This outbreak of EEE developed over a large geographic area in Maine. First identified in Maine in 2001, the virus was previously known only from southwestern regions of the state (Cumberland and York Counties). Despite the abundance of mosquito vectors32,33
and the primary habitat associated with EEEv, forested freshwater wetlands such as red maple or cedar swamps, EEEv activity has historically been rare, and when present, very focal.34
Although the neighboring state of New Hampshire has seen increased EEEv activity in recent years, including several human cases (two of which resulted in death), they have been largely restricted to southern coastal regions.35
Previously, EEEv activity has also been reported from southeastern Canada, in the provinces of Quebec.36
With Maine sandwiched between a regional foci in coastal Massachusetts/New Hampshire and one in Quebec, Canada, it is, therefore, not surprising to find viral activity occurring.
Livestock fatalities occurred over a large area of Maine. Although sampling for vector mosquitoes occurred only after reports of illness were confirmed, habitat at most of the sites of illness were areas near forested wetland or other habitats capable of supporting production of bridge mosquito populations and both epizootic and enzootic transmission. In addition, all of the equine fatalities were either unvaccinated or only partially vaccinated against EEEv and West Nile virus infection (). In Maine, vaccination of livestock against these two viruses may be performed by livestock owners and is not dependent on veterinarians. Although not a mandated and reportable status, vaccination of these animals is highly advised to prevent such infection, especially in consideration of the fact that an effective and moderately priced combination EEEv/West Nile virus vaccine for horses exists. Even though camelids may be given the vaccine without apparent ill effects,37
to date, there is no evidence of its efficacy in protecting these animals from infection. In keeping with the traditional season for mosquito-borne disease in northern New England, date of onset for livestock cases began August 3 and carried through to October 6 (, ), implying that the enzootic cycle amplified in late spring and early summer in avian hosts before spilling over into the mammalian population. Although Maine discontinued dead bird testing for arbovirus surveillance in 2006, data up to that point suggested that bird mortality began in early summer. Despite this, mortality in captive pheasants was not noticed until September. The high number of Cs. melanura
found in September 2009 might be the reason for this, as ring-necked pheasants are reported to be highly susceptible to EEEv23
and should be exposed to the same ornithophilic mosquitoes as wild birds. Data from two sites in nearby towns showed that Cs. melanura
was abundant late into the season (). Pheasants may be vaccinated using an equine EEEv vaccine, to protect valuable bloodlines, however the efficacy of the vaccine has been questioned.24
Status of the birds reported here is unknown. The sole EEEv-positive wild turkey was captured in Penobscot county and screened as part of a successful restoration plan for this game species in Maine. The bird was translocated and results were not known until after the cessation of the project but it speaks to the importance of screening for potential pathogens during such a survey. Although the reservoir potential of wild turkeys for EEEv is not known, positive birds have been found among captive flocks.38
Further research into this species' potential role in the cycle of EEEv is warranted.
Eastern equine encephalitis virus (EEEv) enzootic and bridge vector mosquito species captured and tested during seasonal and rapid response surveys, July–September 2009
Similarly, despite extensive trapping in areas of previous viral activity and recent cases, positive Cs. melanura were not detected until September. Sites for mosquitoes were chosen, in large part, on the abundance of overwintering Cs. melanura larvae crypts. Sites with more productive larval populations of Culiseta were chosen for adult surveys. Since exploratory trapping at sites began during the last week of June, and continued through September, there was an expectation that positive mosquitoes, if present, would have been detected earlier in the season.
Although the collection of EEEv-positive Cs. inornata
in 2008 can be considered historical data for this report, this is the first record of this species occurring in Maine and testing positive for the virus. Even though several other species in the genera Culiseta
have been reported in the state,32
is collected locally, in a discreet location, in York County, but is not widely found. Collection sites where it is found are typified by fresh water, forested wetlands containing alders (Alnus rugosa
), winterberry holly (Ilex verticillata
), and red maple.
There are several possible reasons for the upsurge in EEEv activity in 2009, one being proximity of vector and bridge species to outbreak sites, and the other being the record-high rainfall. Mosquito surveys revealed that at certain sites, the abundance of vector species were quite high in 2009 (). Culiseta melanura
, Ae. Canadensis
, and Cq. perturbans
, common mosquito species associated with freshwater wetlands, are ubiquitous during light trap surveys (). Aedes canadensis
inhabits a variety of wetland habitats, including forested wetlands, whereas Cq. perturbans
is commonly associated with cattail (Typha latifolia
). Both are efficient bridge vectors for EEEv.39
Recent work by Molaei and others40
examining blood meal analysis of mosquitoes, showed that herbivores form an important component of both species' diets. Although there is a clear link between EEEv cycling and abundance of Cs. melanura,
the known bridge vector Ae. canadensis
was found consistently at all survey sites, including those sampled during rapid responses (). The lack of positives from any species except the pools of Cs. melanura
collected in September indicate that more extensive and rigorous surveys need to be conducted, employing techniques that will target gravid female enzootic and bridge vectors (i.e., resting boxes). What role, if any, Ae. canadensis
played during the outbreak cannot be determined with the small number of specimens collected during the surveys.
In addition to the high precipitation in the fall of 2008 (), record-high precipitation in summer 2009 and particularly in July may have created conditions favorable for the EEEv epizootic; rainfall in excess of 20 cm above a 25-year average has been associated with EEE outbreaks in neighboring Massachusetts.16
Summer 2009 rainfall was a 29.1 cm departure from the 30-year average in Portland, Maine.31
By June and July of 2009, 43.1 cm of rain had accumulated when late-season mosquitoes typically emerge, and the correlation analysis suggested a link between July precipitation and the notably high abundance of Cs. melanura
. Although speculative, a link between the occurrence of EEEv and precipitation was previously shown.16,41
Although this outbreak of EEEv activity occurred over a large area of Maine, it is uncertain whether such activity will persist; however, increased surveillance for the virus and potential vectors is warranted, especially in emergent areas of the state such as Waldo, Kennebec, and Penobscot counties. Two communities that showed viral activity in 2009 have a recent history of the disease (York and Lebanon), although it is unknown, at this time, whether areas within these two towns can constitute true established foci. Certainly, if constantly present, the virus is active at low levels. Future surveillance activities, however, will concentrate on these areas and will provide data for the development of a more comprehensive surveillance program statewide.