West Nile Virus (WNV) has been back in the news, in 2012, due to the resurgence in the number of human and horse cases being reported. As of the first week of September 2012, there were 1,993 human cases with 87 fatalities and 187 equine cases reported. Thirty-three states have confirmed cases of the virus.

What is West Nile Virus?

Once found only in Italy and Israel, West Nile Virus was first diagnosed in the United States in 1999. The first cases occurred in New York and Pennsylvania and it wasn’t long before the virus spread across the country.

The virus is a mosquito-borne disease that causes encephalitis (inflammation of the brain) and or meningitis (inflammation of the lining of the brain and spinal cord). The common house mosquito, Culex pipiens, is the species most commonly infected. Birds play a critical role as a reservoir for the virus. The impact of the disease in birds varies, with American crows frequently dying from the infection. Many other bird species survive infection with mild or no indication of disease. The birds can then pass the virus onto a mosquito that bites them. After the mosquito picks up the virus it takes 10-14 days for it to reach the mosquito’s salivary glands. Once in the salivary glands, the mosquito can transmit West Nile Virus to people, horses, or birds that it feeds on. West Nile Virus can also be transmitted via blood transfusion, through breast milk, and from mother to child (mare to foal) during pregnancy. Horses have not been shown to be able to transmit WNV to humans.

What are West Nile Symptoms?

Infection with West Nile Virus does not always lead to signs of illness in people or animals. In people, mild infections may be common and include fever, headache, and body aches, often with a skin rash and swollen lymph glands. In those susceptible to disease, signs can be severe and may include headache, high fever, weakness, neck stiffness, stupor, disorientation, coma, tremors, convulsions, paralysis, and possibly death.

Horses appear to be a species that is susceptible to illness after infection with the virus. Severity of the disease varies from horse to horse ranging from mild illness to neurological signs to death.

Clinically ill horses may exhibit the following signs:

  • Fever
  • Weakness of the hind limbs
  • Paralysis of the hind limbs
  • Ataxia (weakness)
  • Down, trouble getting up
  • Head pressing
  • Inability to swallow
  • Convulsions
  • Coma
  • Hyperexcitability
  • Depression
  • Loss of Appetite
  • Aimless wandering
  • Circling
  • Bowing

Not all horses with these signs have West Nile Virus. There are other neurological diseases that present with similar signs. It is imperative to have a horse with any of these signs examined by a veterinarian and have proper testing done.

How Do We Test for West Nile?

Diagnosis of WNV infection in horses involves testing the blood serum for antibodies. Horses vaccinated for WNV and foals of positive-testing mares are likely to have a positive blood test for the virus. Veterinarians consider blood test results, clinical symptoms and the possibility of other neurological diseases, including rabies, Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), Equine Rhinopneumonitis (equine herpes virus type 1), and Equine Protozoal Myeloencephalitis (EPM), before making a diagnosis.

What is the Treatment for West Nile?

Treatment for clinically ill horses is mostly supportive. Anti-inflammatory medications are used to reduce the inflammation. Plasma containing WNV antibodies can be administered to provide passive immunity to aid in treatment. Other supportive treatments would include keeping the horse in a safe, well padded stall, using a head bumper (as necessary), administering IV fluids and balanced nutrition. Treatment will not guarantee improvement or survival. One third of infected horses will die or need to be euthanized due to the severity of their signs.

How Can We Prevent West Nile?

The best way to protect your horse from WNV is to vaccinate and to eliminate mosquito habitats. Vaccination is very safe and effective. No vaccine is 100% effective, but when administered according to manufacturer recommendations, the vaccine is very good at preventing disease. As important as vaccination is keeping mosquitos away. Eliminate all sources of standing water around your farm. Old tires are a favorite breeding ground for mosquitoes, so make sure they are removed. Mosquitoes can breed in any puddle that is around for 4 days, so think of where water pools and try to eliminate those areas. Where possible, also try to discourage birds from coming into the barn. If you notice any dead birds, report them to your department of health. Do NOT touch them with bare hands. Other means of deterring mosquitos include stall fans, barn fly spray system, and fly sprays for your horse.

Registered 2012 by Equestrian Collections
Author: Sallie S. Hyman, VMD, DACVIM, CVA

Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian, such as TEVA, for their horses' medical care.

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