After 4 deaths last year, mosquito-borne illnesses worry state health officials this summer

Anxious about a repeat of last year, when four people died from mosquito-borne illnesses in Massachusetts, state health officials offered warnings and advice Monday on getting through a buggy summer.

Friday marked the official start to “mosquito season,” which runs from when the first infected bugs are found until the first hard frost, when mosquitoes begin dying off.

The discovery of mosquitoes infected with West Nile virus in Whitman was on the “earlier side” of when the first infections are usually noted in Massachusetts, said Catherine Brown, the state public health veterinarian. But she said it’s difficult to predict the upcoming season’s severity.

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West Nile virus and a second worrisome illness, Eastern equine encephalitis, both depend on “weather, populations of mosquitoes, and the presence of infected birds,” Brown said. It’s a complicated ecological system that changes yearly.

What does give officials pause this year is last summer’s wave of EEE, the more dangerous of the two viruses, which resulted in seven cases and three reported deaths.

“If there was intense EEE the year before, we get very nervous about what the current year is going to show,” Brown said.

EEE follows a “burn-out” pattern, meaning a viral strain gets introduced, spreads, and then eventually burns itself out. So last year’s strain could return this year, or it could have burned itself out last fall.

The commonwealth historically has the second-highest incidence of EEE, after Florida. Surrounding states have rarely seen the disease, but last summer Vermont reported its first cases of the virus, Brown said.

Initial symptoms are flu-like but the illness usually progresses to more serious symptoms including coma or seizures. The disease has a 33 percent mortality rate, according to the Centers for Disease Control and Prevention. Neither EEE nor West Nile is transmitted person to person.

The state Department of Public Health will continue trapping and testing mosquitoes over the next few months to track the threat.

Several signs suggest the summer isn’t on track to repeat 2012. Mosquitoes thrive in heat and humidity, and last year was Boston’s hottest ever, according to the National Weather Service.

Last summer, 33 cases of West Nile virus were reported in Massachusetts—the highest in the state’s history—resulting in one known death.

About 20 percent of individuals bitten by a West Nile-infected mosquito experience symptoms, according to the CDC: usually a fever and flu-like symptoms, including vomiting or diarrhea. A small percentage suffer serious or fatal complications involving the central nervous system.

There is no vaccine or cure for either virus, only treatment for symptoms. Brown said Massachusetts residents should always wear insect repellent and cover up skin while outside at dusk or dawn—peak mosquito time. And people should see a doctor if flu-like symptoms appear between 3 and 14 days after a bite.