EEE and West Nile in Maine and New Hampshire: The Late Summer Risk Window

Maine’s mosquito surveillance program has a date written into it. August 15. That is when the state shifts into Phase III of its testing season and starts screening a far longer list of mosquito species than it did through July. The date can also arrive early: a first detection of Eastern equine encephalitis or West Nile anywhere in Maine or New Hampshire opens Phase III ahead of schedule. A positive batch of mosquitoes in Manchester changes what Maine’s state lab tests.

The risk window has a start date, and it is on paper

Most seasonal health advice runs on a feeling. This one runs on a protocol. Maine CDC’s 2025 Municipal Arboviral Plan, which tells towns what the state is doing and when, splits the year into phases. An early season, May 15 to July 1, tests only for Jamestown Canyon virus. Phase II, July 1 to August 15, screens five kinds of mosquito: Culiseta melanura, Culiseta morsitans, Culex pipiens, Culex restuans and the pipiens/restuans group. Phase III runs to October 1 and widens the list well past those enzootic species.

That widening is the point. The five species tested in midsummer are the ones that keep these viruses cycling quietly in birds. The mosquitoes that carry EEE across to people and horses are a different set, and they show up later.

The edges are blurrier than that suggests, and Maine’s own documents disagree about them. The weekly surveillance report says collection begins in June and runs through October. The municipal plan sets May 15 to October 1 across its phases, then hedges that the season “may extend through the end of fall.”

What both states are finding this year

Through August 29, Maine had tested 393 mosquito pools. Zero positive for West Nile. Zero for EEE. Three carried Jamestown Canyon virus, collected in Augusta and Orono in June. Forty-seven people were tested, none positive for any of the three. The statewide Skeeter Meter, Maine’s public risk gauge, reads Low.

Birds are where Maine’s West Nile has turned up. Of 37 tested, four came back positive: an American Crow in Cape Elizabeth on July 11, a Sharp-shinned Hawk in Glenburn on July 24, then on August 24 an American Crow in Brunswick and a Cooper’s Hawk in Waterboro. Cumberland, Penobscot and York counties, so not one corner of the state.

New Hampshire’s picture looks similar off a much larger sample. Through the week ending August 29, NH had tested 4,453 batches: three positive for West Nile, none for EEE, six for Jamestown Canyon. Twenty-four people tested, no positives. Across its 2025 season, June 1 to October 15, the state ran 5,381 batches and found West Nile in 11 (nine in Manchester) and EEE in one, also Manchester, with no human or animal case either.

Two cautions on those zeros. Confirmatory testing runs through the federal CDC lab in Fort Collins and results lag the testing, so current-season human counts are preliminary by design. And the volume gap between the states is real (4,453 batches against 393 pools, in near-equivalent units) but neither report explains it, and guessing would be exactly that. This is one piece of a wider seasonal picture, mapped in our guide to environmental health in Maine and New Hampshire.

Why the risk stacks up at the end of summer

The mechanism explains the calendar better than any date can. EEE spends most of the year in a loop between birds and one specialist mosquito, Culiseta melanura, which took 94.3 percent of its blood meals from birds in a study of the northern New England focus. People are not part of that loop. Human and horse cases need a bridge, a mosquito that bites both, and those feed the other way around: Anopheles quadrimaculatus at 97.0 percent mammalian, Anopheles punctipennis at 92.8. The same researchers noted that epidemic transmission typically happens toward the end of the transmission season.

The bird side of that loop builds all summer, and someone measured it here. Sampling young, locally hatched songbirds in southern Maine across 2013 and 2014, researchers found EEE antibodies in 12.9 percent of birds in midseason (June through August) and 45.6 percent late in the season (September and October). More virus in more birds, later.

None of this is imported for the summer. White-tailed deer sampled at tagging stations between 2012 and 2017 carried EEE antibodies in all 16 Maine counties, at 6.6 to 21.2 percent, and moose tested positive in Aroostook, Somerset, Piscataquis and Franklin. The virus is endemic here, and it has had loud years: a 2009 outbreak crossed five Maine counties between July and September, with virus isolated from 15 horses, a llama and captive pheasants.

What the two illnesses do, and how rarely

West Nile is mostly quiet. An estimated 80 percent of people infected never develop symptoms they would attribute to it, and up to 20 percent develop West Nile fever. The federal CDC puts the severe end at more than 1,300 people a year nationally with illness affecting the central nervous system, and more than 130 deaths, with symptoms usually starting two to six days after a bite.

EEE is rarer and much worse. CDC counts an average of 11 human cases a year across the entire country. Symptoms typically appear four to ten days after an infected mosquito bites, and about one in three people who develop severe EEE die, usually two to ten days after symptoms begin. There is no specific treatment and no vaccine. Severe illness means supportive hospital care and nothing that targets the virus.

Maine CDC has put a number on what a single human case costs: from about $21,000 for mild, transient illness to as much as $3 million for someone left with permanent neurologic damage.

New Hampshire’s recent history shows how uneven this gets. In 2024 the state found EEE in 15 of 4,454 mosquito batches, seven of 11 animals tested, and five of 69 people tested. Announcing one of those human infections that August, NH DHHS noted the state’s previous reported case was in 2014, when it identified three infections including two deaths. Then 2025 passed with a single positive batch.

The window closes with frost, and the states hedge on that too

Frost ends it. That is the honest summary, though both states are careful about how they say so. New Hampshire’s state epidemiologist, Dr. Benjamin Chan, put it plainly in that 2024 announcement: “The risk will continue into the fall until there is a hard frost that kills the mosquitos. Everybody should take steps to prevent mosquito bites when they are outdoors.”

