It’s a record-breaking year for West Nile virus in Massachusetts. So far, 42 cases have been confirmed, the highest number ever reported in the state in a single year. The previous record was 33 in 2012.
The state Department of Public Health on Friday announced four new cases in humans, all of whom became sick enough to require hospitalization: a woman in her 50s; two men in their 60s from Middlesex County; and a woman in her 60s from Plymouth County. A horse from Franklin County died from the disease.
West Nile is usually transmitted through the bite of an infected mosquito. This year, the hot and humid weather has promoted mosquitoes’ growth and their ability to spread the virus. In comparison, there were six human cases in 2017 and 16 in 2016.
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“Mosquito season is winding down, but mosquitoes will still be active on warmer and more humid days,” Public Health Commissioner Monica Bharel said in a statement. And, she cautioned, mosquitoes at this time of year are more likely to carry West Nile virus.
“Long sleeves and pants will help reduce bites, and mosquito repellent may still be necessary when it is warm and humid,” Bharel advised.
The risk of contracting West Nile virus is listed as “moderate” for most of the state, meaning that infection is likely or has already occurred. But for Boston and a few towns north and west, including Newton, Cambridge, Medford, and Lynn, the risk is high, meaning many people may get infected.
People older than 50 or who have weakened immune systems are advised to stay indoors from dusk to dawn if they live in high-risk areas.
Most people infected with West Nile have no symptoms, but about 20 percent suffer fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a rash on the torso. Rarely, West Nile can cause severe illness.
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To learn more, visit the Arbovirus Surveillance Information web page at mass.gov/dph/mosquito or call the DPH Epidemiology Program at 617-983-6800.
Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer