Press Release  State health officials announce season’s first human case of West Nile virus and first animal case of EEE in Massachusetts

Boston — The Massachusetts Department of Public Health (DPH) today announced the first human case of West Nile virus (WNV) and the first animal case of Eastern equine encephalitis (EEE) in the state this year.

The individual is a male in his 40s who was exposed to WNV in Hampden County. The EEE infection was diagnosed in a horse exposed in Plymouth. Mosquito samples collected in Dedham also tested positive for EEE. These findings raise the EEE risk level to high in Plymouth and to low in Dedham.

This is in addition to the two communities currently at high risk for EEE: Carver and Middleborough. There are 12 communities already at moderate risk for EEE: Amesbury, Groveland, Halifax, Haverhill, Kingston, Merrimac, Newburyport, Plymouth, Plympton, Salisbury, Wareham, and West Newbury.

The risk of human infection with WNV is moderate in the Greater Boston area (Middlesex, Norfolk, and Suffolk counties), and in parts of Bristol, Essex, Hampden, Plymouth, and Worcester counties.

The first WNV-positive mosquitoes in Massachusetts this year were announced on July 2 from Quincy. On July 3, the season’s first EEE-positive mosquitoes in Massachusetts were announced from Carver. There have also been EEE-infected mosquitoes previously identified in Halifax and Kingston in Plymouth County, Barnstable in Barnstable County, and Amesbury and Haverhill in Essex County.

“Historically, August and September are the months when most people are exposed to West Nile virus in Massachusetts. This is also the first year with elevated EEE activity since our last outbreak in 2019 and 2020,” said Public Health Commissioner Robbie Goldstein, MD, PhD. “Populations of mosquitoes that can carry and spread these viruses are large this year and we continue to see increases in the number of EEE- and WNV-positive mosquito samples throughout the state. Residents should continue to take precautions to avoid mosquito bites.”

WNV is usually transmitted to humans through the bite of an infected mosquito. There were six human cases of WNV and no animal cases in 2023. No animal cases of WNV have been detected so far this year.

EEE is a rare but serious and potentially fatal disease that can affect people of all ages. EEE is generally spread to humans through the bite of an infected mosquito. There were 12 human cases of EEE in Massachusetts in 2019 with six deaths, and five human cases with one death in 2020. There were no human cases of EEE in Massachusetts in 2021, 2022, or 2023. No human cases of EEE have been detected so far this year.

“We continue to recommend taking steps to prevent mosquito bites until the first hard frost,” said State Epidemiologist Dr. Catherine M. Brown. “People should use mosquito repellent with an EPA-registered active ingredient, clothing to reduce exposed skin, and consider rescheduling outdoor activities that occur during the evening or early morning.”

People have an important role to play in protecting themselves and their loved ones from illnesses caused by mosquitoes.

Avoid Mosquito Bites

Apply Insect Repellent when Outdoors. Use a repellent with an EPA-registered ingredient, such as DEET (N,N-diethyl-m-toluamide), permethrin, picaridin (KBR 3023), or oil of lemon eucalyptus (p-menthane-3,8-diol (PMD) or IR3535) according to the instructions on the product label. DEET products should not be used on infants under two months of age and should be used in concentrations of 30 percent or less on older children. Oil of lemon eucalyptus should not be used on children under three years of age.

Be Aware of Peak Mosquito Hours. The hours from dusk to dawn are peak biting times for many mosquitoes. Consider rescheduling outdoor activities that occur during evening or early morning in areas of high risk.

Clothing Can Help Reduce Mosquito Bites. Wearing long sleeves, long pants and socks when outdoors will help keep mosquitoes away from your skin.

Mosquito-Proof Your Home

Drain Standing Water. Mosquitoes lay their eggs in standing water. Limit the number of places around your home for mosquitoes to breed by draining or discarding items that hold water. Check rain gutters and drains. Empty unused flowerpots and wading pools and change the water in birdbaths frequently.

Install or Repair Screens. Keep mosquitoes outside by having tightly fitting screens on all windows and doors.

Protect Your Animals

Animal owners should reduce potential mosquito breeding sites on their property by eliminating standing water from containers such as buckets, tires, and wading pools – especially after heavy rains. Water troughs provide excellent mosquito breeding habitats and should be flushed out at least once a week during the summer months to reduce mosquitoes near paddock areas. Horse owners should keep horses in indoor stalls at night to reduce their risk of exposure to mosquitoes. Owners should also speak with their veterinarian about mosquito repellents approved for use in animals and vaccinations to prevent WNV and EEE. If an animal is suspected of having WNV or EEE, owners are required to report to the Department of Agricultural Resources, Division of Animal Health by calling 617-626-1795, and to the Department of Public Health by calling 617-983-6800.

More information, including all WNV- and EEE-positive results, can be found on the Arbovirus Surveillance Information web page at Mosquito-borne Diseases | Mass.gov, which is updated daily, or by calling the DPH Division of Epidemiology at 617-983-6800. 

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