A Comparative Study of the Pathogenesis of Western Equine and Eastern Equine Encephalomyelitis Viral Infections in Mice by Intracerebral and Subcutaneous Inoculations

Intracerebral inoculation of western equine encephalitis (WEE) virus caused a fatal infection in mice. Virus was recovered in the brain and spinal cord. There was no viremia, and very little if any virus was recovered from other tissues. Viral antigen was detectable by immunofluorescent staining in the cytoplasm of neurons and glial cells. By the subcutaneous route, WEE virus produced a low-grade viremia, with persistence of virus at the inoculation site for 3-4 days. Regardless of the dose of virus given, fewer than half of the inoculated mice developed infection that progressed to the central nervous system. Eastern equine encephalitis (EEE) was far more virulent. A small inoculum of less than 10$MLD50$inoculated intracerebrally produced a rapidly fatal disease with virus present in all tissues, although the highest titers were found in the brain. Subcutaneous inoculation also produced a pantropic infection, but a larger amount of virus was required to initiate infection and the incubation period was proportionately longer. In spite of the presence of significant amounts of infectious EEE virus in many body tissues, viral antigen detectable by fluorescent antibody staining was localized only in the central nervous system and was not seen in other non-nervous tissues. Fluorescent conjugate prepared from rabbit antiserum was satisfactory for the detection of WEE viral antigen, but not EEE viral antigen, in infected mouse brains. However, an anti-EEE fluorescent conjugate prepared from rooster serum was satisfactory. The reason for this discrepancy was unclear.