Eastern Equine Encephalitis: The Mosquito that bit the Snake

by Hillary on October 11, 2012

A Cottonmouth, or Water Moccasin. (C) Patrick Feller, Flickr Creative Commons

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 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.

A Copperhead. (C) Greg Hul, Flickr Creative Commons

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.


John Spevacek October 11, 2012 at 12:19 pm

A great post. It explains the situation, proceeds like a detective to chase down the killer, and then expounds on why it is/was so difficult to crack the mystery wide open. Excellent!

Hillary October 15, 2012 at 8:52 am

Thanks! I was really excited when this paper came out!

Margaret October 11, 2012 at 3:55 pm

Thank you for writing this. This answered several of the questions I had from your first post. Until now, I had never heard of EEE.


Hillary October 15, 2012 at 8:53 am

Thank you for reading!

Tim October 11, 2012 at 6:13 pm

the mosquito that bit the snake that harbored the virus that then got bitten by another mosquito that bit the human that then got sick….chad gadya chad gadya…
anyway, well written and informative post. The blog brought up many good points with the studies to back them up.
I learned a lot about EEE and EEEV and the cycle of viruses.

Elizabeth Fryer October 12, 2012 at 1:28 am

I loved seeing the first-person set off in parantheses!

The first paragraph is quite long. Breaking it into at least three will give readers appropriate pausing points to absorb what they’ve read thusfar. White space is important too.

Humans and equines are dead-end hosts but snakes are not, it seems. That point should probably be made when you give the mosquito-biting-snake explanation.

“Amphibians and/or reptiles as the winter reservoir of EEE is not a recent research question.”—I had to read this several times before I got the meaning. “Amphibians and/or reptiles as the winter reservoir of EEE” is not a recent research question or any type of question. “Whether … serve as the …” is a question. Or split it into two sentences: Do … serve as the …? That is not a recent…

Hilary, your second to last paragraph is in first person. I liked the post omitting first-person, but I also like that first-person paragraph. You can’t have both, and I think in this case your excitement about the subject is a positive for the writing. That being said, you need to insert first person early in the first paragraph so as not to jar readers with the change in perspectives—your Chad Gadya comment REALLY jarred me, after such a long third-person intro, but you set it off appropriately.

“Which answers the ultimate question”—Which what?
Oh, are you using “which” as a pronoun? You shouldn’t start a paragraph with a pronoun that refers to something in the previous paragraph. (Please share that tip with your class as I find I’m making the same general comments on most of the posts.) That means no starting paragraphs with “This means…” You can sometimes get away with starting wtih “He said…” or “They complained…” if the writing is about whoever “he” or “they” refers to and the proper nouns have been used a lot before.

This reminded me of something I might read in “National Parks Magazine” minus the first-person, which is making me rethink my advice about changing the perspective to first person. [Look! I just made the mistake I just told you not to make! I started my paragraph with “This” leaving readers to wonder, This what? I caught it on my proofread. I should have written “This blog post…”] I suppose for your blog audience it would be appropriate. But this is informative stuff that would be of interest to other scientists—who would not appreciate the first-person language.

I look forward to more :)

Rose October 16, 2012 at 11:33 am

I found the article easy to read and as a med student/future physician, useful to know and be able to explain to pts. Could you do an article on swine flu?

Hillary October 16, 2012 at 1:26 pm

I’ll see what I can find on the new variant that emerged this summer. That could be a great topic :-)

Angela October 17, 2012 at 9:12 pm

Thanks for the post. I have to say it took me a bit to get into it, maybe because the first two sentences feel a bit technical/textbook-like. But I really get your excitement about the transmission ‘species gap’ – I think it works really well for communicating the topic. And – blimey – there is a Emerging Infectious Diseases class? I hope it’s not on Monday mornings…

Hillary October 17, 2012 at 9:49 pm

I think it was Tuesday/Thursday, but it’s been awhile. I love EID work, so I probably would have signed up even it was 0900 on a Monday.

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