450 BIOLOGY OF THE LABORATORY MOUSE 



The pulmonary lesions did not differ significantly from those described 

 above. Consolidated areas varied in extent and were hilar in distribution 

 with radiations outward along the bronchi. The histopathological appear- 

 ance was essentially the same. In the great majority of instances (85 per 

 cent) the lungs were sterile and such bacteria as were found had no etiological 

 significance. It is of interest to note that pleuropneumonia-like organisms 

 were isolated with ease and in approximately the same numbers from the 

 lungs of normal mice as from those infected with the murine and influenzal 

 viruses. These organisms did not reproduce the disease, nor did rabbit 

 antiserum containing agglutinins neutralize the murine virus. 



The virus was found to be strictly pneumotropic for mice and to increase 

 in virulence with the first few serial passages. Routes other than intra- 

 nasal failed to produce infection, and the virus could not be obtained from 

 the brain after intracerebral inoculation, nor from the liver following intra- 

 peritoneal injection. Attempts to transmit the infection by contact were 

 unsuccessful. Ten other species of animals, including rabbits, ferrets, 

 guinea pigs, and rhesus monkeys, were resistant to infection. The virus 

 has been cultivated in tissue culture with considerable loss in virulence. 



Active immunity in mice was readily obtained by two intraperitoneal 

 injections of living virus or by intranasal inoculation of amounts insuflicient 

 to produce death. All strains of this murine virus were identical immu- 

 nologically and were easily distinguished from the human and swine strains 

 of influenza by cross immunity and neutralization experiments. The virus 

 was neutralized, however, by approximately one-third of 67 human sera, 

 although in later experiments no association could be made with any of the 

 respiratory diseases common in humans. 



In suspensions of infected mouse lung the virus was inactivated at 

 56°C. in 30 minutes, and decreased in titer rapidly at room temperature 

 unless protected by the addition of 10 per cent normal horse serum. No 

 decrease in activity occurred when frozen and stored at — 76°C. It was 

 readily filterable through Berkefeld V and N filters but not through the 

 Seitz filter. By the use of graded collodion membranes, its diameter was 

 found to be approximately 100 to 150 m/x. 



It thus seems probable that experimental pneumonias in mice, though 

 similar pathologically, may be due to different viruses. The agents do not 

 cause spontaneous illness and not all mouse stocks are infected with them. 

 The original source is not known, but may be human (104), if neutralization 

 of this virus is as specific as it is with other viruses. The primary impor- 

 tance of this disease to investigators, however, is its similarity to that pro- 



