Why We Vaccinate - Eastern & Western Equine Encephalitis (“Sleeping Sickness”)

Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE) are both neurological diseases spread to horses and humans from infected wild birds and rodents via blood sucking mosquitos.  Circulating levels of the virus in an affected horse are low, however, so it is highly unlikely that a horse would be a source of infection for a human.  In horses, both EEE and WEE viruses can cause fever, depression, incoordination, staggering and blindness.   Other signs may include teeth grinding, head pressing, and/or hyperexcitability, and the symptoms are usually progressive.  The disease typically culminates in a profound depression that characterizes these diseases in the late stages, and gives them the common name of “sleeping sickness.”  Horses can become paralyzed as the disease progresses.  Nearly all horses showing clinical signs of EEE die (90%), and among those that survive, many will experience permanent neurological impairment.  WEE demonstrates a much higher survival rate, typically 70-80%, although mortality was as high as 50% in one outbreak.

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Eastern Equine Encephalitis is typically seen in the southern, eastern, and southeastern states, however there have been some reports in the Midwest (Wisconsin, Michigan, and Ohio) and Eastern Canada.  As suggested by the name, Western Equine Encephalitis is more commonly seen in the Western and Midwestern United States and Canadian provinces, although there have been sporadic cases in the Eastern US and Florida. 

Although not seen in our area, we wanted to share this video about Eastern Equine Encephalitis, put together by our friends at Myrtle Beach Equine.  They have seen a number of cases of EEE recently, due in large part to lapses in vaccination, however the vaccination recommendations listed in the video are for horses in their area.  The symptoms of WEE can be very similar.

WARNING: Some people may find this video difficult to watch.

Before vaccinations were developed, outbreaks of WEE and EEE were much more common, and although they varied in severity, they could be devastating.  An outbreak of WEE in 1937-1938 was estimated to have involved 350 000 horses and mules.  In Louisiana in 1947, an outbreak of EEE killed approximately 12 000 horses.  As of late, widespread vaccination and immunity gained from subclinical exposure have resulted in a much decreased prevalence of the two diseases.  However, the two viruses have not been eradicated, as evidenced by the recent resurgence of clinical EEE cases in the Eastern US, as well as the detection of WEE virus in mosquitos and birds in the Western United States.    There has been an increased number of EEE cases reported in Eastern Canada and the US in the past few years, likely due to decreased vaccination rates as vigilance against the disease has waned.  WEE has been reported only minimally in the last 2 decades, but the continued presence of the virus in mosquitos and birds could mean more cases could be detected if vaccination rates fall.

Vaccines against EEE and WEE appear to be very efficacious and safe to use.  They are very commonly combined with tetanus vaccines in a "3-way" combination, and are considered “core” vaccinations by the American Association of Equine Practitioners, strongly recommended for all adult horses.