Article courtesy of Larry Frischke DVM, Wyeth Animal Health.

This spring, horse owners need to be vigilant about West Nile Virus (WNV), Eastern Equine Encephalomyelitis (EEE) and Western Equine Encephalomyelitis (WEE).

When I speak with horse owners about the threat posed by these three diseases, there are two distinct reactions. First, just the mention of West Nile Virus (WNV) results in deep and immediate concern. As I see it, horse owners are very aware of West Nile because of the extensive publicity and public information that have surrounded it in recent years. However, the mention of Eastern Equine Encephalomyelitis (EEE) and Western Equine

Encephalomyelitis (WEE) both draw blank looks from most horse lovers.

WNV has a 30% mortality rate. Surviving horses frequently suffer neurological damage and are often never the same. While WNV is the disease with most awareness, EEE and WEE are in fact even deadlier. The mortality rate of WEE is 50% and in the case of EEE, it’s 90%.

That’s why horse owners should treat all three diseases as serious potential threats against which their horses should be vaccinated. In fact, as of January 2008, the American Association of Equine Practitioners now classifies all three diseases as requiring core vaccines that should be administered to all horses.

Just one bite

Let’s briefly review these three diseases. The transmission of WNV, EEE and WEE occurs via mosquitoes, which contract the viruses by feeding on infected wild birds. All three are encephalitic diseases that can affect the central nervous system of the horse. With WNV, the clinical course of the disease will progress from non-specific signs such as fever, loss of appetite and depression to severe neurological signs related to damage to the central nervous system.

With EEE and WEE, horses typically show signs of weakness and depression. Other symptoms may include fever, walking in circles or into obstacles, refusal to eat and standing with its head lowered. These diseases incubate in the horse for one to three weeks before signs are evident.

As I mentioned earlier, EEE and WEE are even more lethal than West Nile, but together the diseases pose a triple threat.

From year to year, it can be difficult to predict what the actual disease threat will be. Spring and summer weather conditions will determine the number of mosquitoes propagated during the seasons and that will drive the level of risk – but you can’t predict that! As well, there is no current mechanism for recording survivors and the incidence of crippling that occurs from West Nile is unknown. Be aware that, even if fewer horses have been positively diagnosed in a given year, the risk to your horse has not necessarily declined.

The margin between safety and infection is extremely small – all it takes is a single mosquito bite. Global climate change is reported to increase the risk of mosquitoborne disease, so there are expectations of living with WNV, EEE and WEE for years to come.

Devastating impact on horses

As a veterinarian, I am sometimes asked by horse owners whether it’s truly necessary to vaccinate against diseases like WNV, EEE or WEE. My answer is an emphatic yes. The impact on horses that contract any one of these diseases is devastating and the cost of treatment is substantial. Meanwhile, the cost of vaccination is modest.That’s why I recommend that owners protect their horses all season long by vaccinating with West Nile-Innovator® + EWT. It’s the only core combination vaccine that guards against WNV, EEE and WEE plus tetanus in a convenient, one-shot booster.

In terms of impact, many horses that are infected with WNV, EEE or WEE lose their ability to stand and walk. This may progress to convulsions or the grinding of teeth from the intensity of the pain being felt in the animal’s head. Even with timely treatment, there is a strong chance that the horse

will die. And if the horse does conquer the disease – as stated, WNV has a lower mortality rate than EEE and WEE – the horse is likely to suffer long-term or permanent residual effects.

So why take the chance? According to a study conducted at the University of California – Davis, it costs around $3,000 to treat a horse for WNV. Prevention by vaccination is a far wiser investment in your horse’s health. As spring approaches, so does the risk of disease for your horse. I urge you to speak with your equine veterinarian about annual spring vaccinations to help protect your horse from West Nile Virus, Eastern Equine

Encephalomyelitis and Western Equine Encephalomyelitis. Here’s to good equine health in 2009.

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