Eastern Equine Encephalitis

From the state website http://www.mass.gov/agr/mosquito/geir_docs/GEIR_MOSQUITO_SPECIES.pdf

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1. Eastern Equine Encephalitis
MCP’s in Southeastern Massachusetts, i.e., Norfolk, Bristol and Plymouth Counties, face the greatest threat
from this disease. During major epidemic years, virus activity extends northward from this enzootic focus into
southern New Hampshire and westward into Rhode Island, Connecticut and Central Massachusetts. All projects
except Berkshire County give considerable continuing attention to this potential problem. Upon occasion, projects
may submit mosquitoes to the SLI for EEE virus analysis.
The enzootic foci of EEE are red maple/white cedar swamps. The largest adult populations of the enzootic
vector, Cs. melanura, occurs in or near the localized swamps where this species develops. Most human and horse
cases also occur in the immediate vicinity of these same swamp habitats. Still, at times this mosquito may disperse
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several miles from its larval habitat (Morris et al. 1980. Nasci 1980. Nasci & Edman 1984) and human/horse cases
occasionally occur in upland areas. This mosquito is unusual in that it overwinters in the larval stage (4th or 3rd
instar). Adults from this generation emerge in late spring (i.e., mid to late May). Two to three summer generations
occur about one month apart, e.g., in late June, July and August, depending on water levels and temperature (Nasci
1980). EEE virus is generally not isolated from this mosquito until late summer. During epidemic years it tends to
be isolated earlier, i.e., beginning in early July, but apparently never from the overwintering generation. The
location of the virus from November to July remains a mystery. Culiseta melanura feeds only after dark and the
vast majority of blood meals are obtained from passerine birds (Nasci & Edman 1981a). This sylvan mosquito
feeds equally at ground level and at higher elevations in the tree canopy. Activity is concentrated just after dark and
just before sunrise (Nasci & Edman 1981b). The morning flight activity peak does not seem to involve
blood-feeding but rather the return to suitable daytime resting sites.
The isolation of EEE virus from the cattail mosquito Cq. perturbans during disease outbreaks (Crans,
personal communication) has focused suspicion on this species at the most likely epidemic vector to horses and
humans. Ae. vexans and Ae. canadensis are two other prime suspects for EEE virus transmission to humans and
horses in Massachusetts. Like Cq. perturbans, they are major pests. Their biologies will be described along with
the other pest species.
A new EEE threat may be developing in New England as Ae. sollicitans, long a known vector in New
Jersey (Crans et al. 1991), was, for the first time, found to be EEE-positive in Connecticut in 1996 (Andreadis
1996). Crans (1991) gave a suggested cycle for EEE transmission to Aedes sollicitans in which Cs. melanura
infected night-roosting glossy ibis, which were then fed upon by Ae. sollicitans while feeding in the salt marsh.
Though the link between glossy ibis and Ae. sollicitans is tentative, there can be no question that Ae. sollicitans is a
potent vector in New Jersey and could be an important vector in Massachusetts as well.