Mosquito woes: Protecting horses from insect-borne illnesses

By James Little, DVM

Equine and companion animal professional services veterinarian

Neogen

 

Mosquitoes are not just those little annoying insects that drive us crazy at dusk. Along with their painful bites, mosquitoes also can carry viruses that can cause deadly diseases in horses. Late summer into autumn is the time of year that we start to see those diseases rear their ugly heads.

West Nile virus

West Nile virus was first reported in the United States in 1999 in New York and spread quickly across the country. Case numbers in horses peaked more than 15,000 cases in 2002 alone. In the last few years, those numbers have decreased due in part to increased awareness, vaccination, and better vector control. However, the threat is not over. As of August 14, 77 equine cases of West Nile have been reported across several states compared to 87 total for 2011. In humans, 693 cases have been reported for 2012 as of August 14 compared to 712 cases for all of 2011. And it’s still early in the season. The point is, the virus is still here, and it’s been a great year for mosquitoes in certain areas.

The West Nile virus normally is found in birds, its natural host. Those pesky mosquitoes transfer the virus between birds. However, as we all know, birds are not the only thing that mosquitoes bite. When a mosquito bites a bird with the virus, it ingests the virus along with its blood meal. The virus then resides in the mosquito’s salivary gland until it is passed to another animal it bites, such as a horse or human. The virus then makes its way into the body of this victim. Horses and humans generally are considered to be “dead end” hosts for the virus, meaning the virus is not the able to be spread from the infected horse or human to another animal. However, the virus can go on to cause disease symptoms in the horse or human. The symptoms seen in horses may include fever, lethargy, decreased appetite and neurologic symptoms. These neurologic symptoms may include any combination of muscle fasciculations (twitching), ataxia, loss of coordination, muscle weakness, paralysis, and a change in mentation. These symptoms can become quite severe resulting in up to 40% of affected horses dying or being euthanized. Treatment is often limited to supportive care and thus prevention is our best weapon against West Nile.

Eastern and western encephalitis

West Nile virus is not the only viral encephalitidy that horses have to face. Eastern equine encephalitis (EEE) and western equine encephalitis (WEE) are neurologic diseases caused by viruses. You may also hear these diseases referred to as “sleeping sickness”. They are transmitted by mosquitoes similarly to the West Nile virus as described above.  Like West Nile, the virus’ natural host is birds and the mosquito transmits the virus from an infected bird to the horse. Symptoms associated with these diseases are similar to West Nile as well and include fever and neurologic symptoms. EEE is usually fatal in up to 90% of cases. WEE is usually less fatal with mortality rates still ranging from 20 – 50%. These viruses have been around in the U.S. since the 1930’s. Unfortunately, I feel that some people may believe these diseases are old and don’t happen much anymore, but the fact is EEE is seen yearly. In fact, numbers of EEE are on the rise in 2012 along with West Nile. As of August 14, the CDC has received reports of 79 cases of EEE in horses in 2012. There were a total of 67 cases in horses reported in 2011. And again, it’s still early in the season. Since 2003, yearly case numbers of EEE in horses have ranged from 60 to 712. WEE is seen less frequently but could pop up again without notice. No cases of WEE have been reported since 2004, according to the CDC. Venezuelan Equine Encephalitis (VEE) is another viral disease that behaves a little differently and usually is limited as a threat to the extreme southern part of the U.S.

So, what do we do?

We have two real preventive strategies in the arsenal when it comes to these diseases. The first is vaccination. Safe and effective vaccines exist that will immunize your horse against West Nile, EEE, WEE, and VEE. They can be found separately or in various combinations. For vaccines to be effective, they must have several things happen. First, they must have been proven safe and effective. Vaccines must be handled and stored properly.  They must be used appropriately, meaning they must be administered correctly to the horse and they must be given on the correct schedule and frequency. Most vaccines require multiple doses initially at specific intervals and boosters periodically thereafter. The frequency of these boosters may depend on your geographic location and the functional length of the mosquito season. Please consult your veterinarian for advice regarding which vaccines are indicated for your horse and the appropriate vaccination schedule for your horse.

The second preventive strategy is vector control. As mentioned in previous articles, vaccination cannot be relied upon solely to prevent disease. Reducing exposure to the disease causing agent also is a valuable tool. With these diseases, it doesn’t matter if the horse travels or comes in contact with other horses. Birds bring the virus to the horse. Mosquitoes play the role of vector and take the virus from the infected bird and transmit it to the horse. Mosquitoes like moist, damp environments with standing water, which provides a place for their larva to become adults and thus grow the mosquito population. The more mosquitoes there are, the greater the chance of spreading these viral diseases. To help control the mosquito population, eliminate areas of standing water. Anything from bird baths to old tires can hold water and act as a reservoir. Dump water out of these types of structures and turn them over so that they can’t collect any more water. Keep horses stalled at dusk and dawn which are peak mosquito periods of the day. Use mosquito repellents that are approved for equine use. Fly sheets and masks are examples of protective barriers that can be used on your horse although it is very difficult if not impractical and potentially dangerous to try to cover every inch of the horse. Fans have also been recommended to keep air moving inside an enclosure such as your barn or a horse’s stall. You may be able to think of other strategies that would benefit your situation. Please consult with your veterinarian before trying anything that may harm your horse.

Final thoughts

Equine viral encephalitides do exist and are a real threat to equine health. Improving your horse’s immune status through vaccination and decreasing its exposure level by controlling the number of mosquitoes in the environment can help reduce the risk of disease. Work with your veterinarian to plan your defensive strategy and the proper course of action if you notice signs of disease.

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