Venezuelan equine encephalitis virus, southern Mexico

Abstract

Equine epizootics of Venezuelan equine encephalitis (VEE) occurred in the southern Mexican states of Chiapas in 1993 and Oaxaca in 1996. To assess the impact of continuing circulation of VEE virus (VEEV) on human and animal populations, serologic and viral isolation studies were conducted in 2000 to 2001 in Chiapas State. Human serosurveys and risk analyses indicated that long-term endemic transmission of VEEV occurred among villages with seroprevalence levels of 18% to 75% and that medical personnel had a high risk for VEEV exposure. Seroprevalence in wild animals suggested cotton rats as possible reservoir hosts in the region. Virus isolations from sentinel animals and genetic characterizations of these strains indicated continuing circulation of a subtype IE genotype, which was isolated from equines during the recent VEE outbreaks. These data indicate long-term enzootic and endemic VEEV circulation in the region and continued risk for disease in equines and humans.

Figure 1

Satellite image of the Pacific coastal areas studied for Venezuelan equine encephalitis virus activity (Landsat thematic mapper). Bands 4, 5, and 1 are displayed as a red-green-blue false-color composite. The villages sampled are indicated in yellow.

Figure 2

Rates of Venezuelan equine encephalitis virus seropositivity by age group for persons living in the La Encrucijada region. Positive samples had 80% plaque reduction neutralization titers of *1:20. The total numbers of serum specimens tested for each age group were as follows: 0–5 years, 10; 6–10 years, 75; 11–15 years, 89; 16–20 years, 35; 21–30 years, 64; 31–40 years, 76; 41–50 years, 54; 51–70 years, 63; 71–90 years, 14.

Figure 3

Maximum parsimony phylogenetic tree derived from partial PE2 envelope glycoprotein precursor gene sequences showing relationships of the newly isolated Venezuelan equine encephalitis virus (VEEV) strains from sentinel hamsters (Mex01-22 and Mex01-32) to other subtype IE strains sequenced previously (19). Strains are designated by country abbreviation followed by year of isolation and strain designation. Numbers indicate nucleotide substitutions assigned to each branch.

Figure 4

Maximum parsimony phylogenetic tree derived from complete genomic sequences showing relationships of the newly isolated Venezuelan equine encephalitis virus (VEEV) strain (MX01-22) to other strains from Mexico and Guatemala. Numbers indicate nucleotide substitutions assigned to each branch. All nodes had bootstrap values of 100%, except the OAX131-37820 (62%) and GU68-GU80 (<50%) groupings. Relative rates tests applied to the branches indicated a rate of nucleotide substitution in Mexico of 2.0–2.9 x 10–4 subst/nt/y since 1993, and 6.8 x 10–5 for the Guatemalan lineage from 1968–1980. These data suggest continuous circulation of VEEV in Mexico since 1993.