Scott C. Weaver

Eastern equine encephalomyelitis virus: epidemiology and evolution of mosquito transmission.

Publisher Summary This chapter focuses on the role of mosquitoes in the enzootic transmission of eastern equine encephalomyelitis (EEE) virus and discusses the historic and epidemiologic knowledge, structure and replication, and transmission cycles of the virus. Studies on mosquito–virus interactions and speculation on the significance of these findings have been summarized in the chapter. Early studies of EEE virus described the disease caused by this virus, identified the virus as an etiological agent, and determined arthropod and vertebrate hosts for enzootic and epidemic transmission. Subsequent studies provided important details on mosquito bionomics, vector–virus interactions, and basic virology. Because EEE virus is easy to work with in the laboratory, the virus has also been used as a model for testing concepts in entomological aspects of arbovirology. Multiple blood-feeding by mosquitoes during a single gonotrophic cycle increases the probability of virus persistence during interepidemic periods and the explosive spread of diseases during epidemics. EEE virus transmission cycles can be used to test the importance of this prediction. Moreover, the techniques that are being developed—histological examination of engorged mosquitoes and alkali metal blood markers in sentinel hosts—have broad application to hematophagous arthropods that transmit a variety of diseases.

Genetic diversity and slow rates of evolution in new world alphaviruses

Alphavirus is a genus of arthropod-borne viruses (arboviruses) in the family Togaviridae. These viruses have a single-strand messenger or plus-sense RNA genome of ca. 11 700 bases. 42S genomic RNA is found in virions and infected cells; a subgenomic 26S RNA species, identical to the 3’-third of the 42S genomic messenger RNA (mRNA), is present only in infected cells and codes only for the structural proteins (Strauss and Strauss 1986; Schlesinger and Schlesinger 1990).

Phylogenetic analysis of alphaviruses in the Venezuelan equine encephalitis complex and identification of the source of epizootic viruses.

We studied the evolution of alphaviruses in the Venezuelan equine encephalitis (VEE) complex using phylogenetic analysis of RNA nucleotide sequences from limited portions of the nsP4, E1, and 3 untranslated genome regions of representative strains. The VEE complex constituted a monophyletic group of viruses (descended from a common ancestor); some serologic VEE varieties such as subtype III formed monophyletic groups while subtype I did not. Subtype II Everglades and variety ID enzootic viruses formed a monophyletic group which also included all epizootic variety IAB and IC VEE isolates. Everglades virus diverged from this ID lineage (colonized North America) ca. 100-150 years ago, followed by divergence of variety IAB and IC epizootic viruses. Variety IAB viruses probably emerged from the variety ID lineage once during the early part of this century, while variety IC viruses evolved at least two times. These results identify the source of epizootic VEE viruses as the variety ID enzootic virus lineage which occurs in northern South America and Panama. Even if variety IAB and IC viruses are extinct, recent, multiple emergences of epizootic viruses from an enzootic lineage suggests that other epizootic VEE viruses may evolve again in the future. The close genetic relationship of subtype II Everglades virus to the variety ID lineage also implies the potential for emergence of equine-virulent VEE viruses in Florida.

Molecular evolution of eastern equine encephalomyelitis virus in North America.

We examined the rate and spatial pattern of eastern equine encephalomyelitis virus (EEEV) evolution in North America using primer-extension sequencing of viral RNA. Nucleotide sequences of the entire 26 S structural gene region of four EEEV strains revealed remarkable conservation between 1933 and 1985, with an estimated 0.7% divergence or 1.4 x 10(-4) nucleotide substitutions per site per year. Sequences from smaller 26 S regions of nine additional strains suggested that EEEV evolves in North America in a single lineage, with genetic exchange regularly occurring among enzootic transmission foci. In these limited 26 S genome regions, only synonymous nucleotide substitutions became fixed between 1933 and 1988, implying a high degree of conservation in protein structure. Short nucleotide sequences from a Panamanian, South American variety isolate revealed a relatively distant relationship to North American serotype viruses. This suggested genetic divergence between antigenic varieties, and independent evolution of EEEV in North and South America. Factors related to replication and epidemiology of EEEV, which may constrain its evolution in nature, are discussed. Possible mechanisms of genetic exchange among enzootic foci are also considered.

Genetic characterization of an antigenic subtype of eastern equine encephalomyelitis virus

SummaryA 1983 human Mississippi isolate of eastern equine encephalomyelitis virus (EEEV), recently identified as an antigenic subtype of the North American variety, was genetically characterized using oligonucleotide fingerprinting and sequencing of viral RNA. This strain was found to be very closely related to other North American EEEV isolates from the same time period. Phylogenetic analysis suggested that this subtype belongs to a single EEEV lineage in North America. Two amino acid substitutions in the E 2 envelope glycoprotein, not seen in eight other isolates sequenced, probably contributed to the antigenic difference with respect to other EEEV strains. These substitutions include threonine for lysine at position 71, resulting in the addition of a potential N-linked glycosylation site, and lysine for glutamic acid at position 147.