Ecology and transmission dynamics of Everglades virus

Everglades virus (EVEV), an alphavirus in the Venezuelan equine encephalitis (VEE) serocomplex, circulates among rodents and vector mosquitoes in Florida and occasionally infects humans, causing a febrile disease sometimes accompanied by neurological manifestations. \r\nEVEV infections of cotton rats from Florida, and from Texas, a non-endemic area were performed to validate their role in enzootic transmission and to evaluate whether the viremia induced regulates EVEV distribution. Cotton rats from both localities developed viremia levels that exceeded the threshold for infection of the vector indicating that rat susceptibility does not limit EVEV distribution. \r\nSusceptibility experiments were performed with Aedes taeniorhynchus and Culex nigripalpus, potential EVEV vectors, to evaluate their permissiveness to EVEV infection. In contrast to the high degree of susceptibility of the established vector Culex (Melanoconion) cedecei, these two species were relatively refractory to oral EVEV infection, indicating that they are probably not important vectors. \r\nPet dogs were used as sentinels of EVEV activity to detect recent circulation and to delineate EVEV distribution. Four percent of Florida dog sera contained EVEV antibody and many animals lived farther north than recorded EVEV activity, indicating that EVEV is widespread in the state and may be a cause of undiagnosed febrile illness in residents.\r\nDespite evidence that enzootic subtype ID Venezuelan equine encephalitis viruses (VEEVs), the closest relatives of EVEV, have emerged to cause significant human and equine disease, EVEV has not caused outbreaks in Florida. Venezuelan equine encephalitis virus emergence can be mediated by adaptation to epizootic mosquito vectors via mutations in the E2 glycoprotein gene. EVEV may accrue similar E2 mutations resulting in epizootic disease in Florida. The role of the EVEV E2 gene in epizootic vector infection was evaluated with a VEEV/EVEV E2 chimera. Insertion of the EVEV E2 into the VEEV backbone reduced infection in Ae. taeniorhynchus by 40%, indicating that the epizootic VEEV E2 is necessary for infection of epizootic vectors. \r\nThe intrinsic plasticity of RNA viruses can facilitate changes in host range that may cause epidemics. However, the evolutionary processes that promote cross-species virus transfers are poorly defined, especially for generalist RNA arboviruses that replicate alternately in arthropods and vertebrates and often exhibit slower evolutionary rates than other RNA viruses with similar mutation frequencies that replicate only in vertebrates. The observed genetic stability of RNA arboviruses may result from constraints imposed by alternating between disparate hosts, where optimal replication in one host involves a fitness tradeoff for the alternate host. Accordingly, freeing RNA arboviruses from the alternate replication cycle, and thereby allowing them to specialize in a single host, will facilitate faster evolution and adaptation. To test this hypothesis in vivo, VEEV was passaged serially in mosquitoes or in vertebrates to eliminate host alteration, or alternately between mosquitoes and vertebrates. Virus lineages allowed to specialize in mosquitoes exhibited increased mosquito infectivity, and vertebrate-specialized strains produced higher viremias. Alternately passaged VEEV exhibited no detectable fitness gains in either host and serial passaged VEEV exhibited fitness declines in the bypassed host. These results support the hypothesis that arbovirus adaptation and evolution is limited by obligate host alternation. \r\n\r\n