Eastern equine encephalitis (EEE) is an arboviral disease (arthropod-borne virus) found in the eastern United States. It is vectored by mosquitoes and is found in swampy coastal areas along the Atlantic seaboard, states bordering Lake Michigan, red maple swamps in the northern Atlantic states and in the southern and Gulf areas associated with cedar forests, salt-water marshes, and loblolly pines (Armstrong & Andreadis, 2002; Greenlee, 2014). The causal agent of EEE is Eastern equine encephalitis virus (EEEV), an RNA alphavirus belonging to the family Togaviridae (Ciota, 2022). Epidemics among horses were first observed in Massachusetts in 1831, but it was not until 1933 that EEEV was first isolated as an equine disease in Delaware, Maryland, New Jersey, and Virginia (Armstrong & Andreadis, 2002).
The first North American human case of EEE was recorded in Massachusetts in 1938, during an outbreak that involved 38 human and 248 horse cases. Human infections are uncommon today (Armstrong & Andreadis, 2002; Corrin, et al., 2021; Greenlee, 2014). In the past two decades, an average of 11 U.S. cases per year have been reported to the Centers for Disease Control and Prevention (Figure 1) (CDC, 2004). A notable 2019 EEE outbreak in the Northeast and Midwest recorded 38 human infections, 23 of which were in the northeastern United States resulting in 14 deaths (Armstrong & Andreadis, 2002; Ciota, 2022; Morens, et al., 2019; CDC, 2004).
States with the highest average annual EEE occurrence from 2003 to 2016 included New Hampshire, Massachusetts, Vermont, Maine, and Alabama (Lindsey, et al., 2018). There has never been a reported human case of EEE in Ohio. Infected mosquitoes, birds, and sentinel animals from 10 Ohio counties have been reported to the CDC (Figure 2) (CDC, 2004).
How is EEEV Transmitted?
In hardwood wetlands, Eastern equine encephalitis virus (EEEV) persists in black-tailed mosquitoes, Culiseta melanura (Coquillett) (Figure 3), and wild passerine birds (Order: Passeriformes) on which they feed (Armstrong & Andreadis, 2002; Morens, et al., 2019; Buckner, et al., 2019). EEEV has been observed in at least 21 different northeastern U.S. mosquito species belonging to the genera Aedes, Anopheles, Culiseta, Coquillettidia, Culex, and Ochlerotatus (Armstrong & Andreadis, 2002; Ciota, 2022). Unlike C. melanura, which prefers to feed on birds, these mosquitoes feed on mammals and birds, and are responsible for mammalian infections (Armstrong & Andreadis, 2002; Ciota, 2022; Corrin, et al., 2021). A small percentage of C. melanura blood meals are taken from mammals, suggesting that this species may in some cases be directly involved in EEEV transmission to mammals as well (Armstrong & Andreadis, 2002; Morens, et al., 2019). When an infected mosquito takes a mammalian bloodmeal, EEEV is transmitted in its saliva (Morens, et al., 2019). Infections occur in the late summer and fall due to increased activity of mosquito vectors (Armstrong & Andreadis, 2002).
The primary reservoir hosts for EEEV are passerine songbirds and perching birds, including sparrows, starlings, thrushes, magpies, crows, swallows, and finches. Wood thrushes, common blackbirds, and American robins have been implicated as significant EEEV spreaders. While EEEV infection does not cause high fatality in passerines, illness and death may occasionally occur in some wading birds (cranes, egrets), some birds of prey, and some game birds (turkeys, pheasants, ducks) (Greenlee, 2014; Williams, et al., 2023; Nemeth & Kunkel, 2024). Mammals (e.g., horses, swine, humans), and non-passeriform birds are “dead-end” hosts, meaning they do not transmit EEEV to feeding mosquitoes (Figure 5) (Morens, et al., 2019).
EEE Symptoms
In humans, EEEV incubates for 3–10 days after which patients may demonstrate non-specific fever, malaise, intense headache, muscle aches, nausea, and vomiting, which are indistinguishable from influenza-like symptoms (Corrin, et al., 2021; Morens, et al., 2019; Williams, et al., 2023). Mild infections can last up to 14 days followed by full recovery (Greenlee, 2014). Patients with a more serious neurological disease will exhibit altered mental status, seizures, meningitis, encephalitis, and coma within 5 days of exhibiting non-specific symptoms (Greenlee, 2014; Morens, et al., 2019). Ninety to 96% of people infected with EEEV are asymptomatic. Of those who have symptoms, 35%–50% exhibit permanent neurological damage, and 33% suffer fatality (Morens, et al., 2019; Williams, et al., 2023). In severe cases, death may occur from 2 to 10 days following the onset of symptoms (Greenlee, 2014). No vaccines or efficacious antiviral therapeutics are available for the treatment of human EEE (Greenlee, 2014). People with confirmed EEEV infection should not donate blood for 120 days after diagnosis (CDC, 2004). People over the age of 50 and those under the age of 15 are at the greatest risk for developing severe disease when infected with EEEV (CDC, 2004).
EEE and Horses
Horses can be significantly impacted by EEEV, which is one of the causative agents of Eastern equine encephalomyelitis (EEE). Signs of EEE in horses include fever, lethargy, decreased appetite, and multiple neurologic presentations such as tremors, difficulty walking or standing, difficulty swallowing, behavioral changes, nerve dysfunction, and seizures. Unfortunately, several other neurologic diseases, such as rabies and equine protozoan myeloencephalitis, present with similar symptoms in horses which makes diagnosis a challenge (American Association of Equine Practitioners, 2023). The American Association of Equine Practitioners, the world’s largest equine veterinary professional organization recognizing the impact of EEEV, recommends vaccination for EEE as part of its core vaccine recommendations for both foals and adult horses (American Association of Equine Practitioners, 2020).
Protecting Yourself from EEE
The best way to avoid Eastern equine encephalitis virus (EEEV) infection is to follow mosquito bite prevention guidelines (CDC, 2004; EPA, 2025):
- Use EPA-registered repellent (e.g., on-skin active ingredients (ai) include DEET, picaridin, and IR3535; on clothes and gear ai 0.5% permethrin). Make sure to read, understand, and follow all label instructions for repellent application. The label is federal law.
- Wear light-colored clothing that covers exposed skin during peak mosquito activity at dusk and dawn.
Eliminate larval mosquito breeding habitats, such as areas of standing water or places where water can collect (e.g., containers, bird baths, gutters, pool covers, etc.), and adult harborages, such as tall grass or overgrown shrubs.- Use EPA-registered insecticides judiciously, such as larvicides (for larvae) in areas that cannot be emptied regularly or spray adulticides (for adults). Make sure to read, understand, and follow all label instructions for pesticide applications. The label is federal law.
- For more information on how to keep yourself, your family, and your animals safe from mosquitoes, check out The Ohio State University fact sheet ENT-88, "How to Control and Prevent Mosquito Bites in and Around Ohio Homes" (ohioline.osu.edu/factsheet/ent-88).
References
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Molaei, G., Oliver, J., Andreadis, T. G., Armstrong, P. M., Howard, J. J. (2006). Molecular identification of blood-meal sources in Culiseta melanura and Culiseta morsitans from an endemic focus of eastern equine encephalitis virus in New York. American Journal of Tropical Medicine and Hygiene, 75(6), 1140–1147.
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Disclaimer: This project was supported by award number NU50CK000637 from the Centers for Disease Control and Prevention. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention.