Global Health

EEE and West Nile Virus: Understanding the Increase in Mosquito-borne Diseases

David Hamer, Jessica Leibler, and Laura White discuss the growing threat of Eastern Equine Encephalitis and West Nile Virus infections, which have been on the rise in Massachusetts and across New England this summer.

On Thursday, September 5, the Massachusetts Department of Public Health announced one additional case each of Eastern Equine Encephalitis (EEE) and West Nile virus (WNV) in Middlesex County, for a total of three confirmed EEE cases, and seven confirmed WNV cases, in the Commonwealth this year.  As a result, nearly two dozen communities in Middlesex and Worcester counties are now at an elevated risk for EEE, and several more counties throughout the state are at a moderate risk for WNV.

The cases are a stark sign of the health consequences of climate change, in which a warming climate, along with more frequent and extreme weather events, have created the perfect breeding conditions for mosquitos to thrive and spread a variety of vector-borne diseases. 

The majority of people who contract these viruses experience no symptoms, but those who do can develop severe illness. EEE, a rare but potentially deadly disease, can cause physical and mental complications, including flu-like symptoms and neurological issues, with a fatality rate of 30 percent or higher. WNV, the most common mosquito-borne illness in the US, is generally more mild, but can also cause serious or life-threatening issues. There are no commercially available vaccines or treatment for either virus. Due to the potential severity of health complications, multiple towns have shut down park access or recommended voluntary curfews to the public during peak mosquito hours at dusk and dawn.

Prior to 2024, the last human cases of EEE—a total of five, with one death—were detected in 2020. The current US total of six EEE infections this year still falls short of the nationwide average of 11 infections per outbreak, but health experts worry those numbers could change swiftly over the next month. The risk of infection will remain elevated until the mosquitos are killed during the first hard frost of the season.

“The total number of EEE infections is not concerning, but if it increases, this will be more worrisome,” says David Hamer, professor of global health. “So far, the cases have been fairly widely distributed, although sadly 3 of the 7 have been in Massachusetts.”

The risk of infection will remain elevated until the mosquitos are killed during the first hard frost of the fall season, an occurrence that is likely to take place later and later each year, thanks to the warming climate.

As we reach a peak in mosquito season, Hamer, along with Jessica Leibler, associate professor of environmental health, and Laura White, professor of biostatistics, discuss the factors contributing to the continued increase in mosquito-borne diseases and what an appropriate public health response should look like to ensure maximum public safety.

Can you expand on the impact that climate change is playing in the frequency and spread of vector-borne diseases? And although both EEE and West Nile are spreading, outbreaks are still sporadic; do we have any idea of what causes the virus to surge and wane? 

LEIBLER: Warmer temperatures and increased precipitation are the primary climate change-driven factors that affect vector-borne disease ecology. Increases to temperatures can accelerate mosquito life cycles, allowing for more reproductive cycles and expand the geographic range of mosquitos to potentially increase the range of human exposure. Increased rainfall and higher humidity can create more breeding environments for mosquitoes.

In terms of the surge in Massachusetts in 2020, it was likely due to a combination of factors, including an unusually wet summer that led to an increase in mosquito populations, in conjunction with an increase in the number of infected wild bird hosts—with birds as the primary reservoir for these viruses—to host the pathogens. There are complex ecological interactions involved with the surges of these viruses, involving vector, host, and climate factors, and these factors most likely intersected in 2020. 

Do you think the park usage restrictions and voluntary curfews that a few towns have implemented are appropriate precautions or should an effective public health response focus on mosquito control measures and education around risk mitigation? Do you worry that people who are still weary from COVID restrictions may not take these warnings seriously?

HAMER: These are reasonable precautions although tough to encourage local populations to adhere since it is late summer, the weather is nice, and a lot of outdoor activities are planned. That said, people in much of the state, especially those living in or near the EEE hot spots should be careful and try to adhere to anti-mosquito precautions.

LEIBLER: To me, public education and awareness campaigns alongside mosquito control measures are better approaches than shutdowns and curfews, which I suspect should be reserved for the most high-probability, high-risk scenarios. Exposure risk to EEE and WNV remains relatively low, and using mosquito repellant and wearing long sleeves and pants when outdoors in the evenings are effective ways to substantially reduce risk of exposure. 

Do you think delayed diagnoses of EEE are common, since many symptoms are associated with other illnesses, and is that a concern? Is it possible that milder symptomatic cases are being overlooked until they become severe?

HAMER: This is a tough question. The problem is that clinicians probably miss most, if not all, cases of mild disease due to EEE. And there are no rapid tests, so all tests—whether they be PCR or serology—need to be sent out to speciality labs or the state. This may add to patient care costs and delayed diagnosis so many clinicians may not even bother to try to test. More likely though is that in the absence of other neurological disease, EEE or WNV are not likely to be considered, and appropriate diagnostic tests are not likely to be sent.

Worcester and Plymouth counties began spraying for mosquitos after the first case of EEE was detected; should counties take a more proactive approach during peak mosquito season and spray before cases are detected? Do you see need for improvement with surveillance or prevention for EEE/WNV?

WHITE: Surveillance is critical for all public health threats and requires investment, innovation and a lot of cooperation between stakeholders. In public health we are always interested in prioritizing prevention which requires being less reactive and more proactive. I think we are still trying to figure out how to do this best for WNV and EEE, as well as many other diseases. I think that there are opportunities to improved technologies for surveillance and monitoring of mosquitos, better data streams, and modeling to improve our capabilities to proactively approach these threats. That takes investment and a lot of collaboration between researchers, public health and investment to fund that work.