Mosquitoes with West Nile virus were found in Billerica and Fitchburg last week as the number of positive mosquito samples rises to 43 in Massachusetts so far in 2026. There are no confirmed human or animal cases, according to the state Department of Public Health.
West Nile virus is primarily transmitted to humans via the bite of a mosquito. The illness carries a range of symptoms including fever, headaches, joint pain, vomiting, diarrhea and rash in milder cases, according to the U.S. Centers for Disease Control and Prevention. In more severe cases where the virus impacts the central nervous system it can cause encephalitis, which is the inflammation of the brain, and meningitis, which is the inflammation of the membranes around the brain and spinal cord.
Symptoms in more severe West Nile cases include a high fever, headache, stiffness of the neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis, according to the CDC, which also says about 10% of patients with a central nervous system infection die.
“The mosquitoes that carry this virus are common throughout the state, and are found in urban as well as more rural areas. While WNV can infect people of all ages, people over the age of 50 are at higher risk for severe infection,” the town of Billerica said in a statement on its website and social media Tuesday evening.
The town offered advice for avoiding mosquito bites altogether, including the use of insect repellent when outdoors, wearing pants and long sleeved shirts and staying inside during peak mosquito hours at dusk and dawn. The town also advised residents to drain standing water outdoors, where mosquitoes readily breed, and to install and repair window screens.
“While Billerica Board of Health continues to work closely with the MDPH and other agencies, locally are working with the Central Mass Mosquito Control Project (CMMCP) to control mosquitoes in Billerica,” said the town’s statement. “In response, the CMMCP will continue to treat catch basins in Billerica that require treatment for larval control. The CMMCP will also conduct a mosquito control spray application next week in the affected area. Notice regarding the upcoming mosquito control spray application will be posted on the Town of Billerica website prior to the application.”
According to the DPH risk map for West Nile virus, the risk in Billerica still remains low, and just one positive mosquito sample was found on July 22, along with samples in Stoneham and Woburn. So far this year there have been no reported human or animal cases of West Nile virus or of eastern equine encephalitis, another commonly tracked mosquito-borne illness. There have also been no reported EEE positive mosquito samples in the state so far this year.
West Nile virus was discovered in a mosquito sample in Fitchburg on July 21, along with samples in Boston, Barnstable, Falmouth, Northfield and Whitman. Another positive mosquito sample was found in Worcester on July 24. The highest current risk for the virus is found in and around the cities of Boston and Worcester, where the DPH map shows pockets of cities and towns with a moderate risk.
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