Diagnosis of eastern equine encephalomyelitis by immunofluorescent staining of brain tissue.

Brain tissues were obtained from 5 horses with clinical encephalomyelitis during an epizootic in southwestern Michigan in August-September 1980. These tissues were tested for virus by intracerebral inoculation of suckling mice and by examination of frozen sections and impression smears by the indirect fluorescent antibody (FA) technique. Eastern equine encephalomyelitis virus was isolated and detected by FA technique in brains of 3 horses which died or were euthanatized within approximately 24 hours of onset of the disease but not from 2 horses at 2 and 3 days after onset. The latter 2 animals had serum-neutralizing antibodies at the time of death. Seven areas of the brain of 1 horse were tested. The proportion of fluorescing cells in frozen sections correlated with infectivity titers. Impression smears were negative. Viral titers ranged from 10(5.7) to 10(10.0) suckling mouse intracerebral median lethal doses/g; highest titers and most intense fluorescence were present in the thalamus and pons, emphasizing the need to obtain selective samples of central brain structures for diagnostic examination. The FA technique appears useful for the rapid diagnosis of fatal eastern equine encephalomyelitis and may be applicable in laboratories not equipped for isolation of viruses.