442 BIOLOGY OF THE LABORATORY MOUSE 



uted in the tissues of infected animals. Different strains vary in the degree 

 of their virulence, but in general the virus is pathogenic for mice, rats, guinea 

 pigs, monkeys, and man. The serum of certain convalescent or recovered 

 animals contains complement-fixing and protective antibodies, as does that 

 of rabbits inoculated with virus suspensions (io6, 147, 251, 252, 253). 



The virus withstands freezing and drying (252), and 50 per cent glycerin 

 for at least i month (291), but rapidly decreases in infectivity when sus- 

 pended in physiological saline solutions at room temperature unless protected 

 by the addition of 2 per cent normal inactivated serum (252). In size the 

 virus particles are not more than 100 to 150 m/i in diameter, as determined 

 by filtration through graded collodion membranes (220). From suspensions 

 of infected tissue, a soluble antigen — apparently protein in nature — has been 

 obtained (252, 253). It is capable of fixing complement and of precipitating 

 when mixed with immune serum. The antibodies which react with this 

 antigen are apparently distinct from those responsible for neutralization of 

 the virus in protection tests. Immunological reactions with both tissue 

 emulsions and soluble antigen are entirely specific, and no qualitative differ- 

 ences have been found between various strains of the virus. 



Diagnosis of the disease. — Since the disease may be present in a mouse 

 colony as a subclinical, latent infection, its existence may not be suspected. 

 Recognition of the infection may be accomplished by several methods, (a) 

 The virus may be isolated by intracerebral inoculation of an "indicator 

 host " (7) — that is, a guinea pig or mouse known to be free from the infection 

 — with blood or emulsion of brain, spleen, or kidney, {b) Demonstration of 

 immunity in a certain number of stock mice is an indication of previous 

 infection. Traub, for example, found that the morbidity rate following 

 intracerebral inoculation of the virus into mice from the infected stock was 

 about 60 per cent and the mortality rate about 40 per cent, (c) Intra- 

 cerebral injection of a sterile, non-infectious agent, such as starch emulsion 

 or bouillon, may be a sufficient stimulus in some of the animals to cause a 

 flare-up of the inactive infection, resulting in a clinical picture similar to that 

 seen in normal mice inoculated intracerebrally with the virus (292, 69, 146). 



Final diagnosis of the disease is made on the basis of the clinical course in 

 mice or guinea pigs, the pathological findings, and immunological identifica- 

 tion of the virus by complement fixation or protection tests, or by inoculation 

 of recovered animals with a known strain of the virus. Immunological 

 methods, for example, afford a clear distinction between the viruses of lym- 

 phocytic choriomeningitis and acute meningo-pneumonitis (81), although 

 the clinico-pathological features following intracerebral injection are similar. 



