INFECTIOUS DISEASES OF MICE 445 



inoculation the clinical picture, if apparent at all, is essentially that described 

 above. 



No significant gross changes are found at autopsy (268, 269, 88, 109). 

 Microscopic lesions occur primarily in the spinal cord and are characterized 

 by perivascular round cell infiltration, acute neuronal necrosis particularly of 

 the anterior horn cells, neuronophagia, and gliosis. Ganglion cells of the 

 posterior root are not involved. The brain shows perivascular cullting 

 to a lesser extent and degeneration of occasional isolated neurons. A 

 decrease in the number of anterior horn cells is found in the cords of recovered 

 mice with residual paralysis. No inclusion bodies have been demon- 

 strated. The virus may most easily be demonstrated in the spinal cord and 

 brain. 



The clinical picture produced by the two more virulent strains of virus 

 isolated by Theiler and Gard (270) differs considerably from that described 

 above. The incubation period following intracerebral inoculation is much 

 shorter (2 to 6 days), the course more rapid (24 to 48 hours), the mortality 

 greater, and the titer of virus in the infected brains higher. With one strain 

 (FA) the signs of infection following intranasal or intracerebral injection 

 resemble those of an encephalitis more than a myelitis, e.g., an appearance of 

 being sick, hyperexcitabihty, ruflied fur, twitching, and tonic convulsions 

 sometimes terminating in death. Weakness of one of the legs may occur but 

 paralysis is rare. The histopathological appearance is that of a marked 

 encephalitis with a minimal meningeal reaction. Following intraperitoneal 

 inoculation, however, flaccid paralysis is usually the predominant sign. 

 With the other strain (GD VII), hyperirritability may be the first sign of 

 infection, but the mice appear well and the signs are referable to lesions of the 

 cord, i.e., flaccid paralyses. The same picture results from intracerebral, 

 intranasal, or intraperitoneal injection. 



Properties of the virus. — Although the several strains of this virus vary 

 in virulence or invasiveness, they are much alike, if not identical, in their 

 other properties (268, 269, 270, 88, 109). The average particle diameter as 

 determined by filtration through graded collodion membranes is 9 to 13 m^^, 

 closely approximating that of the viruses of human poliomyelitis and of 

 foot-and-mouth disease of cattle. Filtration through all grades of Berkefeld 

 filters is accomplished with ease. The virus may be preserved in 50 per cent 

 glycerin at 2° to 4°C. for more than 150 days and is most stable at pH 8.0 or 

 pH 3.3. It withstands the action of ether and precipitation by ammonium 

 sulphate, but is destroyed or inactivated by a temperature of 50°C. with 

 rapidity, by 20 per cent ethyl alcohol in 45 minutes in the icebox, by i per 



