Don Lyman November 10, 2023
There were no human cases of EEE and only six cases of West Nile virus in Massachusetts this season.
JAIME SALDARRIAGA/ASSOCIATED PRESS
Despite a rainy summer that produced lots of standing water where mosquitoes breed, there were not many cases of Massachusetts’ two most notorious mosquito-borne diseases: eastern equine encephalitis and West Nile virus.
While this is good news, it seems counterintuitive. More rain should equal more mosquitoes, which could equal more EEE and WNV, but there were no human cases of EEE this season and only six of WNV.
“There were relatively large populations of various species of mosquitoes,” said Dr. Catherine Brown, Massachusetts state epidemiologist. “And there was plenty of virus in the mosquitoes. It is unusual we haven’t seen large numbers of corresponding EEE cases.”
Brown speculated part of the reason there were so few cases this year also could be due to behavior — of both mosquitoes and people.
“Mosquitoes aren’t flying during heavy rainstorms,” Brown said. “And rain doesn’t just affect mosquitoes. It also affects people and their behavior. Maybe all the rain kept people indoors enough so that we’ve seen fewer human cases.”
EEE popped up late in mosquitoes this year.
“It usually shows up in early to mid-July,” said Brown. “But despite conducting mosquito trapping and testing for both WNV and EEE since mid-June, there was no evidence of the EEE virus until the first positive mosquitoes collected on Aug. 30.”
Brown said human cases of EEE are usually seen from late August until mid-September, but the species of mosquitoes most commonly responsible for spreading the EEE virus to humans — Coquillettidia perturbans — started to die off in early September, before the insects could spread the disease.
According to the Central Mass. Mosquito Control Project, there were 12 cases of EEE in Massachusetts in 2019, five in 2020, and zero in 2021, 2022, and 2023.
“EEE usually occurs in outbreaks,” Brown explained. “The pattern is 2-3 years with multiple human cases followed by some number of years with no or very few cases. 2019-2020 were outbreak years, and we have been in the period between outbreaks since then.”
West Nile first showed up in mosquitoes in July, which is common, Brown said. According to the Massachusetts Arbovirus Update, six human cases occurred from August to November in Worcester, Middlesex, Hampden, and Norfolk counties, Brown said. Massachusetts usually has 10 to 15 cases of WNV per year. On its Mosquito-borne Diseases web page, the Massachusetts Department of Public Health said there are 51 mosquito species in the state, but not all of them bite people. Some species feed on different kinds of animals, including frogs, turtles, and birds. Mosquitoes that bite both birds and mammals acquire EEE and WNV from infected birds, then spread the disease to humans when they bite them. Just eight mosquito types are listed as “common species that can spread disease.”
Only female mosquitoes bite to suck blood, which they use to make eggs, according to DPH. The main food source for both male and female mosquitoes is nectar from plants and juice from fruit. Female mosquitoes are attracted to carbon dioxide that humans and other animals breathe out, as well as lactic acid on skin, and even certain scents and fragrances. Different mosquito species are active at different times, but most are busy from around dusk until dawn.
Mosquitoes are trapped and sent to laboratories to test for EEE and WNV, Brown said. “For EEE, they trap mosquitoes in wetland areas,” said Brown. “For WNV, mosquitoes are trapped in urban environments. Suffolk and Middlesex counties have the most cases of WNV because of urban habitat and human density.”
Culex pipiens, the primary mosquito that transmits West Nile to humans, thrive in urban environments, said Brown, and like to breed in water in artificial containers like gutters and plastic swimming pools.
There is no treatment for EEE or WNV, said Brown. Doctors provide supportive care and treat symptoms.
“EEE is universally worse than WNV, and has a 50 percent fatality rate,” Brown said. Symptoms of EEE include fever, stiff neck, headache, and lack of energy, which occur three to 10 days after a bite from an infected mosquito, according to DPH. Encephalitis, which involves swelling and inflammation of the brain, is the most frequent serious complication. Approximately 80 percent of people infected with WNV have no symptoms, DPH said. The remaining 20 percent may experience fever, headache, body aches, nausea, vomiting, swollen lymph glands, and a rash on the chest, stomach, and back.
“Less than 1 percent of people infected with WNV will develop severe illness, including encephalitis or meningitis,” said DPH.
Two human cases of Jamestown Canyon virus — a rare mosquito-borne illness that also can cause encephalitis — occurred in New Hampshire this summer. Brown said Massachusetts does not currently test mosquitoes for JCV, but people may be tested for it if they test positive for EEE or WNV.
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