INFECTIOUS DISEASES OF MICE 443 



No specific measures are as yet available for effective immunization of 

 mice or for prevention of the disease. General preventive measures 

 should be taken to protect a disease-free colony or to stop spread of the 

 infection. 



Encephalomyelitis of mice (Theiler). — Spontaneous encephalomyelitis 

 of mice is a virus disease which rarely produces clinical signs under natural 

 conditions. The active agent, however, is widespread in distribution. It 

 may be obtained with great regularity from normal mice of certain age 

 groups, or may be. encountered in animals inoculated with other agents. 

 Since Theiler (268) first described the disease and demonstrated its etiology 

 the virus has been found in several strains of mice in the United States (269, 

 231, 270), as well as in Germany (88), Japan, (109), and Palestine (189). 



Occurrence.— Tht incidence of the natural disease is difficult to deter- 

 mine, but is probably very low. Various figures have been given : i in about 

 2000 Swiss mice purchased from various dealers (269); i or 2 per 1000 mice 

 of the Rockefeller strain (231) although no cases were found among a series 

 of 5000 animals observed later (189). The low incidence does not indicate 

 lack of contact with the infective agent, however, since the virus has been 

 demonstrated in the intestinal contents of almost all (66 to 100 per cent) 

 mice between the ages of i and 2 months (187, 271, 188, 189). 



The natural disease. — Spontaneous illness in mice (268, 269) may 

 be recognized by the development of flaccid paralysis of the hind legs without 

 other apparent signs. No reports of the course and mortality are available. 

 Pathological examination of the central nervous system reveals scattered 

 necrosis of ganglion cells and perivascular infiltration, most marked in the 

 spinal cord but also present in the brain. The disease seems to become 

 evident chiefly in young mice — approximately 6 to 7 weeks of age — some of 

 which are apparently highly susceptible to invasion of the central nervous 

 system. There is no evident reason why certain animals should be afflicted 

 while the great majority escape, yet practically all at this age are carriers of 

 the virus. In paralyzed animals, the virus is present in the spinal cord in 

 highest concentration and in the brain. It has not been demonstrated in the 

 blood. 



The virus is regularly found in the contents and walls of the gastro- 

 intestinal tract, in the mesenteric glands, and in the feces, but not in the 

 central nervous system, salivary glands, or other organs of normal mice 

 between the ages of 4 and 8 weeks. It is absent or irregularly present in 

 animals younger than 20 days or older than 6 months. Excretion of the 

 virus may persist up to 53 days after the first isolation (271). 



