THE LIMITED NEUROTROPIC CHARACTER OF THE ENCEPHALITIS VIRUS (ST. LOUIS TYPE) IN SUSCEPTIBLE MICE

Abstract

St. Louis encephalitis virus injected intracerebrally into susceptible mice multiplies there to reach a titre of 109 intracerebral lethal doses. It is found also in the blood in small amts. im-mediately following injection and preceding death. Injected intraperitoneally or subcut. the virus circulates in the blood for several hrs. and survives in the spleen for days. It does not multiply in the brain and cause encephalitis, however, unless overwhelming doses are injected or the brain is traumatized. Virus dropped into the nares is demonstrable in the olfactory bulbs at 24 hrs., in the piriform lobes at 24-48 hrs., in the remainder of the brain at 3 days, and in the spinal cord at 4 days. In the brain it reaches a titre of 10* in 6 days. Virus is not readily demonstrable in the blood, but is present in the spleen after 48 hrs., where it survives and multiplies. Lesions following nasal instillation of virus appear first in the olfactory bulbs on the 3d day, in the piriform lobes on the 4th, and in Ammon''s horn on the 5th day. The character of the lesions in order of their appearance is exudation of mononuclear cells about superficial blood vessels and in the pia, hyperplasia of the endothelium of the pia, and necrosis of nerve cells of the olfactory tract.