Risks posed to humans by rare but deadly mosquito-borne disease, explained by veterinary expert

Newswise — Eastern equine encephalitis (EEE), a rare but potentially deadly mosquito-borne disease that can spread between people and animals, caused the death of a previously healthy middle-aged New Hampshire resident, according to health authorities. This tragedy, and a documented case of EEE in a hospitalized Massachusetts resident, caused communities in Massachusetts to reschedule public events, curb outdoor activities, and close public parks as measures to reduce risks of human exposure to mosquito carrying the virus.

“The potential harm from EEE is worsened by a lack of both a human vaccine to prevent infection and specific drugs to treat infection,” said Sierra Guynn, a clinical assistant professor with the Virginia-Maryland College of Veterinary Medicine.

“Without vaccines or treatments available to reduce human disease, state and local authorities have a duty to reduce conditions that optimize mosquito breeding and feeding and to minimize contact with humans. Though park closures and limitations on outdoor activities might seem disruptive, authorities must account for the greater risk of death and the decreased quality of life for the community as a whole,” Guynn said.  

She answered further questions about the history of EEE and the problems it poses.

Q: How long has EEE been known as a danger to humans? 

“Eastern equine encephalitis was recognized as a formal distinct disease that causes illness in horses in 1933 during an outbreak in coastal Delaware, Maryland, New Jersey, and Virginia, though there is evidence of it in the U.S. as far back as 1831. The disease was recognized as a human pathogen in 1938, when it infected 38 humans in eastern Massachusetts. Two-thirds of those infected died.”

Q: Where have cases been recorded? 

“Since 1938, the northeastern United States has experienced irregular outbreaks with most of them occurring in Massachusetts, where a large wetland system encompasses the southeastern portion of the state. Wetlands are natural refuges for wild birds and make excellent habitat for mosquitoes to breed. EEE is spread to humans by the bite of over 30 types of mosquitoes, which get the virus from feeding on more than 200 kinds of birds.”

Q: What makes EEE a threat to public health? 

“Though cases are rare, there are several factors that make EEE a significant public health threat for people. The virus can cause two syndromes in humans — either a systemic disease with very non-specific ‘flu-like‘ symptoms, or in less than 5 percent of infected individuals, it can progress to severe inflammation of the brain, called encephalitis. 

“The symptoms of encephalitis, such as headaches and altered mental status, only manifest after severe damage to the brain has already occurred. Approximately a third of patients who progress to encephalitis from EEE do not survive, and those that do survive often have severe brain damage.  People less than 18 years of age and older individuals have an increased risk of developing severe encephalitis from the virus.” 

Have any conclusions been reached as to why cases are increasing?

“Research into EEE and other vector-borne zoonotic diseases in humans continues at federal and regional levels. It is hard to determine appropriate and effective mitigation efforts without an understanding of how and why increased infections are occurring. Theories that need further examination include the effects of increasing temperatures due to climate change and whether that allows insects to sustain and thrive in environments where they previously did not — and also whether the effects of human development encroaching into wetland areas puts more humans at risk of contracting the virus.” 

About Guynn 
An alumna of the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Tech, Guynn joined the faculty in 2012 as a clinical assistant professor of production management medicine in the Department of Large Animal Clinical Sciences. Since 2019, Guynn has expanded her own education and research into public health, large animal veterinary preventative medicine, and One Health at the intersection of large animals and humans. Read more about her here.

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