Eastern Equine Encephalitis: The Mosquito that bit the Snake

Guest post by Hillary Craddock

Last week a new study regarding Eastern Equine Encephalitis (EEE) was published online (Bingham et.al.). EEE is a mosquito-borne virus that can cause serious, and sometimes deadly, disease in humans and equines. In warmer parts of North America, the virus is spread year-round, but in areas where mosquitoes get killed off in the winter it has been something of a mystery as to how the virus makes it from year to year. Humans and equines are both dead-end hosts, which means that a mosquito can not be infected from biting an infected person or horse. Researchers in Alabama found that wild snakes in the Tuskegee National Forest were positive for  Eastern Equine Encephalitis virus (EEEV), which could explain how EEE was maintained after the first frosts killed off infected mosquitoes. Essentially, what would happen is that an infected mosquito bites a snake, probably during the summer or early fall, and the snake harbors the virus in its blood during the winter. Then, in the spring, an uninfected mosquito (which overwinters as a larva) bites the snake and acquires the virus. This now-infected mosquito can bite a horse or a human, who can then get sick. (I’m sensing a Chad Gadya theme here. Just me? Ok…)

Amphibians and/or reptiles as the winter reservoir of EEE is not a recent research question. A book, Reptiles as possible reservoir hosts for eastern encephalitis virus, (which I was unfortunately unable to get my hands on, since apparently only the University of Alberta has an available copy) was published in 1961, and another  study in 1980 by Smith and Anderson stated that two New England species of turtles could be infected by the virus. Interestingly enough, a 2012 study by Graham et. al. (same research group as Bingham et.al.) found that, out of 27 species surveyed, only snakes showed high seropositivity (positive for virus antibodies in the blood), while amphibians, turtles, and lizards had low to no seropositivity. A 2004 study by Cupp et.al., also in Alabama, found that mosquitoes carrying EEEV had fed on amphibians and reptiles in addition to birds and mammals. Now, it’s all well and good to show that a reptile can act as a host, but just because something can be the host doesn’t mean that it is the host in the actual system. The crucial step was testing their hypothesis in a wild population.

And test they did. The researchers were careful to state that the question of snakes acting as reservoir hosts is “unresolved,” but there is “mounting evidence” that snakes are the winter hosts of the virus. Cottonmouths (Agkistrodon piscivorus) were the most common snake sampled, making up 41% of sampled reptiles. They were also frequently seropositive, with 35.4% testing positive for EEEV. Of the five species sampled, one other, the copperhead (Agkistrodon contortrix) was found to be positive. The researchers tested for active infection in addition to antibodies, and found that some snakes were actively infected. This means that, if a mosquito bit the snake, the mosquito could possibly acquire the virus and pass it on to other creatures.

So why am I so excited? When I took my first Emerging Infectious Diseases class in college, the professor explained to us that zoonotic infectious diseases were most likely to jump between closely related species. Granted, I’m using the word “close” loosely here. She meant that diseases were far more likely to jump mammal to mammal or bird to mammal than, say, fish to mammal or reptile to mammal. I was also taught that if you can understand how a disease is transmitted, you’re one step closer to controlling it.

Which answers the ultimate question – so what does this all mean? When we better understand how a disease is transmitted, it’s easier to control it. Further research in other parts of the country is needed to see if snakes are harboring the virus in the North East and Midwest regions, but the implications for disease control are there. If we understand where or when snakes congregate, we might be able to better predict disease dynamics, specifically outbreaks. If the first outbreaks in the summer originate from mosquitoes biting snakes, then it’s possible that scientists could conduct heavier surveillance in areas where snakes are known to congregate. In this case, we have two entire categories of experts – herpetologists (reptile specialists) and wildlife scientists – that public health practitioners can work with to try to control the disease. This paper is amazing because it unlocks a whole new cavalcade of questions and potential solutions.


This post was republished with permission by the author, and was originally published at Mind the Science Gap.

Hillary is a second year master's student in Epidemiology at the University of Michigan, and she is currently working in influenza research. Her primary interests include zoonotic, emerging, and vector-borne infectious diseases, disaster preparedness and response, and public health practice.

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Interesting post, Hillary. We have had an outbreak of equine West Nile in Iowa, with 21 cases reported by the USDA as of October 2, and by September, I am aware of at least two horses that had to be put down in eastern Iowa. I am unclear if Equine West Nile and EEE are carried by different mosquitos - the CDC has conflicting information - and I am very surprised to learn that any mosquitos bite reptiles. The thickness of reptile skutes/scales would seem to protect them. In considering prevention and further investigation, an appropriate train of thought would be to consider the typical equine environment along with snake environments .... snakes (in Iowa, the most likely is probably the black rat snake [elaph obsoleta obsoleta]). LIke horses, they live in barns, where they feed on rats and other rodents. One problem to consider in the Midwest is that many people have horses that live in pastures (of varying conditions, from muddy to grassy to dry) and I am constantly surprised at few of these horses are used much for work or pleasure. In well-kept stables where people keep dressage or hunter/jumper horses (or even western pleasure horses), the horses are well-cared for and well trained. The loose horses that seem to roam around many farms along the back roads are unlikely to be trained and, in my opinion, pose an injury risk to people, especially children. Untrained horses are big, dangerous animals. Those same horses are likely to live in the same environment as grass and other snakes, although (fortunately) Iowa is not home to cottonmouth snakes. Thinking about these linked environments, including the existence of standing water in puddles or ponds, may help us control the spread of EEE.

By Anne Wallis (not verified) on 19 Oct 2012 #permalink