Presence of rare eastern equine encephalitis virus has Northeast towns on high alert

Officials in Massachusetts and surrounding states are on high alert after recent tests detected a reemergence of the rare but deadly eastern equine encephalitis (EEE) virus in the region.

The town of Plymouth, Massachusetts closed its parks and fields from dusk until dawn on Aug. 23 after the mosquito-borne virus was detected in a horse in the area. A week earlier, an 80-year-old man from Oxford, Massachusetts contracted the disease – the first human case in the state since 2020.

While rare, EEE is very serious. Approximately 30% of people who contract EEE die, and many survivors have ongoing neurologic problems, according to the U.S. Centers for Disease Control and Prevention. Symptoms can include fever, headache, vomiting, diarrhea, seizures, behavioral changes, and drowsiness.

So far this year, five cases have been reported nationwide. Most cases have been reported in the Northeast with one case each in Massachusetts, New Jersey, New Hampshire, and Vermont. A single case has been reported in Wisconsin. On Tuesday (Aug. 27) a New Hampshire resident died of a brain infection after being infected with eastern equine encephalitis virus. The person had been hospitalized with severe central nervous symptoms prior to succumbing to the disease.

Seven cases were reported nationally last year, according to the CDC. The highest yearly total to date is 38, which was reported in 2019. Despite the small overall total that year, 12 people died, including three people in Connecticut.

Recent random tests have identified the presence of EEE in mosquitos in Connecticut and Rhode Island. Horses in New York and New Jersey have tested positive for EEE as have a deer in Connecticut and a turkey in Maine, heightening concerns. Massachusetts and New York have begun spraying pesticides to prevent EEE in high-risk areas while other states continue to monitor the situation.

Yale School of Public Health Associate Professor Nathan Grubaugh, an infectious disease epidemiologist, said there were early signs that 2024 could potentially be a bad year for EEE. Researchers found high rates of EEE when they randomly tested mosquitos that had been caught in traps in July.

“There is a direct correlation between finding infected mosquitoes and human cases during this time of year,” Grubaugh said. “In fact, finding infected mosquitoes in July - early season for EEEV - is a really good predictor of human cases in late summer / early fall.”

According to the CDC, EEE is caused by a virus that is spread to humans by the bite of an infected mosquito. Anyone who spends time in a place where the EEE virus has been detected is at risk, health officials say.

Grubaugh said that all of the infected mosquitos found in Connecticut were along the state’s eastern border with Rhode Island.

The fact that EEE has reemerged this year may be due in part to timing and weather conditions. Mosquitos can thrive in summer if there is a warm and wet spring, as many states in the Northeast experienced this year. It has also been five years since the last EEE outbreak (2019), which matches the outbreak patterns of EEE, Grubaugh said.

Eastern equine encephalitis has a 5–8-year cycle based on its unique ecology, Grubaugh explained. The virus is maintained in the Southeast, particularly the Florida panhandle, and needs to be constantly re-introduced into the Northeast by migratory birds.

“A few years after introduction, when the mosquito populations can support high transmission, we see an outbreak,” Grubaugh said. “Then, after the outbreak, there is a buildup of immunity to EEEV in birds (its host), and the viruses go locally extinct. The relatively short lifespan of the immune birds keeps the virus at bay for a few years until enough susceptible new birds are born and/or migrate in, and then a new introduction can take hold and start the cycle again.”

Grubaugh said spraying is an effective way to limit transmission.

“There is a saying in our field that an "old mosquito is your worst enemy,” Grubaugh said. “That is because only relatively "old" mosquitoes will transmit viruses - they need time to take a first bloodmeal from an infected bird, then 4-10 days for the infection to reach the mosquitoes salivary glands and for it to lay eggs (so that it wants to take another bloodmeal), then potentially feed on a human while infected. So, the idea of spraying is that you kill old adult mosquitoes.”

But killing the older mosquitos through the application of pesticides comes with a price.

“This is the perfect time to spray because its these potentially older infected mosquitoes that you want to eliminate,” Grubaugh said. “Spraying is pretty safe to humans. The larger concern is off target killing, like bees and other pollinators. Plus, the more you spray, the higher the likelihood that the mosquitoes develop resistance. So, there is a fine line between spraying too often and not enough to be effective.”

There are no vaccines to prevent or medicines to treat the EEE, but people can prevent infection by protecting themselves from mosquitoes. The CDC offers the following recommendations:

• Use Environmental Protection Agency-approved insect repellents. Those that include DEET as an active ingredient are very effective.
• Wear loose-fitting, long-sleeved shirts and pants.
• Treat clothing and gear with Permethrin, an insecticide that kills or repels mosquitoes. Permethrin-treated clothing provides protection after multiple washings. Do not use it directly on skin.
• Take steps to control mosquitoes indoors as well as outdoors. Use screens on windows and doors, and repair holes in screens when needed. Use air conditioning if available. To keep mosquitoes from laying eggs in or near water, empty out items that hold water, such as tires, toys, pools, birdbaths, flowerpot saucers, and trash containers.