Abstract
Five years after the apparent end of the major 1995 Venezuelan equine encephalitis (VEE) epizootic/epidemic, focal outbreaks of equine encephalitis occurred in Carabobo and Barinas States of western Venezuela. Virus isolates from horses in each location were nearly identical in sequence to 1995 isolates, which suggests natural persistence of subtype IC VEE virus (VEEV) strains in a genetically stable mode. Serologic evidence indicated that additional outbreaks occurred in Barinas State in 2003. Field studies identified known Culex (Melanoconion) spp. vectors and reservoir hosts of enzootic VEEV but a dearth of typical epidemic vectors. Cattle serosurveys indicated the recent circulation of enzootic VEEV strains, and possibly of epizootic strains. Persistence of VEEV subtype IC strains and infection of horses at the end of the rainy season suggest the possibility of an alternative, cryptic transmission cycle involving survival through the dry season of infected vectors or persistently infected vertebrates.
Highlights
Five years after the apparent end of the major 1995 Venezuelan equine encephalitis (VEE) epizootic/epidemic, focal outbreaks of equine encephalitis occurred in Carabobo and Barinas States of western Venezuela
Suspected VEE was reported in Obispos, Zamora, Pedraza, and Miranda municipalities before these VEE virus (VEEV) isolates were confirmed at the Instituto Nacional de Investigaciones Agropecuarias in April 2000
Of 5 major hypotheses proposed to explain the source(s) of strains of subtypes IAB and IC responsible for all major VEE outbreaks [7], 2 are supported by previous studies: 1) several of the later VEE outbreaks caused by subtype IAB strains were probably initiated by the use of incompletely inactivated vaccines produced from early, wild-type, equine-virulent isolates [8,33]; and 2) all subtype IAB and IC strains evolved independently from an enzootic lineage of subtype ID VEEV that circulates in western Venezuela, Colombia, and northern Peru [2,22]
Summary
Five years after the apparent end of the major 1995 Venezuelan equine encephalitis (VEE) epizootic/epidemic, focal outbreaks of equine encephalitis occurred in Carabobo and Barinas States of western Venezuela. The second hypothesis, supported by genetic studies, is that epizootic/epidemic ( called epidemic) subtype IAB and IC VEEV strains arise through mutation from enzootic subtype ID VEEV strains [9]. The strongest such evidence links a small, 1992-93 Venezuelan VEE outbreak caused by a subtype IC virus strain to sympatric strains of enzootic subtype ID virus [3,10]. After the apparent end of the 1995 outbreak, no confirmed epidemic or epizootic VEE occurred in South America for >4 years
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