Issue: October 2020
Disclosures: Shapiro and Staples report no relevant financial disclosures.
September 29, 2020
3 min read

Climate change a possible factor in last year’s increase in EEE

Issue: October 2020
Disclosures: Shapiro and Staples report no relevant financial disclosures.
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Between 2003 and 2018, the United States recorded an average of eight cases of eastern equine encephalitis per year. Last year, there were 38 cases of the mosquito-borne disease, including 15 deaths.

According to Infectious Diseases in Children Editorial Board Member Eugene Shapiro, MD, a professor of pediatrics and epidemiology at Yale School of Medicine in New Haven, Connecticut, climate change may have been a factor in the increase.

Encaphalitis infogrpahic
Source: Denlinger DL, et al. Annu Rev Entomol. 2014;doi:10.1146/annurev-ento-011613-162023.

“The first frost used to occur mid-October. The leaves used to turn around here 2 weeks earlier than they do now,” Shapiro said in an interview. “I think the increase has to do with the fact that we don't get a freeze until a lot later, and it was particularly warmer last year.”

The transmission of eastern equine encephalitis virus (EEEV) to humans occurs through the bite of an infected Culiseta melanura mosquito after it has bitten an infected bird. This primarily takes place around cedar or hardwood swamp areas where these mosquitoes and birds both reside but where human population is usually limited, according to the CDC.

“These mosquitoes are surviving for many more weeks, and therefore they are more likely to become infected with the virus. So consequently, a higher number of people get infected,” Shapiro said.

Older adults, younger kids most susceptible

Eugene Shapiro
J. Erin Staples

According to the CDC, only 4% to 5% of human EEEV infections result in EEE, which causes death in around 30% of patients and ongoing neurological issues in many others.

“It’s not unusual to see increases or upticks in the number of cases from year to year,” J. Erin Staples, MD, PhD, a medical epidemiologist in the CDC’s Arboviral Disease Branch, told Infectious Diseases in Children.

“For eastern equine encephalitis, we saw similar increases from 2004 to 2006 and 2010 to 2012,” Staples said. “As with most mosquito-borne diseases, there are several factors that contribute to years with higher than average case counts. This could include changes in the bird and mosquito populations and immunity levels, weather patterns, and even human behaviors, including increased awareness and testing.”

Shapiro noted that, during late summer, some mosquitoes enter a sort of hibernation called diapause, which allows them to survive through the winter months and into the next year. An article in the Annual Review of Entomology classified diapause as “a dominant feature in the life history of many mosquito species, [offering] a mechanism for bridging unfavorable seasons in both temperate and tropical environments and [serving] to synchronize development within populations, thus directly affecting disease transmission cycles.”

If it stays warm for a longer period of time, the surviving mosquitoes are the same generation but they have more time to become infected and spread the virus, Shapiro said.

The two groups most susceptible to severe EEE are patients aged 50 years or older and children aged 15 years or younger. These age groups have shown higher risks for developing severe disease compared with younger adults and adolescents.

“Clinicians should consider the diagnosis for EEE in children who have been in areas where the virus is known to occur and present with signs and symptoms of either meningitis or meningoencephalitis during the summer and early fall,” Staples said.

She noted that the signs and symptoms of neurologic disease include fever, headache, vomiting, diarrhea, seizures, behavioral changes, drowsiness and coma.

“In infants, neurologic disease often occurs soon after onset. In older children, encephalitis may occur after several days of systemic illness,” Staples said. “Some individuals can experience only a systemic febrile illness characterized by fever, chills, malaise, arthralgia and myalgia. The diagnosis is typically made by detecting [immunoglobulin M] antibodies in cerebrospinal fluid or serum.”

Shapiro said “it’s not a mystery” that EEE is “one of the possible causes” if a pediatric patient develops encephalitis, but noted the small proportion of people who are infected who go on to develop encephalitis.

“There are plenty of people who get infected, and we know from serostudies that they're infected but don't get sick,” Shapiro said.

Another surge?

Through Sept. 22, the CDC had received reports of seven confirmed cases of EEE so far this year, including four in Massachusetts, which recorded 22 cases of EEE from 2010 through 2019 — more than any other state. Michigan (17) and Florida (13) also reported double-digit cases during that time frame.

According to Shapiro, cases do not typically appear until August or September, so if there were to be another year of a high number of cases, it would not be noticeable until after that. He said the possibility remains higher than normal for an above-average amount of cases.

“When there is a big spike in the number of cases, the preceding year often can have an above-average number of cases as well,” Staples said. “Because of the number of factors that can influence mosquito-borne diseases, however, it is hard to predict what any given year will look like.”

The main way to prevent EEE is to minimize or try to prevent any mosquito bites by wearing long-sleeved shorts and long pants and using EPA-registered insect repellents, the CDC recommends.


Denlinger DL, et al. Annu Rev Entomol. 2014;doi:10.1146/annurev-ento-011613-162023.

Lindsey NP, et al. MMWR Morb Mortal Wkly Rep. 2020;doi:10.15585/mmwr.mm6902a4.