Virological and serological studies of Venezuelan equine encephalomyelitis in humans

During the 1971 epidemic of Venezuelan equine encephalomyelitis (VEE) in south Texas, 203 suspect VEE cases were evaluated by the Center for Disease Control. Sixty-seven were confirmed as cases of VEE. Laboratory confirmation was accomplished by isolation of VEE virus from a serum specimen taken during the acute illness in 50 (75%) of the confirmed cases. Serological confirmation was obtained in 17 cases (25%). Virus isolations were most often obtained from sera collected during the first 3 days of illness. Peak serum virus titers (algebraic mean, 10(5-7) suckling mouse intracranial 50% lethal doses [SMICLD50] per ml) occurred on day 2 of illness. One-half of the sera from which virus was isolated contained at least 10(5) SMICLD50/ml, which has been shown to be sufficient to infect some vector mosquitoes. Blood from 13 virus-positive VEE cases was obtained 1 and 11 months after illness. Hemagglutination-inhibiting, complement-fixing, and neutralizing antibodies were formed by all 13 patients 1 month after illness. Hemagglutination-inhibiting antibody titers were essentially unchanged 11 months after illness. Complement-fixing antibody was undetectable 11 months after illness in 23% of cases and was detectable at dilutions of 1:8 or 1:6 in 77%. Neutralizing antibody (measured by log neutralization index) was not detectable 1 year after illness in one person (8%); titers had declined from 1.0 to 2.0 in 46%, were unchanged in 39%, and were not tested in one person (8%). No evidence of intrafamilial spread of VEE virus was obtained in either of two illness and antibody surveys. A randomized household illness and antibody survey of 681 Port Isabel residents revealed an inapparent infection ratio of 1:11 and an overall antibody prevalence of 3.2%.