With the arrival of Labor Day and the unofficial end of summer, officials are warning the public about the risks presented by the mosquito, a summertime nuisance but also a vector for West Nile virus and other diseases.

On Monday, the Massachusetts Department of Public Health announced the fourth human case of West Nile virus in the state this year, a woman in her 50s from Middlesex County who was not hospitalized for her illness. Three other cases were reported on Aug. 24.

Earlier in August, a horse had to be euthanized in Worcester and an alpaca in Milford because of the severity of the reaction to contracting West Nile, the state said.


Public health officials raised the risk level for West Nile from moderate to high in 11 cities and towns: Arlington, Boston, Belmont, Brookline, Cambridge, Chelsea, Everett, Medford, Newton, Somerville, and Watertown. The risk level for the rest of the state was considered moderate.

“Several individuals from the same area have developed West Nile virus,” said Dr. Monica Bharel, state public health commissioner, in a statement. “That means that there is an increased risk in this specific area and that additional people could become infected.”

As of Wednesday, 363 mosquito samples from a dozen counties had tested positive for West Nile. It has so far been quiet in Essex County, with just two positive samples reported through Aug. 24: July 17 in Lynnfield and Aug. 13 in Newbury.

“It is extremely important for people to take steps to avoid mosquito bites, including using repellents, wearing clothing to reduce exposed skin, dumping standing water, and moving indoors when you notice mosquitoes biting you,’’ State Epidemiologist Dr. Catherine Brown said in a statement Monday.

There were no reported human cases of Eastern equine encephalitis, a more deadly mosquito-borne virus, and no findings of EEE in the mosquitoes sampled from around the state.


Traditionally, West Nile virus has thrived in urban environments, where the culex species of mosquito (the primary carrier of the virus) breeds in the dirty, shallow water of catch basins, old tires, empty plastic containers, and other man-made items that hold dirty, shallow water.

“This year we’re finding [WNV samples] everywhere we look,” Brown said. “It’s a different pattern than we’re used to seeing.”

As always, experts believe the numbers are driven by weather, in this case a hot summer featuring significant rain events.

“Heat really draws the virus,” said Brian Farless, superintendent of both the Suffolk County and East Middlesex Mosquito Control projects, which treat about 100,000 catch basins to try to control the population in 28 communities, including Boston.

Brown was hopeful that there would continue to be no EEE findings, a continuation of an historical trend of on-again, off-again EEE. There was seven human EEE cases in 2012, and no human cases since 2013.

In 2017, there were six human cases of West Nile virus identified in Massachusetts.

West Nile virus can infect people of all ages, but people over 50 are at higher risk for severe disease, according to public health officials. Symptoms tend to include fever and flu-like illness, but in rare cases more severe illness can occur. Most people who are infected have no symptoms.

To prevent infection, officials recommend using mosquito repellent with an EPA-registered ingredient; being aware of peak mosquito activity from dusk until dawn; and wearing long sleeves, pants and socks.


They can also drain gutters, flower pots, etc.; change the water in bird baths; and install/repair window screens. More can be found at mass.gov/dph/mosquito or by calling 617-983-6800.

“In my mind, it’s like when it’s 10 degrees in the winter, you put on your coat and hat to prevent frostbite,” said John Morris, public health director in Amesbury and Salisbury and chairman of the Northeast Massachusetts Mosquito Control and Wetlands Management District. “In the summer, you’ve got to protect yourself in a different way.”

David Rattigan can be reached at drattigan.globe@gmail.com.