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Risk of West Nile Virus Infection Raised to High in Cambridge and Other Communities

September 10, 2021

On September 10, the Massachusetts Department of Public Health raised the risk of West Nile virus infection from “moderate” to "high" in Cambridge and 26 other Greater Boston communities. The rest of the state is currently at low or moderate risk.

So far this summer, one mosquito sample in Cambridge has tested positive for West Nile virus and no WNV infections have been reported among Cambridge residents. The Massachusetts Department of Public Health's decision to raise the risk level in the 27 communities was based on multiple factors including recent human cases in Middlesex County, an above average population of the mosquito species responsible for most human infections, and heavy rains and continued weather favorable for mosquito activity.

Four human cases of West Nile virus have been reported in Massachusetts so far in 2021. All four individuals were exposed or likely exposed to the virus in Middlesex County.

West Nile virus is usually transmitted to humans through the bite of an infected mosquito. Most human West Nile virus infections are mild, but a small number of people—fewer than 1% of those infected—become very sick. People over 50 are at greatest risk for serious illness, especially those with weakened immune systems.

Four human cases of West Nile virus have been reported so far this season in Massachusetts. All four individuals were exposed or likely exposed to the virus in Middlesex County. 

The best way to avoid West Nile virus is to prevent mosquito bites. Some tips:

  • Apply insect repellent when outdoors. Use a repellent with DEET, permethrin, picaridin, oil of lemon eucalyptus or IR3535. DEET products should not be used on infants under two months of age and should be used in concentrations of 30% or less on older children. Oil of lemon eucalyptus should not be used on children under three years of age. Follow the directions on the package.
  • 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 the evening or early morning.
  • Clothing can help reduce mosquito bites. Wearing long-sleeves, long pants, and socks when outdoors will help keep mosquitoes away from your skin.
  • Get rid of mosquito breeding sites in your yard. Mosquitoes lay their eggs in still water. Empty standing water from flower pots, buckets, barrels, and children’s pools. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out.
  • Make sure that window and door screens fit tightly and are in good condition.

State and county information about West Nile virus and reports of West Nile virus activity in Massachusetts can be found on the state health department website.

For local West Nile virus updates and prevention tips, call the Cambridge Public Health Department at 617-665-3838 or visit the department's West Nile virus web page.

Suzy Feinberg, MPH
Public Information Officer

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