In the midst of the coronavirus outbreak, the last thing we want to think about is another virus. But now that summer is upon us, so are mosquitoes. And with mosquitoes comes the risk of eastern equine encephalitis (EEE), a rare but deadly virus. Last fall in Massachusetts, there were 12 confirmed cases of EEE, including six deaths. This marked the largest number of EEE cases in the state since the 1950s. And experts say it’s likely the disease will make a return visit this season.
What is Eastern equine encephalitis (EEE)?
EEE is a virus spread by the bite of infected mosquitoes. Inflammation of the brain, or encephalitis, is a frequent and life-threatening complication of EEE that may also lead to permanent neurological damage or coma.
“People often associate dangerous infectious diseases with exotic, faraway places but you don’t have to go to West Africa or the Congo to find a dangerous virus with a 50 percent mortality rate,” says Dr. Ahmed. “This is one of the most dangerous viruses on the planet, and it’s right here in our own backyard.”
Why were there so many cases of EEE last year?
Every summer, Massachusetts surveys the state for mosquitoes that carry the virus. Historically, the areas of peak activity are found in Southeastern Massachusetts between Bristol and Plymouth counties near an area called the Hockomock swamp, which is the traditional breeding ground of the mosquito that amplifies the virus in nature. But when the risk is very high, other non-traditional areas become higher risk areas, which is why we were hearing about EEE in the middle of the state, the northeast corner and even Western Massachusetts last summer.
Families should remember that EEE is very rare, even during peak years.
Ahmed thinks that periodically there are introductions of novel strains of the virus in our local environment that the normal bird reservoir hosts in nature lack immunity to. Because of this lack of bird immunity to these new stains, the virus builds up in the bird population and the mosquitos that feed on them. In addition, “certain ecological conditions likely favor the abundance of the type of mosquito that spreads the virus in nature,” he says. “During those times, the virus can take root in less traditional places and those become little factories for amplification of the virus.”
Will EEE be bad again this year?
Dr. Ahmed says it’s always hard to predict future risk, but notes that EEE epidemics have tended to occur in two- to three-year clusters, and then wane for several years. 2012 was the last big year, with seven human cases. The last time Massachusetts had a cluster of several years of increased human cases was 2004, 2005 and 2006. These patterns should be studied, he says. “When we’re in the middle of an epidemic, we’re thinking about how to best keep people safe, but there does need to be that pivot to potential causative factors once we’ve handled the acute period.” He acknowledges that this kind of study would require a sizeable infrastructure and coordination across different states, disciplines and industries.
“I hope this high level of EEE activity inspires a national coordinated effort among physicians, scientists, and public health officials at the CDC, NIH, and state health departments to investigate the factors that lead to these periodic EEE epidemics — such as the ‘novel viral strain hypothesis.’ They have the infrastructure and resources in place to take a systematic, broad approach and figure out what’s going on,” says Dr. Ahmed.
What are the signs and symptoms of EEE?
It’s important to distinguish between early warning signs that may begin 4 to 10 days after a mosquito bite, and the neurological symptoms specific to EEE — and understand how they relate to each other.
|Early symptoms||Neurological symptoms|
|General malaise||Stiff neck|
|Nausea||Sensitivity to light|
|Altered or declining mental status|
In a 2013 research paper published in Emerging Infectious Diseases, Ahmed and co-authors examined 15 cases of children treated for EEE at Boston Children’s between 1970 and 2010. They found that patients with a longer “prodrome” or period of early symptoms experienced a slower progression of neurologic symptoms, likely because their immune systems were more quickly triggered to fight and contain the infection. “But if the time between early symptoms and neurological symptoms was short, the prognosis was very, very poor,” says Ahmed.
How can I protect my family?
In seasons where there are high levels of the virus in the environment, families should not panic, but be smart by taking protective measures. And remember that EEE is very rare. The four most important things you can do to protect yourself and your family:
- Use insect repellant
- Wear clothing that covers the arms and legs
- Avoid peak mosquito activity from dusk until dawn
- Drain any area around your house, however small, that collects water
What months are the highest risk for EEE?
Historically, the end of August through mid-September presents the highest risk. But that risk continues until the first frost, so it’s important to use protective measures through October and into November.
Learn more about Eastern equine encephalitis (EEE).
About our expert: Dr. Asim A. Ahmed is a physician and researcher in the Division of Infectious Disease at Boston Children’s. He has a long-standing clinical and scientific expertise in EEE and other arboviruses. His most recent research efforts are focused on the use of next-generation sequencing (or “deep-sequencing”) for pathogen detection to assist in the diagnosis of infectious diseases.
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