Venezuelan Equine Encephalitis in Panama: Fatal Endemic Disease and Genetic Diversity of Etiologic Viral Strains

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Abstract
Venezuelan equine encephalitis (VEE) is a reemerging, mosquito-borne viral disease of the neotropics that is severely debilitating and sometimes fatal to humans. Periodic epidemics mediated by equine amplification have been recognized since the 1920s, but interepidemic disease is rarely recognized. We report here clinical findings and genetic characterization of 42 cases of endemic VEE detected in Panama from 1961–2004. Recent clusters of cases occurred in Darien (eastern Panama) and Panama provinces (central Panama) near rainforest and swamp habitats. Patients ranged from 10 months to 48 years of age, and the more severe cases with neurological complications, including one fatal infection, were observed in children. The VEE virus strains isolated from these cases all belonged to an enzootic, subtype ID lineage known to circulate among sylvatic vectors and rodent reservoir hosts in Panama and Peru. These findings underscore endemic VEE as an important but usually neglected arboviral disease of Latin America. Venezuelan equine encephalitis (VEE) is a mosquito-borne viral disease that has caused major epidemics in many parts of Latin America and has even spread into Texas on one occasion. These epidemics result from spillover to humans of a horse-mosquito-horse amplification cycle that has occurred periodically since the 1920s. However, between these equine-mediated epidemics, little attempt is typically made to detect VEE in humans. Here, we show that VEE virus strains that typically circulate in a mosquito-rodent cycle, termed enzootic strains, also produce many cases of severe and sometimes fatal disease in Panama in the absence of apparent epidemics. These endemic infections are probably rarely detected because they are difficult to distinguish clinically from dengue fever, another mosquito-borne viral illness common in the tropics. Our findings underscore endemic VEE as an important but usually neglected arboviral disease of Latin America.