Summer Mosquitoes Are Spreading Disease Into the Fall

Mosquito image via Shutterstock

The Massachusetts Department of Public Health (DPH) confirmed the seventh case of Eastern Equine Encephalitis (EEE) and announced two new cases of West Nile virus this week. A second death from EEE was confirmed on Monday, and the towns of Marshfield and Scituate have been urged by the DPH to cancel all planned evening outdoor activities until the first hard frost.

“Summer may be over, but the threat of mosquito-borne illness is not. We can expect to continue seeing mosquito activity until the first hard overnight frost,” says DPH State Epidemiologist Dr. Al DeMaria. “People need to continue to use insect repellant, cover up exposed skin, and avoid being outdoors at dusk and after nightfall when mosquitoes are at their most active.”

So what can we do to protect ourselves? We spoke to Dr. Asim Ahmed from the Division of Infectious Diseases at Boston Children’s Hospital to get more answers.

How can we tell the difference between a regular cold or flu bug and something more serious like EEE or West Nile?

Some of the early symptoms of both WNV and EEE can overlap with the flu, like fever, malaise, muscle aches, and sore throat. But patients with EEE will often develop other very severe symptoms in 2 to 3 days like seizures, altered mental status, confusion, coma, and even death. About 80 percent of people who get WNV will have what’s called a sub-clinical infection with very mild or even no symptoms. But 20 percent of people with WNV will have mild self-limited symptoms that can mimic the flu. Most of these patients don’t see a doctor, never have a test sent, and will recover fully from the infection. A small subset of people with WNV, about 1 in 150 to 1 in 250, will have severe disease which may result in confusion, altered mental status, polio-like flaccid paralysis [isolated weakness of a limb], seizure, and coma. One out of 10 patients with the severe form of the disease will succumb to their illness.

What is the difference between EEE and West Nile?

They are both transmitted by mosquitoes, occur in the summer and early fall, and cause overlapping types of diseases, such as encephalitis or brain infection, so they often get lumped together. The major difference between EEE and WNV is that EEE is much more severe—30 to 50 percent mortality rate. Another difference is that historically within Massachusetts, EEE has mostly been found in the corridor southeast of Boston in close proximity to the wetland swamps that straddle Plymouth and Bristol counties, whereas WNV has been found throughout the state. This year, however, has been an unusual year for EEE, which has been found in several uncharacteristic parts of the state: Franklin, Worcester, Essex, and Middlesex counties.

Why is this happening so late in the season?

These viruses are normally amplified in bird populations. As long as ecological factors favor mosquito abundance there is a risk of transmission of these viruses from birds to humans and horses and this risk exists until the first hard frost of the season, which breaks the cycle by eliminating mosquitoes from the circuit.

Who is most at risk?

WNV tends to be more virulent in older individuals. This is likely related to generally weaker immune systems in older people. EEE effects both adults and children.

Is there a treatment for EEE and West Nile?

The only treatment is supportive. There are no specific anti-viral medications that are effective against WNV or EEE.

That’s scary. So what can we do?

Prevention is key. There are important things you can do protect yourself, your children and your parents from these mosquito-borne viruses. Be aware of the public health advisories in your particular town or area. Avoid being outside during times when mosquitoes are most active: dusk and dawn. Wear long clothing to protect yourself from bites. Drain standing water in and around your property because these are breeding grounds for mosquitoes. And use insect repellants like DEET.