Boston West Nile Virus Risk Level Has Been Raised

At the end of last week the Boston Public Health Commission (BPHC) announced that Boston’s West Nile Virus risk level has been raised from moderate to high. This designation by the Massachusetts Department of Public Health is based on the high number of positive mosquito samples in the City and its surrounding towns, the rainfall and favorable weather for mosquito breeding, as well as the number of human cases in surrounding towns.

“West Nile Virus in humans is rare in Boston. However, we’ve seen a lot of rain this summer, leading to a higher number of mosquitoes in our City,” said BPHC’s Director of Infectious Disease, Dr. Sarimer Sanchez “When you’re outside, you and your family can take precautions to protect yourself from mosquito bites and the dangerous diseases mosquitoes may carry. That includes using an approved mosquito repellent and wearing clothing that covers your body. Keep window screens in good condition to keep mosquitos out of your home and drain standing water from your yard to prevent mosquitos.”

A rainy summer has made mosquitoes a nuisance all summer in Boston. Residents have complained that mosquitoes were worse than ever this season. Many residents complained that all hours of the day–morning, noon and night–mosquitoes were abundant and inflicting itchy bites on those trying to do yard work or enjoy a cookout with family members.

Back in August the BPHC partnered with the Suffolk County Mosquito Control Project (SCMCP) to protect Boston residents from mosquito-borne disease transmission.

The SCMCP performed an aerosol spray of Boston neighborhoods around and applied a formula that contains the pesticide, sumithrin, to help control mosquitoes.

However, the efforts and results by the BPHC and SCMCP were short lived as mosquitos continue to be a problem in the area.

Now that Boston has raised the West Nile Virus risk level to high, the BPHC is recommending that people over 50 years of age and those with compromised immune systems consider avoiding outdoor activities during the peak mosquito times between dusk and dawn.

West Nile Virus is a member of the family Flaviviridae, from the genus Flavivirus, which also contains the Zika virus, dengue virus, and yellow fever virus. The virus is primarily transmitted by mosquitoes. The primary hosts of WNV are birds, so that the virus remains within a “bird–mosquito–bird” transmission cycle. The virus is genetically related to the Japanese encephalitis family of viruses.

About 80% of those infected with West Nile Virus (WNV) show no symptoms and go unreported but about 20% of infected people develop West Nile Fever (WNF). The symptoms of WNF vary in severity, and begin 3 to 14 days after being bitten. Most people with mild symptoms of WNV recover completely, though fatigue and weakness may last for weeks or months. Symptoms may range from mild, such as fever, to severe, such as paralysis and meningitis. A severe infection can last weeks and can, rarely, cause permanent brain damage. Death may ensue if the central nervous system is affected. Medical conditions such as cancer and diabetes, and age over 50 years, increase the risk of developing severe symptoms.

WNV is usually detected in Boston mosquitoes during the summer and fall months from June to November every year since 2000.

So far this year there have been no human cases in Boston nor were there any human cases of WNV infection diagnosed in Boston residents in 2020 and 2019.  In 2018, there were seven human cases of WNV infection diagnosed in Boston residents.

The BPHC said the best way to protect against WNV is to protect against mosquito bites. They suggest using repellents containing DEET, oil of lemon eucalyptus, picaridin or IR3535.

Another tip is to cover up when outside when mosquitoes are most active. This includes wearing protective clothing such as long-sleeved shirts, long pants and socks whenever possible.

For more information about the sprayings, contact SCMCP at 781-899-5730. For a full list of any upcoming spraying, and for West Nile Virus and Mosquito-Borne Illness Fact Sheets in Chinese, English, Haitian Creole, Portuguese, Spanish and Vietnamese, go to

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