4o6 BIOLOGY OF THE LABORATORY MOUSE 



the virus of vaccinia. The L5 organism isolated from these animals was 

 culturally and immunologically identical with the type A of Sabin, although 

 the production of an exotoxin by the L5 strain has not yet been established. 



Sabin's findings differ from those described above in that the majority 

 of the animals recovered in a few days; some, however, showed a relapse or 

 continued to exhibit choreiform movements for months. Variations in the 

 clinical picture were noted with the age of the mice — no signs occurred in 

 the majority of animals younger than 15 days or older than 2 months, 

 although infection occurred; and arthritis developed in about 30 per cent 

 of mice more than 2 months of age. The characteristic lesion was acute 

 necrosis of the caudal pole of the cerebellum and of the tissues around the 

 lateral ventricles. Cerebellar involvement was absent in animals showing 

 no signs of infection, but periventricular involvement was regularly present. 



Intraperitoneal or intrathoracic injection of the L5 or A strain produces 

 convulsions or other signs of involvement of the central nervous system 

 in 20 to 40 per cent of mice (225). Death usually follows in 17 to 48 hours. 

 At autopsy, lesions are found only in the brain, while organisms are demon- 

 strable in the viscera but not in the blood or brain. The explanation of 

 this finding was revealed by the demonstration of a toxin, which passes 

 through a Seitz filter and, injected intravenously, produces nervous signs 

 within I or 2 hours. Most of the animals die in a few hours, but those 

 surviving for longer periods exhibit the same acute degeneration of the 

 cerebellum described above. No organisms can be demonstrated in such 

 animals (225). 



Serial intranasal inoculations of an emulsion of lung from a "normal" 

 mouse by the method of bhnd passage, that is, employing the pulmonic 

 tissue of one animal as the inoculum for the next, may result in a pneu- 

 monia apparently due to pleuropneumonia-like organisms after a variable 

 number of passages (264). The infection progresses rapidly with ruffling of 

 the fur, anorexia, weight loss, and dyspnea. Death occurs after 4 or 5 days 

 in about one-third of the mice; if the animals survive for 7 days recovery 

 usually takes place. At autopsy, purple areas of pneumonic consolidation 

 are found involving one or more lobes or an entire lung. Pleuritis may 

 occur. Microscopically, the picture is one of an interstitial pneumonia, 

 congestion, and infiltration chiefly with mononuclear phagocytes. In 

 recovered mice, pneumonic areas may persist for as long as 3 weeks, or a 

 cystic degeneration, similar to that occurring in rats naturally infected with 

 pleuropneumonia-like organisms, may take place. Organisms can be 

 isolated by culture of the emulsified lesions. In addition to the Type A 



