1999 Workshop Abstracts | Virus Evolution Home Page | Noble Foundation Plant Biology Home Page

Phenotypic and genetic characterization of a newly emergent epizootic VEE subtype IE virus in Mexico

Aaron C. Brault, Ann M. Powers, and Scott C. Weaver
University of Texas Medical, Department of Pathology, 300 University Blvd., Galveston, TX 77555-0609

In 1993 in Chiapas and 1996 in Oaxaca, the two southern most states of Mexico, outbreaks of equine encephalitis were reported as the result of a VEE subtype IE virus that had never before been associated with equine disease. Sequence analysis of the E2 envelope glycoproteins of four isolates made during the 1993 and 1996 outbreaks has indicated that they are most closely related to enzootic IE viruses isolated along the Pacific coast of Guatemala.

In order to ascertain specific nucleotide sequence differences which could be associated with the emergence of an epizootic subtype IE virus from an enzootic predecessor, we sequenced the entire genome of strain 93-42124, an equine brain isolate from the 1993 outbreak in Chiapas, and compared the sequence to that of strain 68U201, an enzootic IE isolate from Guatemala in 1968. Although these viruses were isolated twenty-five years apart, their sequences were less than three percent different at the nucleotide level. Furthermore, in the entire structural portion of the genome, there were merely four amino acid differences (three in the E2 glycoprotein), one of which encodes a new N-linked glycosylation site. Site-directed mutagenesis of an infectious clone derived from 68U201 has been performed to independently introduce each of the three amino acid substitutions from 93-42124.

Experimental infection of animal models with the enzootic and epizootic VEE subtype IE viruses resulted in no significant difference in viremia or virulence. The lack of a small animal model for equine virulence may necessitate the use of experimental infection of equines to definitively determine which substitutions are responsible for its epizootic phenotype. In order to phenotypically characterize these isolates, using in vitro markers associated with equine virulence, sensitivity to interferonµ/b as well as plaque size differences were determined for the four epizootic IE viruses and the most closely related enzootic IE viruses. Results indicated that the epizootic IE viruses were extremely insensitive to interferonµ/b and produced slightly smaller plaques than did enzootic IE viruses. These results are consistent with results obtained from epizootic subtype IC VEE viruses and closely related enzootic ID viruses. In order to further investigate the role of the three E2 amino acid substitutions, interferon sensitivity and plaque size determination will be performed on each of the site-directed mutants.

Abstract - Presented at the Virus Evolution Workshop
Ardmore, OK
October 21 - 24th, 1999

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e-mail: mroossinck@noble.org

Dr. Marilyn Roossinck
Plant Biology Division
The Noble Foundation
P.O. Box 2180
Ardmore, OK 73402

phone: 580 224-6630