EEE. It's Here!

Last week I talked with a veterinarian friend in southwest Michigan who mentioned that he had recently seen a case of suspected EEE. As of 2 days ago, there were 3 confirmed cases of EEE in horses in SW Michigan, as well as several other highly suspicious cases, that are awaiting the completion of further testing to confirm the diagnosis. Infected horses have been identified in Cass, Barry & Calhoun counties.

Veterinary medicine is notorious for using letter abbreviations for disease names. This alphabet soup may seem a little silly, but with diseases like Eastern Equine Encephalitis you can see why we all call it EEE. The more common name for this disease is Sleeping Sickness. That name is a bit of a misnomer since it implies a peaceful, resting state. It is anything but. This is a very nasty disease, characterized by neurologic deficits with symptoms including fever, depression and weakness that rapidly progress to lack of coordination, head pressing, a “sawhorse” stance, circling, paddling and/or convulsions. Affected horses can be very irritable with aggressive behavior, they may be excitable, blind, and/or have abnormal sensitivity light and sound. 75 to 90% of affected horses will die from the disease.

It can be difficult to sort out EEE from some other neurologic diseases. Rabies is the most significant other possibility for horses exhibiting these symptoms. Other possibilities include West Nile Virus, Western EE and Venezuelan EE (VEE is not know to exist in the northern regions of the U.S.) Because of the potential human health concerns with these viruses, definitive conformation of the disease is critically important. While there are blood tests for some of these diseases, a brain specimen (post mortem) is required to definitively confirm the diagnosis.

The fortunate thing about EEE, WEE, WNV & Rabies is that we have very effective and safe vaccines readily available to prevent infection. The caveat to vaccination is that they must be properly administered using a properly handled and stored product well in advance of exposure to the bug. Immunization against these diseases is a staple of our current vaccine programs. These are vaccines all horses should always get, every year, regardless of where they go or what they do. Vaccination is by far and away the most effective single measure to undertake to prevent infection.


EEE, WEE, and WNV all are viral diseases transmitted by mosquitoes. Because of that, we usually do not start to see cases until late summer (August or September) because it takes a while to get enough virus circulating through mosquitoes to result in an infection. EEE is carried by birds, that usually are not affected by the virus. Mosquitoes bite the bird and then transmit it to horses or humans. Because mosquitoes are the major vector in the spread of the disease, measures to control the mosquito population are crucial to successful control strategies. But given that several cases have been reported while it is still relatively early in the summer, means that we are sure to see more cases as the summer progresses.

The other reason these disease are of such great concern is that humans can also be affected. With EEE, affected horses are not a source of infection to people or other animals, but Rabies certainly can be. That said, if a horse is affected with EEE it is a pretty good indicator that the virus is present in the mosquito population in that area. So other horses in the same proximity are certainly at risk as well.

One of the scariest things about these cases, is that it is still relatively early in the summer to start to see this mosquito vector borne disease. The importance of proper vaccination cannot be overemphasized. It is not too late to get your horse(s) vaccinated if it has not already been done. Annual vaccination is considered protective. The other disease prevention strategies really center around limiting exposure to mosquitoes. This has been a very wet summer, especially in SW Michigan, where these cases are emerging. It would be prudent to eliminate sources for standing water around your barn or property. Bringing horse into the barn, especially around dusk can be helpful as well. Fly and mosquito sprays can be beneficial, but none of them seem to last very long. Stay tuned for further updates.