healthHealth and Medicine

Potentially Fatal Brain Infection Found In Four American Horses. Should You Worry?


Dr. Katie Spalding

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer

horse and child

Horse and human, the two types of EEE victim. Image credit: Alla-Berlezova/

A recent alert from the Wisconsin Department of Health Services (DHS) reminded people to take steps to avoid mosquito bites after four horses across the state were diagnosed with Eastern equine encephalitis (EEE).

EEE is a rare brain infection caused by a virus – only a few cases are reported in the US each year, with most occurring in eastern or Gulf coast states, per the CDC.


Wisconsin is home to 56 different species of mosquito –annoying though they are, they’re vital to the local ecosystem. Many don’t even bite humans and aren’t dangerous unless you’re a frog or white-tailed deer. However, the mosquitoes that do bite humans can be very dangerous indeed. 

The EEE virus is spread from Culiseta melanura mosquitoes to humans and horses, and in quite an interesting way: rather than going from C. melanura to H. sapiens in one shot, it instead percolates for a while in the local bird population. It takes a different mosquito species – one that actually bites humans (or horses) – to act as a “bridge” between infected birds and uninfected mammals – but even then, only four or five percent of those bitten will go on to develop EEE.

Although the infection is rare, that doesn’t mean it’s not scary. If you get it, there’s about a one in three chance it’ll kill you within a couple of weeks, and even if it doesn’t, you may be left comatose or with permanent neurological issues. According to the CDC, patients who recover from EEE can be left with anything from severe intellectual impairment, to personality disorders, to seizures, and even cranial nerve dysfunction such as paralysis and numbness.

Since the EEE virus is only transmitted by a mosquito bite – it isn’t contagious between people or horses (or horses to people, or vice versa) – the four cases shouldn’t be cause for alarm. What it does mean, though, is that the EEE virus-transmitting mosquitoes are definitely in the area – and locals need to start taking common-sense precautions to reduce their chances of getting bitten. According to the Wisconsin DHS, these include:

  • Using an EPA-registered repellent, such as DEET, picaridin, oil of lemon eucalyptus, or IR3535 and apply according to label instructions.
  • Using permethrin-treated clothing and gear.
  • Wearing loose-fitting long-sleeved shirts and long pants to keep mosquitoes away from your skin.
  • Avoiding spending time outdoors at dawn and dusk when mosquitoes are most active.
  • Taking steps to control mosquitoes outside your home.

While living with the mosquitoes may be a pain, biologists say it’s just the price we have to pay for flourishing local habitats. The buzzing bloodsuckers are a food source for birds, bats, ducks, and other animals, and when mosquito numbers fall – or humans overzealously try to “control” them – so too do the populations of those species.

And remember: it’s only temporary. By October, temperatures will likely have fallen below freezing – the so-called “killing frost” – and you can finally crack out those short sleeves and hot pants without fear of getting bitten.



healthHealth and Medicine
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  • viruses,

  • horses,

  • mosquitos,

  • America,

  • wisconsin,

  • Eastern equine encephalitis