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Venezuelan equine encephalomyelitis (VEE) viruses, of the genus Alphavirus of the family Togaviridae, cause disease ranging from mild febrile reactions to fatal encephalitic zoonoses in Equidae and humans. They are transmitted by haematophagous insects, primarily mammalophilic mosquitoes. The VEE complex of viruses includes six antigenic subtypes (I-VI). Within subtype I there are five antigenic variants (variants AB-F). Originally, subtypes I-A and I-B were considered to be distinct variants, but they are now considered to be identical (I-AB). Antigenic variants I-AB and I-C are associated with epizootic activity in equids and human epidemics. Historically, severe outbreaks have involved many thousands of human and equine cases. The other three variants of subtype I (I-D, I-E, IF) and the other five subtypes of VEE (II-VI) circulate in natural enzootic cycles. Equidae serve as amplifying hosts for epizootic VEE strains while enzootic VEE viruses cycle primarily between sylvatic rodents and mosquitoes. Enzootic variants and subtypes have been considered to be nonpathogenic for equids, but can cause clinical disease in humans. During 1993 and 1996 however, limited outbreaks of encephalitis in horses in Mexico were shown to be caused by enzootic VEE viruses of subtype I-E. Identification of the agent: Diagnosis of VEE virus infection can be confirmed by the isolation, identification, and antigenic classification of the isolated virus.

The factors which determine image contrast in optical and electron micrographs are discussed in relation to a new metal shadow-casting technique whereby the contrast of images is greatly increased by depositing obliquely a thin film of metal on the microscope preparations. Further advantages of the use of shadow-casting are described, and an estimate is made of the lower limit of size of objects which should be observable by shadow-casting, Examples are given and illustrations are shown of the applications of this technique to the electron micrography of particles of macromolecular dimensions, of replicas of such particles, and of surface replicas prepared in several ways.