Maine builds the same idea into its own operations with a wider margin. The state’s arboviral Emergency Operations Center stands down on one of three conditions: public demand drops to a manageable level, the emergency is declared over, or two weeks after the second hard frost. Not the first. The second, plus a fortnight.

Neither state pins a hard frost to a temperature, so there is no thermometer reading to watch for, and one cold night in late September is not an all clear. If you will be outdoors into October anyway, our look at the fall adult deer tick season covers what else is active then.

What the guidance actually asks you to change

Not much, most of the time, and that deserves saying out loud. Maine’s Skeeter Meter has four tiers (Low, Moderate, High, Severe) and the guidance only asks you to move an evening plan at the top two. At High it advises considering a move of outdoor evening activities to another time when mosquitoes are less active. At Severe it advises moving them, or providing EPA-approved repellent for everyone attending. Mosquitoes are most active at dawn and dusk, per Maine CDC, which is also when a September cookout is at its best.

On repellent the two states do not say the same thing, which is worth knowing if you read guidance from both. NH DHHS advises that repellents containing 30 percent or less DEET are the ones advised for children and adults, and that children should not apply DEET themselves. Maine CDC and the federal CDC set no such ceiling, pointing instead to EPA-approved products containing DEET, picaridin, IR3535 or oil of lemon eucalyptus. Two states, two answers, neither of them a recommendation from us. They are also the products already in most mudrooms here for ticks (our comparison of DEET, picaridin and permethrin covers how the active ingredients differ).

New Hampshire also names what has not been shown to prevent bites: vitamin B, ultrasonic devices, incense and bug zappers.

So: a start date the state writes down, a stretch when infected birds and bridge-feeding mosquitoes overlap, and an ending that arrives with cold weather instead of a calendar page. Quiet so far in both states, which is the ordinary case rather than a guarantee.

Sources

• Maine Center for Disease Control and Prevention. Maine Mosquito-Borne Illness Guidance for Towns and Communities (2025 Municipal Arboviral Plan). Maine DHHS. 2025. https://www.maine.gov/dhhs/mecdc/sites/maine.gov.dhhs.mecdc/files/2025-Municipal-Arboviral-Plan.pdf
• Maine Center for Disease Control and Prevention. Maine Weekly Arboviral Surveillance Report, September 1, 2026 (week 34). Maine DHHS. 2026. https://www.maine.gov/dhhs/mecdc/sites/maine.gov.dhhs.mecdc/files/epi-updates/2026-09/Arbo2026wk34.pdf
• Maine Center for Disease Control and Prevention. Mosquitoes and the Skeeter Meter. Maine DHHS. https://www.maine.gov/dhhs/mecdc/diseases-conditions/insect-borne-diseases/mosquitoes
• Maine Center for Disease Control and Prevention. Eastern Equine Encephalitis (EEE). Maine DHHS. https://www.maine.gov/dhhs/mecdc/diseases-conditions/insect-and-animal-borne-diseases/mosquitoes/eee
• New Hampshire Department of Health and Human Services. NH DHHS Identifies Person Infected with Eastern Equine Encephalitis Virus in New Hampshire. NH DHHS. August 27, 2024. https://www.dhhs.nh.gov/news-and-media/nh-dhhs-identifies-person-infected-eastern-equine-encephalitis-virus-new-hampshire
• New Hampshire Department of Health and Human Services. 2026 NH Arbovirus Testing Results, Arbovirus Surveillance Bulletin #13. NH DHHS. September 3, 2026. https://www.dhhs.nh.gov/sites/g/files/ehbemt476/files/documents2/arbotestresults2026.pdf
• New Hampshire Department of Health and Human Services. 2025 Arboviral Surveillance Summary. NH DHHS. April 9, 2026. https://www.dhhs.nh.gov/sites/g/files/ehbemt476/files/documents2/arbosummary2025.pdf
• New Hampshire Department of Health and Human Services. Mosquito-Borne Illnesses. NH DHHS. https://www.dhhs.nh.gov/programs-services/disease-prevention/infectious-disease-control/mosquito-borne-illnesses
• U.S. Centers for Disease Control and Prevention. About West Nile Virus. CDC. https://www.cdc.gov/west-nile-virus/about/index.html
• U.S. Centers for Disease Control and Prevention. Data and Maps for Eastern Equine Encephalitis. CDC. https://www.cdc.gov/eastern-equine-encephalitis/data-maps/index.html
• Elias SP, Lubelczyk CB, Rand PW, et al. Seasonal Patterns in Eastern Equine Encephalitis Virus Antibody in Songbirds in Southern Maine. Vector Borne and Zoonotic Diseases. 2017;17(5):325-330. https://pubmed.ncbi.nlm.nih.gov/28287934/
• Kenney JL, et al. Eastern Equine Encephalitis Virus Seroprevalence in Maine Cervids, 2012-2017. American Journal of Tropical Medicine and Hygiene. 2020;103(6):2438-2441. https://pubmed.ncbi.nlm.nih.gov/33146118/
• Molaei G, et al. Insights into the recent emergence and expansion of eastern equine encephalitis virus in a new focus in the Northern New England USA. Parasites and Vectors. 2015;8:516. https://pubmed.ncbi.nlm.nih.gov/26453283/
• Lubelczyk C, et al. An epizootic of eastern equine encephalitis virus, Maine, USA in 2009. American Journal of Tropical Medicine and Hygiene. 2013;88(1):95-102. https://pubmed.ncbi.nlm.nih.gov/23208877/

This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making any health decisions.