440 BIOLOGY OF THE LABORATORY MOUSE 



signs of infection, but more commonly the animals become hyperactive so 

 that even a slight stimulus will cause them to leap into the air or will induce 

 a convulsion. If the mouse is lifted by the tail, a convulsion frequently 

 follows, characterized by rapid clonic movements of the fore legs, terminat- 

 ing in a sustained tonic extensor spasm of the hind limbs, and lasting from 

 one to several minutes. Convulsive attacks also occur spontaneously. 

 Death may result in the first or subsequent attacks. If the animals survive 

 for 3 or 4 days after the onset of signs of the disease, complete recovery with- 

 out residual paralysis usually occurs. Blood counts are within normal 

 limits (292). This same clinical course may be seen in naturally infected 

 mice injected intracerebrally with sterile starch emulsion or bouillon. 



Intranasal and subcutaneous inoculations produce no signs of the disease, 

 but the virus may be demonstrated in the blood and the animals acquire an 

 immunity to subsequent intracerebral inoculation. Mice inoculated intra- 

 peritoneally or intravenously may show labored respiration 5 to 10 days 

 later for a period of a week or more. Convulsions do not occur. A few of 

 the mice die, but the majority recover and are resistant to a second inocula- 

 tion. The virus may persist for weeks or months in mice recovering from 

 experimental infection and has been demonstrated in the brain, blood, liver, 

 spleen, kidneys, lungs, adrenal, nasal passages, and urine. No neutralizing 

 antibodies, however, have been observed in the blood of recovered mice 



(293)- 



The pathological picture varies with the route of inoculation. Meso- 

 dermal tissues are primarily involved with the production of a hyperplastic 

 reaction. Following intracerebral inoculation, congestion is apparent 

 grossly in the surface vessels of the brain, in the liver, and in the spleen, 

 which may be slightly enlarged. Microscopically, there is infiltration of the 

 meninges of the brain and spinal cord, the cellular exudate being composed 

 chiefly of lymphocytes, and to a less extent, of mononuclear and polymorpho- 

 nuclear cells. Infiltration is most marked at the base of the brain (Fig. 170), 

 but the choroid plexuses and the ependyma are quite constantly involved. 

 Perivascular round cell infiltration is present if the animals survive for 2 or 

 3 weeks. Involvement of the nervous tissue proper is minimal, and no 

 inclusion bodies are found. Changes in the other organs are minor ; irregular 

 hyperplasia of reticulo-endothelial (Kupffer) cells and slight lymphocytic 

 infiltration in the liver, and small areas of interstitial bronchopneumonia in 

 the lungs are the chief findings. 



Mice developing signs of infection after intraperitoneal or intravenous 

 inoculation show visceral lesions but infrequently there is evidence of even 



