Venezuelan equine encephalitis virus activity in the Gulf Coast region of Mexico, 2003-2010

Abstract

Venezuelan equine encephalitis virus (VEEV) has been the causative agent for sporadic epidemics and equine epizootics throughout the Americas since the 1930s. In 1969, an outbreak of Venezuelan equine encephalitis (VEE) spread rapidly from Guatemala and through the Gulf Coast region of Mexico, reaching Texas in 1971. Since this outbreak, there have been very few studies to determine the northward extent of endemic VEEV in this region. This study reports the findings of serologic surveillance in the Gulf Coast region of Mexico from 2003-2010. Phylogenetic analysis was also performed on viral isolates from this region to determine whether there have been substantial genetic changes in VEEV since the 1960s. Based on the findings of this study, the Gulf Coast lineage of subtype IE VEEV continues to actively circulate in this region of Mexico and appears to be responsible for infection of humans and animals throughout this region, including the northern State of Tamaulipas, which borders Texas.

Figure 1. Geographical distribution and frequency of equids in the municipalities of three Gulf States of Mexico (Tamaulipas, Veracruz, Tabasco) with positive serology by ELISA, PRNT, and/or HI for Venezuelan equine encephalitis virus (VEEV), 2003–2004.

Map inset shows the locations of the States of Tamaulipas, Veracruz, and Tabasco relative to the other states in Mexico. Table inset provides a location key for each municipality. Key inset shows the relative frequency (%) of positive samples based on color intensity.

Figure 2. Geographical distribution and frequency of bovids and equids in the northern municipalities of the State of Veracruz with positive serology by ELISA and PRNT for Venezuelan equine encephalitis virus (VEEV), 2010.

Maps show the distribution and seroprevalence for VEEV in bovids (A) and equids (B). Map inset shows the location of the State of Veracruz relative to the other states in Mexico. Table inset provides a location key for each municipality. Key inset shows the relative frequency (%) of positive samples for based on color intensity.

Figure 3. Geographical distribution of humans in municipalities of the State of Veracruz with positive serology by ELISA for IgM/IgG, PRNT, and/or HI for Venezuelan equine encephalitis virus (VEEV), 2008–2009.

Red indicates municipalities of the State of Veracruz that were IgM positive for VEEV, and blue indicates municipalities that were positive for VEEV by IgG ELISA, PRNT, and/or HI. Map inset shows the location of the State of Veracruz relative to the other states of Mexico. Table inset provides a location key for each municipality, number of total samples, number of positive samples, seroprevalence (%), and the serology assays that were performed on the samples.

Figure 4. Geographic distribution of subtype IE Venezuelan equine encephalitis virus (VEEV).

Based on phylogenetic analyses of these strains, Pacific IE genotypes are represented by blue, Gulf/Caribbean IE genotypes are represented by green, and Panama IE genotypes are represented by red. Recent isolates (2008–2010) are printed in bold.

Figure 5. Neighbor joining (NJ) phylogenetic tree of Venezuelan equine encephalitis virus (VEEV) strains based on a 1677 nt fragment, which extends from the 3′ end of the capsid to the 5′ end of E1 within the structural protein genome region.

The tree shows the clustering pattern of the sequences obtained from this study as well as representative strains that were available from the GenBank database. VEEV subtype IIIA Mucambo virus was used as the outgroup. Trees generated using maximum parsimony and maximum likelihood (ML) methods had identical topologies. The numbers adjacent to the nodes indicate NJ bootstrap values for 1000 replicates. The scale bar represents the number of nucleotide substitutions per site, and the brackets indicate the genotypes of the subtype IE VEEVs and some geographical/temporal data within the Gulf/Caribbean IE genotype. Sequence names are marked by VEEV subtype, a two-letter abbreviation for the country of origin, a two digit number indicating year of isolation, and the strain name. Pacific IE genotypes are represented by blue, Gulf/Caribbean IE genotypes are represented by green, and Panama IE genotypes are represented by red. Recent isolates (2008–2010) are printed in bold.