Worms and Germs Blog : Animal Health & Diseases : University of Guelph : Ontario Veterinary College : Zoonotic Diseases : Guelph, Hamilton, Ontario

A horse in Ontario was recently diagnosed with Eastern Equine Encephalitis (EEE), a serious neurological disease caused by a virus of the same name, which is transmitted by mosquitoes.  The horse was from the North Durham region.  The last reported cases of EEE in Ontario were in 2004.  A few weeks ago, the Worms & Germs Blog talked about a large number of cases of EEE that have been reported in Florida this year.

Here are some of the key points to remember about EEE:

• Like West Nile, EEE is a seasonal disease.  It is more common in warmer areas, especially some regions of the southeastern US.  It is rare in cooler climates, but occasionally EEE is found in horses in Ontario.
• EEE is usually fatal in horses, and there is no effective treatment.
• EEE can also occur in people, and can be fatal in some cases.
• Infected horses cannot transmit the EEE virus to people, but if a horse gets EEE from the mosquitoes in the area, then people could also potentially be exposed to the virus by mosquitoes.
• A vaccine for EEE is available for horses, but most horses in Ontario are not vaccinated for EEE because it is so rare.  Nonetheless, vaccination can be considered because the disease is so devastating when it occurs.
• As for West Nile virus, avoiding mosquitoes - for both horses and people - is an important preventative measure for EEE.

For more information, see the Worms & Germs Blog post "Eastern Equine Encephalitis – Not Just For Horses", or the CDC's website on arboviral encephalitides.

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Over 50 horses have died from Eastern Equine Encephalitis in Florida this year. The disease, caused by a virus of the same name, affects the brain, resulting in a broad range of clinical signs from behaviour changes to blindness to irregular gait. The disease is also sometimes called “sleeping sickness” because some horses may become severely depressed, with low head carriage and droopy eyes, ears and lips. Almost all horses that develop neurological signs from this infection die. Only 35 cases were reported in Florida in 2006 and 2007 combined.

There are actually three related equine encephalitis viruses – Eastern, Western and Venezuelan – which are called EEE, WEE and VEE for short. VEE is found in South and Central America and Mexico, and occasionally in the southern United States, but has never been reported as far north as Canada (VEE is a reportable disease in Canada). It is unique among the three diseases as the only one in which an infected horse will carry enough virus in its bloodstream to infect a mosquito, which could then pass the virus on to another animal. The EEE and WEE viruses, just like the West Nile virus, do not reach high enough levels in the bloodstream of horses to do this. The mosquitoes usually pick up the viruses from passerine birds, which do not become ill from the viruses (unlike West Nile virus in birds from the family Corvidae).

People can also be infected by EEE, WEE and VEE. About 10 fatal cases of EEE in people are reported in the United States every year. But horses cannot transmit EEE or WEE to humans, even if they’re bitten by the same mosquito. A higher number of cases in horses, however, may mean a higher number of mosquitoes that are carrying the virus. There is no vaccine for these viruses for humans, but there are vaccines available for EEE, WEE and VEE for horses.

In the end, EEE is just one more good reason to make sure you wear mosquito repellent when you’re enjoying the great outdoors during the summer. Visit the Health Canada website for safety tips on using personal insect repellents. EEE is very uncommon in Ontario, but horses that live in or travel to the southern United States should be vaccinated. Talk to your veterinarian about whether or not your horse should be vaccinated. Remember that fly control is also important for our equine companions (and also helps protect them against West Nile!).

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