The Louisiana Animal Disease Diagnostic Laboratory identified the state's first case of West Nile Virus in a horse. Earlier this June, the virus tested positive in a horse from St. Landry Parish. Most animals exposed to the virus die…..this horse is still alive.

What is West Nile Virus?
West Nile Virus (WNV) is a mosquito-borne virus that causes inflammation of the brain and/or inflammation of the lining of the brain and spinal cord. The virus was first found in the U.S. in the fall of 1999 in New York City, and has since spread to midwestern and southern states, including Louisiana.

Birds: the Primary Reservoir Species for WNV
WNV infects and multiplies in birds, which then serve as the reservoir species for the virus. WNV is spread from bird to bird by mosquitoes, when they bite, or take a blood meal, from birds that are infected with the virus. Mosquitoes are also capable of spreading the virus to horses, humans, and other mammals. Migrating birds may have a significant impact on the spread of the virus across the U.S. Birds that are infected with WNV may show signs such as the inability to fly, incoordination, abnormal movements, and death.

How to Protect Your Horse from WNV
A vaccine is available as an aid in control of WNV in horses. The vaccine has been shown safe for use in horses. The WNV vaccine is similar to vaccines against Eastern Equine Encephalitis and Western Equine Encephalitis and is only available through licensed veterinarians. Horses vaccinated against Eastern, Western, and Venezuelan equine encephalitis are not protected against infection with West Nile Virus. Horse owners are advised to consult their veterinarian concerning West Nile Virus vaccination of their horses. Vaccinated horses will test positive on WNV tests.

In addition to the vaccine for horses, simple insect control measures should be utilized. Consider the use of insect repellents, and place horses in barns/stables under fans during dusk, dawn, and other times when mosquitoes are present. Eliminate opportunities for mosquito breeding by draining wet areas of pasture, filling puddles, repairing, gutters, etc., clearing any containers that might hold even small pools of water, and draining water tanks once or twice weekly.

West Nile Virus in Horses
Most horses bitten by carrier mosquitoes do not develop disease. Of those that do, approximately one-third develop severe disease and die or are so affected that euthanasia is required. The incubation period, or the time between the bite of an infected mosquito and when clinical signs appear, ranges from seven to 14 days.

Symptoms of West Nile
Although horses do not usually develop clinical symptoms of West Nile Virus, horses that do become ill may show symptoms ranging from very mild signs to deadly illness. Typical signs include incoordination, stumbling, and limb weakness that either appear suddenly or appear gradually and worsen sleepiness, dullness, listlessness, droopy eyelids, lower lip, and an inability to rise. Some horses may develop mild fevers, blindness, muscle trembling, seizures, and other signs. Horses can not spread the disease to humans, but humans are susceptible to the disease if bitten by a carrier mosquito.

Diagnosis of WNV
Diagnosis of West Nile virus in horses involves testing the blood serum for antibodies against the virus. Since 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), and Equine Protozoal Myeloencephalitis (EPM), before making a diagnosis.

Blood samples should be collected by a veterinarian and sent to the State Animal Health Diagnostic Laboratory at Louisiana State University. The heads of horses that die or are euthanized due to neurological disease should be submitted for testing to the diagnostic lab.

Treatment and Long term Effects of WNV
Currently, there is no specific treatment for the disease caused by West Nile Virus. Supportive therapy should be administered by the horse's veterinarian to reduce clinical signs and the possibility for secondary infections. When a horse becomes infected with the West Nile Virus, that horse develops antibodies in response to the infection. If the horse develops encephalitis, there may not be full recovery and the horse may possibly have permanent central nervous system damage.

For more information contact Renita Marshall, DVM, director of livestock programs, (225) 771-2242 ext. 330.

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