Ecology and Epidemiology

Eastern equine encephalitis (EEE) virus is an alphavirus endemic to the United States. EEE virus is maintained in the environment mainly by cycling between birds and mosquitoes inhabiting freshwater hardwood swamps (Morris 1988).

Transmission risk is highest in lowland regions where its principal enzootic vector, the ornithophilic mosquito Culiseta melanura readily occurs. Most cases occur in eastern or Gulf Coast states. As with other zoonotic arboviruses, EEE virus persists in a complex ecological web of host species, alternating between mosquito vectors and vertebrate amplification hosts. Habitats that pose a threat to humans and other vertebrates are those that support mosquito species that can serve as vectors and vertebrate hosts that develop viremia of sufficient magnitude to infect mosquitoes (amplification hosts). Reptiles and amphibians have also been implicated in enzootic transmission, particularly in Southern states, perhaps serving as an over-wintering refuge for EEE virus (Graham et al. 2012).

Freshwater swamp and bog habitats are a source ecosystem for EEE virus even in periods of low-level transmission (Miley et al. 2021, Skaff et al. 2021). These freshwater habitats provide pools of water for Cs. melanura larval growth and development and plant nectar for adult mosquito survival. In the Northeast region, EEE virus infections in mosquitoes are correlated with proximity to forested wetland habitat dominated by red maple (Acer rubrum) and Atlantic white cedar (Chamaecyparis thyoides). The dominant trees shift to bald cypress (Taxodium distichum) and water tupelo (Nyssa aquatica) in the southeast bottomlands. In the Great Lakes region, Cs. melanura favor sphagnum bogs for oviposition and larval development. Primary buttressed trees in these habitats include black spruce, red spruce, yellow birch, and tupelo.

In the late summer, bird populations disperse after breeding, causing declines in easily available avian blood meals. Swamp-inhabiting mosquitoes may then seek blood outside of the swamp habitats, increasing risk that the virus will spill over to infect other vertebrates and secondary vectors. Surveillance for EEE virus-infected hosts routinely detects vertebrate and vector infections within the swamp habitat, and during epizootic and epidemic transmission outside their primary habitat. Some secondary vectors act as bridge vectors, feeding on birds and mammals and transmitting virus from enzootic to epizootic and epidemic hosts. Once spillover occurs, surveillance can detect EEE virus infections in other potential vector mosquito species.

Vectors suspected of transmitting EEE virus to horses and humans vary geographically. Culiseta melanura can act as both an enzootic and epidemic vector (Armstrong and Andreadis 2010). Primary bridge vectors include Coquillettidia perturbans, Aedes canadensis, Ae. sollicitans, and Culex salinarius in the Northeast and Mid-Atlantic regions (Armstrong and Andreadis 2010; Armstrong and Andreadis 2022; Crans 1977), Cq. perturbans, Ae. canadensis, Cx. salinarius, and Cx. erraticus in the Southeast (Burkett-Cadena et al. 2022), and Cq. perturbans in the Midwest (Nasci et al. 1993). Bridge vectors are competent to transmit virus and infected mosquitoes of these species are frequently detected during epizootic and epidemic periods. Horses and humans are considered dead-end hosts because they do not develop high enough levels of EEE virus in their blood to infect feeding mosquitoes.

The first human EEE disease cases were recognized during a 1938 outbreak in southeastern Massachusetts (Feemster 1938). Subsequent outbreaks were then reported in New Jersey during the 1950s (Goldfield and Sussman 1968). Since then, EEE cases have occurred sporadically and in small clusters, most around freshwater hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region (Goldfield and Sussman 1968; Przelomski 1988; CDC 2006). Human infections typically occur during summer months, with >80% of reported human cases having an illness onset during July through September (Lindsey et al. 2018). During 2003–2018, an average of eight EEE disease cases were reported annually in the United States (range = 4–21 cases/year) (Lindsey 2018; CDC 2021). However, in 2019, 38 cases were reported nationally (Vahey et al. 2021). The reasons for this increase are unknown but are likely related to several factors, including weather, abundance of birds and mosquitoes that can transmit the virus, human behavior, and clinical awareness and diagnostic testing practices.

References

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Vahey GM, et al. West Nile virus and other domestic nationally notifiable arboviral diseases–United States, 2019. MMWR 2021;70:1069–1074.