Eastern Equine Encephalitis (EEE) is Back—Should You Be Worried?

Yale experts urge people to take extra precautions against mosquito bites.

people hiking in a forest, concerned about EEE

Eastern equine encephalitis (EEE) is a rare but deadly mosquito-borne virus. Because a EEE-positive mosquito has recently been detected in Connecticut, experts say people should take extra precautions.

Credit: Getty Images

The COVID-19 pandemic isn’t the only worrisome health news we need to keep track of right now. Connecticut has already started to see mosquitos testing positive for eastern equine encephalitis (EEE), also called “Triple E.”

Last year, this dangerous mosquito-borne virus dominated the news due to a five-fold growth in severe cases nationally, with several cases reported here in Connecticut. As winter approached, we were told to be on the lookout for the virus in the summer of 2020. Then came COVID-19.

But EEE remains a serious health threat, especially in the late summer and early fall months—the height of mosquito season.

Last year’s EEE outbreak was the largest in half a century. Compared to a national average of about seven severe cases per year, there were 38 reported last year, with 19 deaths, according to the Centers for Disease Control and Prevention (CDC). Four severe cases occurred in Connecticut and all patients were treated at Yale New Haven Hospital. Three died and the surviving patient suffered severe neurological damage, says Yale infectious disease specialist Joseph Vinetz, MD.

“This is a very bad virus,” Dr. Vinetz says. “There’s no treatment or vaccine for EEE yet. We need to carry out extensive fundamental research on EEE to protect people.”

Compared with other mosquito-borne viruses, EEE is exceedingly rare. Last year’s outbreak translates to 1 case per 10 million people in the U.S.

However, EEE is also exceptionally dangerous. About one in three people who become severely ill with the virus die. Severe cases occur when people develop the virus’s most fatal symptom: brain inflammation or swelling, called encephalitis. About half of all survivors of encephalitis suffer permanent neurological damage.

Anyone can develop a serious case of EEE, but children younger than 15 and adults older than 50 are at greater risk, due to weaker immune systems, Dr. Vinetz says.

How EEE affects the body

After a EEE-infected mosquito bite, people usually begin to experience symptoms within 4 to 10 days. (Reassuringly, only about 5% of people infected with the virus develop a life-threatening illness.) Some people may experience flu-like symptoms like fever, chills, lack of energy, and body aches. After about one or two weeks, the symptoms resolve on their own without the need for medical attention. Still others with EEE are asymptomatic and won’t feel sick at all. 

Signs of a serious infection that can lead to encephalitis include vomiting, seizures, drowsiness, behavioral changes, and coma. 

“Patients who are hospitalized receive supportive care, like oxygen and IV fluids,” Dr. Vinetz says. “We can diagnose the encephalitis through a spinal tap. After that it’s ‘watch and wait.’” 

Detecting the EEE virus requires highly specialized lab equipment. Yale Medicine’s Clinical Virology Lab can run a preliminary blood test, Dr. Vinetz says. However, only the CDC can give an official diagnosis after additional testing in its labs. (Mild EEE infections that resolve on their own are not tracked by the CDC.) Sometimes weeks can pass between the onset of symptoms and a final diagnosis, Dr. Vinetz says.

A EEE outbreak could happen again

“Because EEE is so rare, if case counts double or triple from one season to the next, that’s considered an outbreak or epidemic of the virus,” says Yale School of Public Health virologist Philip Armstrong, ScD, who leads the state’s mosquito surveillance program at the Connecticut Agriculture Experiment Station.   

Armstrong and his team found the state’s first EEE-infected mosquito in Stonington on August 5 this year. 

“In Connecticut, the virus is usually detected later in the season starting in late August or early September—if at all,” Armstrong says. “Early emergence of the virus extends the transmission season and leads to higher levels of virus amplification in the mosquito population.”

During the 2019 season, the team caught EEE-infected mosquitoes in 28 towns throughout the state. Most were from the southeastern region near the Massachusetts border, which is consistent with previous years, he says.

“Once EEE is seeded in our area, it can linger for one to five years before going locally extinct again,” Armstrong says. 

Predicting what will happen the rest of the 2020 season is very difficult, he says.

That’s due mostly to the virus’s complex life cycle. Each year, migratory song birds return to the Northeast after wintering in warmer states where mosquitoes are active year-round. These birds can introduce a slightly different strain of EEE (than was seen in previous years) to native birds. If native birds don’t have immunity to the virus, it actively circulates in the bird population. So begins a vicious cycle of infected birds passing the virus on to mosquitoes and vice versa. EEE gets further amplified during seasons with ideal mosquito breeding conditions, like warmer temperatures and higher rainfall.  

Once local birds become immune to the virus, they cannot pass it on to mosquitoes. The transmission between mosquitoes and birds—which both serve as hosts in biology-speak—drops off. (A human, which cannot spread this particular virus, is called a dead-end host.)

How to protect yourself from EEE

Most severe EEE human cases happen between late August through September in rural areas, Armstrong says. 

Mosquitoes rely on a sophisticated sensory system that homes in on the carbon dioxide we exhale with every breath. So, while we cannot escape notice of mosquitoes, we can prevent their bites. 

The CDC and CT.gov recommend taking these precautions: 

  • Minimize time outdoors at dawn and dusk when mosquitoes are most active. 
  • Wear light-colored socks, long pants, and a long-sleeved shirt to cover up your skin. 
  • When outdoors, use an Environmental Protection Agency (EPA)-registered insect repellent with one of the following active ingredients:

                   - DEET 

                   - Picaridin

                   - IR3535

                   - Oil of lemon eucalyptus (OLE)

                   - Para-menthane-diol (PMD)

                   - 2-undecanone 

  • Remove any sources of standing water near your home. 
  • Make sure window screens are tight-fitting. 
  • Cover cribs, strollers, and baby carriers with mosquito netting.

“Everyone knows to do a tick check,” says Dr. Vinetz, “but not as many people think about preventing mosquito bites.” 

For more information on how to prevent mosquito bites, view the publicly available information from the CDC and CT.gov.