Ask the Vet: What’s up with West Nile Virus?

West Nile Virus has had a busy summer, infecting horses around the country. HN’s vet-in-residence Dr. Jen Johnson gives us the DL on this deadly virus and a theory about why it’s on the rise.

Photos: Wikimedia Commons

From Dr. Jen:

As of September 4, 2012, every state except Alaska and Hawaii had reported at least one human case of West Nile Virus (WNV) to the Centers for Disease Control. And where there are human infections, horses are also at risk. Large veterinary centers throughout the Midwest and south are also reporting equine cases. So what’s the deal with this virus and why is it still a problem after so many years?

Until 1999, WNV was considered a foreign animal disease, meaning it there were no reported cases in the United States. First discovered in New York, the virus spread west across the continental United States in about 6-7 years. The primary transmission cycle is between birds and mosquitoes: The virus replicates in a bird and reaches very high levels in the blood. Then a mosquito bites an infected bird, picks up the virus and then passes it along to another bird via another bite. Humans and horses are considered incidental hosts. This means that, if infected, we may get sick, but the virus never reaches high enough level in our bloodstream to infect another mosquito. The virus maintains itself in the wild bird population through this cycle, and isn’t going away any time soon. This means that every time your horse is bitten by a mosquito, there is a chance of contracting WNV.

WNV causes a disease known as encephalitis, meaning inflammation (swelling) of the brain. Swelling of the brain is bad for two reasons. 1) The brain is the master control for the whole body, when it isn’t working right, many other body systems suffer and 2) the brain is surrounded by a hard case (the skull). When the brain starts to swell, there isn’t a lot of room for expansion. The brain can begin to press on the skull, which causes further damage. Needless to say, encephalitis is a very bad thing.

Horses infected with WNV can show a range of neurologic clinical signs from being a little lethargic to stumbling and head tilts to full recumbency and seizures. Unfortunately, because this is a virus, there are limited options for treatment. Antibiotics that work against bacteria won’t work against this (or any) virus. There are a few antivirals available for use in human medicine, and some were administered to horses during the height of the initial WNV outbreaks in the early 2000s. However, due to the size of most horses, this can become very expensive very quickly. Treatment is aimed at supportive care to help the horse while the body tries to fight off the virus. This usually includes IV fluids, antibiotics to prevent secondary infections, feeding tubes, slings if the horse can’t stand on its own, aggressive anti-inflammatory therapy to reduce brain swelling and addressing any specific needs an individual horse may have. The nursing care of a neurologic horse, due to their size, can be very demanding and very expensive. Sadly, many horses with WNV are euthanized because of financial reasons; their owners simply can’t afford to keep up the intensive nursing care required to see the horse through the disease.

Since WNV is spread by mosquitoes, one way to prevent disease is to completely mosquito-proof your horse. Not terribly practical, but one strategy for management is to bring horses in at times when mosquitoes are at their worst, keep fly sheets on and reduce mosquito breeding grounds by addressing standing water around your farm. Another approach is through the use of one of the WNV vaccines. There are several now on the market, and they work in different ways but are almost universally effective in preventing disease from the virus. Vaccinated horses that do show signs of disease are known to show much less severe disease, and there is no reported case of a properly vaccinated horse dying from WNV (that I could find at the time of writing this article).

One theory about why we’re seeing an upswing in equine cases this summer is that owners may have become complacent about WNV vaccination. Unfortunately, we know that the shots do not provide long lasting immunity, which means we’re going to have to keep boostering our horses, especially in areas with high virus loads and areas with year-round mosquito populations. Although it may be tempting to think I’ve vaccinated my horse religiously for the last five years, he should be immune by now this is just not always the case. Be sure to discuss your specific vaccination protocol with your vet, who will help you tailor your approach to your specific type of vaccine and your geographical region.

Dr. Jen is a veterinarian, and an eventer and dressage rider. She been involved in horses in some form or another since she was 4 years old and has experience in gaming, pleasure, H/J and, for the last eight years, eventing and dressage. The majority of her practice has been with horses, both in referral centers and field service. She has also served as official show veterinarian for several horse trials and other shows in the Midwest.