What is West Nile Virus?
West Nile Virus (WMV) is a mosquito-borne virus that causes Encephalitis (inflammation of the brain) and/or Meningitis (inflammation of the lining of the brain and spinal cord). Outbreaks of West Nile Virus have occurred in Egypt, Asia, Israel, Africa, and some parts of Europe and Australia. The virus was first found in the U.S. in the fall of 1999 in New York City and has spread to southern and Midwestern states, including Michigan.

What animal carries the virus?
BIRDS: THE PRIMARY RESERVOIR SPECIES FOR WEST NILE VIRUS. WNV infects and multiplies in birds, which then serves as the reservoir species for the virus. At least 76 types of birds are capable of carrying the virus, with American Crows frequently dying from the infection with mild or no indication of the disease.
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 United States.
Birds that are infected with the WNV may show signs such as the inability to fly, incoordination, abdominal movements and death.

West Nile Virus in Horses
Most horses bitten by a carrier mosquitoes do not develop the WNV disease. Of those that do, approximately 1/3 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 7 to 14 days.

Although horses do not usually develop clinical symptoms of WNV, horses that do become ill may show symptoms ranging from very mild signs to deadly illness. Typical signs include Atazia (incoordination, stumbling, limb weakness) that either appears suddenly or appears gradually and worsens, sleepiness, dullness, listlessness, facial paralysis (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.

West Nile Virus in Other Mammals
WNV may be capable of infecting mammals other than horses or humans. Antibodies have been found in blood samples from bats, cats, chipmunks, Gray Squirrels, domestic rabbits, Eastern Striped Skunks, Cows, sheep, and pigs. Although some of these animals may become ill, unlike horses and humans, they do not show signs of or develop Encephalitis.
There is no evidence that infected hoses, humans, or other animals are able to transmit the virus to other animals, people or mosquitoes. Only a wild bird/mosquito transmission has been proven as a means of transmitting West Nile virus.

Diagnosis of WNV in Horses
Diagnosis of WNV 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 National Veterinary Services Laboratory or to the Animal Health Diagnostic Laboratory (AHDL) at Michigan State University. The heads of horses that die or are euthanized due to neurological disease should be submitted for testing to the AHDL. The Michigan Department of Agriculture may provide transportation of the specimen to the AHDL, and may also cover the laboratory expenses.

TREATMENT AND LONG-TERN EFFECTS OF WNV.
Currently, there is no specific treatment for the disease caused by WNV. Supportive therapy should be administered by the horse owner's veterinarian to reduce the clinical signs and possibility of secondary infections. When a horse becomes infected, with or without clinical disease, that horse develops antibodies in response to the infection. Infected horses can acquire long lasting immunity to WNV after recovery due to these antibodies. If the horse develops Encephalitis, there may not be full recovery and the horse may possibly have permanent central nervous system damage.

PROTECTING YOUR HORSE FROM WNV
A vaccine is available as an aid in control of WNV in horses. The vaccine has been shown safe for the use in horses. Effectiveness has not yet been proven; the vaccine has been approved for release under a provisional license until effectiveness is shown, due to the rapidly emerging nature of this disease. The WNV vaccine is a killed adjuvented product similar to vaccines against Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE) 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 animals will test positive for WNV tests.

In addition to the vaccine for horses, simple insect control measures should be utilized. Consider the use of insect repellants, 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 eve troughs, gutters, etc. Clearing any containers that might hold even small pools of water and draining water tanks once or twice weekly.

REPORTING WNV AND OTHER REPORTABLE ANIMAL DISEASES
West Nile virus is a reportable animal disease. If you suspect that an animal may have WNV, you must immediately contact your veterinarian, state veterinarian, or federal veterinarian.