Morgan Murphy

The Potential Deadly Threat of Mosquito Transmitted Disease - West Nile Virus & Chikungunya in Horses

Mosquitos are bad news! Not only are they irritating to you and your horse, they are also carriers of some deadly and dreaded diseases - West Nile Virus and Chikungunya have both been found in the USA and are potentially fatal for any horse.  Malaria is a well-known mosquito-borne illness but, alas, it’s not the only problem to be transmitted by these winged pests. Other illnesses spread by mosquitos include canine heartworm, Eastern Equine Encephalomyelitis and Western Equine Encephalomyelitis and the emerging exotic diseases Chikungunya and Venezuelan Equine Encephalitis Virus. These are not just everyday infections, they are complex and potentially fatal. Mosquito-borne infections are a continually evolving threat to horses around the world, although fortunately most of these diseases are now preventable through immunization.

Small yet deadly - the mosquito*

One notable and increasingly common mosquito-borne condition, West Nile Virus, has only very recently become established in the USA. The first North American case was diagnosed in 1999 and the virus quickly spread to multiple mosquito species, leading to widespread distribution of the disease throughout the US. An effective preventative vaccine was released in 2001 and is now widely in use; however, as yet not everyone has adopted a regular vaccines protocol, thus leaving their horses vulnerable to this potentially deadly disease.

In order to assist you in making an informed decision whether or not to inoculate your horse against Western Equine Encephalomyelitis, we have compiled a beginner’s guide to the virus, explaining its symptoms and prognosis.


What is West Nile Virus?

West Nile Virus (WMV) causes inflammation of the spinal cord and brain in horses. The disease is carried in the blood stream of wild birds and primarily spread by mosquitos. WMV is transmitted when an infected mosquito, known as a vector, bites into a horse or other host. The infection spreads quickly and between 3-14 days later the horse will start to show clinical signs of disease. Investigatory blood work at this time would reveal the infection to have spread throughout the bloodstream, reaching the spinal cord and brain.

What are the signs of West Nile Virus?

WMV characteristically causes severe neurological disturbances in an afflicted horse. Symptoms of illness include:

  • fever

  • wobbling and stumbling

  • lethargy

  • reluctance to feed

  • muscle tremors

  • inability to stand after lying down

  • paralysis of the lower lip

  • difficulty swallowing

  • excess sweating

  • colic

  • convulsions or collapse, in severe cases.

Contact your veterinarian immediately if you suspect your horse has contracted WMV.


Can West Nile Virus be treated?

Unfortunately, there is a limited chance of recovery in horses that contract West Nile Virus; with a mortality rate of around 40%, it is a serious and severe threat to horses’ lives. Even those horses who do recover are often left with residual neurological complications, and this presents a strong case for opting to have your horse vaccinated. Your veterinarian will take a blood test in order to diagnose the disease but, given WMV's rapid progression, it is likely that your vet will start symptomatic treatment while awaiting the results.

There is no single cure for WMV. Instead, treatment involves a combination of intravenous fluid therapy, anti-inflammatory drugs and intense nursing. Research into the use of interferons and other anti-viral products in the treatment of WMV is currently taking place but this is still at an early stage. Sadly around 10% of cases that initially improve later go onto relapse, resulting in an overall low recovery rate.

What is the West Nile Virus vaccination protocol?

Vaccinating your horse against WMV is the only way to safeguard him from contracting the disease. The American Association of Equine Practitioners (AAEP) recommends that all foals and horses in the US are vaccinated at least annually. For horses in the northern US, vaccination should take place in the spring, prior to mosquito season; in the south it may be necessary to vaccinate more frequently (twice a year) as mosquitos are often found all year round in that climate. To find out which vaccine protocol is best suited to your horse, speak to your veterinarian for personalized advice.

How can I minimize the number of mosquitos around my horse?

Reducing the number of mosquitos in the vicinity of your barn and horses will drastically reduce the risk of transmission of West Nile Virus and other nasty illnesses. There are a number of actions you can take to minimize the mosquito population and reduce your horse’s exposure to these unwanted irritants:

  • Stable your horse during dawn and dusk, which are peak mosquito times.

  • Reduce areas of stagnant water near your horse.

  • If you have areas of stagnant water invest in Spalding PreStrike Mosquito Torpedo – this is a tablet which can be dropped into standing water to prevent mosquitos from reaching maturity. Not only does the Spalding Mosquito Torpedo eliminate 95% mosquitos but it is safe for humans, livestock and fish.

  • Situate fans in the barn to promote air movement; mosquitos struggle to fly in powerful air currents.

  • Avoid bright lighting in the barn at night, which serves to attract bugs.

Understanding West Nile Virus clearly illuminates the need to have your horse protected against this potentially deadly disease. To prevent your horse from contracting WMV, consult your veterinarian about vaccination today.

*Image courtesy of Wikimedia Commons