Maine health and agriculture officials on Friday reported that a horse from Somerset County tested positive for eastern equine encephalitis, the latest in a string of mosquito-borne illnesses discovered in animals across the state this summer.

The sharp rise in mosquito-borne diseases – in Maine and beyond – comes as scientists warn that the diseases will likely become more common because of climate change.

The horse is the third domestic animal to have tested positive for EEE. The other two were emus in Penobscot County.

Also, 15 wild birds have tested positive for West Nile virus, and Jamestown Canyon virus has been found in four mosquito pools. A mosquito pool is a group of up to 50 mosquitoes tested as part of routine surveillance.

In 2023, there were 17 cases of EEE and three cases of West Nile virus in animals, according to the Maine Center for Disease Control and Prevention.

With 18 cases of mosquito-borne disease in animals through Sept. 5, there’s a good chance Maine will surpass last year’s totals. Additional cases can occur in September and October, when mosquitoes are still biting and before frosts kill them.

Such reports were uncommon in Maine before last year.

From 2014 – the first year data is available in Maine – to 2022, there were three confirmed cases of the viruses infecting animals in Maine, including two animals with EEE and one with West Nile virus.

The same disease-carrying mosquitoes that transmit the viruses to animals also bite and infect people.

Confirmed human cases of the diseases are rare, but they are likely circulating in people more than reported because humans often have no symptoms or mild ones and are often not tested for the viruses. In some cases, however, the viruses can be deadly or cause severe conditions such as brain swelling.

A New Hampshire man died of EEE in August, according to state officials there.

Several towns in Massachusetts have urged people to avoid going outdoors at night this summer to protect against mosquitoes that might carry diseases.

The Maine Climate Council, in its 2024 report, linked the increase in mosquito-borne diseases to climate change.

“Maine has relied on hard frosts in October to end the mosquito season, but climate change in Maine has manifested as earlier degree-day accumulation, more extreme rain events in spring and summer, and extended frost-free falls,” according to the report. All of those factors contribute to an increasing risk of mosquito-borne diseases in Maine, it said.

“Lengthening of the fall season can allow for extra breeding opportunities for mosquitoes, which can increase mosquito populations,” said Griffin Dill, integrated pest management specialist at the University of Maine Cooperative Extension. Wetter and warmer springs also can lead to more mosquitoes.

“Anywhere in the state of Maine there is potential for these pathogens, so precautions should be taken throughout the entirety of the state,” Dill said.

He said some mosquitoes can even overwinter, especially in damp, protected places, such as cellars.

Dill said Maine is not yet warm enough for the mosquitoes that carry tropical diseases, such as the Zika virus, to survive.

Shannon LaDeau, a senior scientist at the Cary Institute of Ecosystem Studies, told Business Insider for an article this week that climate change is contributing to the proliferation of mosquito-borne diseases.

“Everything about mosquito development speeds up with temperature,” LaDeau said. “How fast they go from egg to adult shortens when it’s warmer. Viruses replicate in the mosquito faster when it’s warmer. So you get more mosquitoes faster, and then their chances of interacting with a human is higher.”

While no locally contracted mosquito-borne diseases have been detected in humans in 2024, humans can contract these diseases from the bite of an infected mosquito.

There have been 10 recorded cases of mosquito-borne diseases in humans over the past decade in Maine: four West Nile virus, four Jamestown Canyon virus and two EEE.

“These positive cases of EEE and West Nile Virus should prompt Maine people and visitors to take action to protect themselves whenever they go outside,” said Dr. Puthiery Va, Maine CDC director, in a statement. “The best way to keep yourself, your loved ones, and domestic animals safe from these serious viruses is to avoid mosquito bites as much as possible.”

Dr. Stefanie Bolas, state veterinarian for the Department of Agriculture, Conservation and Forestry, said that the positive EEE case “serves as a reminder of the importance of vaccinating horses against this deadly virus. While horses cannot transmit EEE to humans or other animals, the high mortality rate makes prevention critical. We urge horse owners to take immediate precautions, including ensuring horses are up-to-date on vaccines, using approved insect repellants and minimizing exposure during peak mosquito activity hours.”

The Maine CDC advises people to avoid mosquitoes by limiting outside activity at dusk and dawn, when mosquitoes are most active, wear long sleeves and pants, use insect repellant and drain sources of standing water on property.

“These illnesses usually start with flu-like symptoms in humans. Severe symptoms can include brain swelling (encephalitis) and spinal cord inflammation (meningitis). If you experience flu-like symptoms, severe headache, behavior changes, or serious drowsiness, call a health care provider, especially if a COVID-19 test is negative,” according to the Maine CDC.