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Repeated outbreaks of Venezuelan equine encephalitis (VEE) in humans and equines in the Guajira region of Venezuela suggested a sylvatic focus of an epizootic subtype of VEE virus. A surveillance system was established, and virus isolations were attempted from 67,760 mosquitoes as well as sentinel hamsters. Sixteen isolates of eastern equine encephalitis (EEE) and a strain of Itaqui virus were recovered from mosquitoes, and 60 isolates of EEE, two of VEE, and two of Itaqui viruses were recovered from tissues of sentinel hamsters. The VEE virus isolates were shown to be closely related antigenically to prototype VEE ID and the EEE virus isolates were shown to be more closely related to the South American than the North American variety of EEE virus. Evidence for the presence of VEE and EEE viruses in small wild vertebrates was obtained from serologic testing. This study showed, for the first time, the enzootic presence of both VEE ID and EEE viruses during a nonepizoodemic period in the Guajira region.

Abstract. During field studies of enzootic Venezuelan equine encephalitis (VEE) viruses associated with epizootic emergence, a large number of virus isolates were made in sylvatic foci of Venezuela and Colombia. To rapidly characterize these isolates, antigenic subtypes were determined by means of immunofluorescence and by single-strand conformational polymorphism (SSCP) analysis by use of an 856-bp fragment from the P62 gene, which we used to distinguish genetic variants. Representative isolates were sequenced to assess the sensitivity of SSCP to detect genetic differences. The SSCP analysis distinguished isolates differing by as little as 1 nucleotide; overall, differences of Ն 1 nucleotide were recognized 89% of the time, and the sensitivity to distinguish strains that differed by only 1 or 4 nucleotides was 17 and 57%, respectively. Phylogenetic analyses of representative sequences showed that all recent isolates from the Catatumbo region of western Venezuela and the middle Magdalena Valley of Colombia were closely related to epizootic subtype IAB and IC strains; strains from Yaracuy and Miranda States were more distantly related. Cocirculation of the same virus genotype in both Colombian and Venezuelan foci indicated that these viruses are readily transported between enzootic regions separated by Ͼ 300 km. The SSCP analysis appears to be a simple, fast, and relatively efficient method of screening VEE virus isolates to identify meaningful genetic variants.

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Virulent and attenuated strains of yellow fever virus were compared for their ability to grow in cultures of unstimulated leucocytes and monocytes derived from human peripheral blood, and of a macrophage-like cell line of human origin, U937. The extent of virus growth in leucocyte cultures varied depending on the strain of virus, multiplicity of infection, presence of diluted antibody in the culture medium but independently of the flavivirus immune status of the donor. The same pattern of differential growth was observed in the three types of cultures used. Although strain related variation in growth occurred within both virulent and attenuated strains, most of the attenuated strains produced higher virus yields than the virulent ones, suggesting that replication in this cell system is not related to the expression of virulence for the host. Replication in human monocytes as an in vitro marker of immunogenity for substrains of 17 D vaccine virus is discussed.