesenteric glands may 

 occur secondarily. Whether the intestinal tract is the focus for distribution 

 of the virus when first introduced, or is but secondarily infected, has not been 

 determined. Theiler and Gard (271) have suggested that the development 

 of antibodies due to infection of the intestinal tract may be responsible for 

 the increasing resistance with age. The failure of an individual animal to 

 form antibodies might then allow invasion of the central nervous system and 

 the production of clinical disease. Experiments with a disease-free stock of 

 mice would be of value in elucidating further the epidemiology and nature 

 of this disease, which, though unimportant as regards mortality, is of con- 

 siderable importance to an investigator employing mice in the study of 

 viruses. 



