West Nile Fever is a disease that is relatively new to the U.S. The first known cases of the disease occurred in the summer of 1999 in New York City. The disease is caused by the West Nile Virus and is therefore frequently referred to simply as "WNV". The disease gets its name from the West Nile District of Uganda in Africa where it was first recognized in humans in 1937. The first cases of WNV in horses were recognized in France and Egypt in the 1960's. WNV occurs routinely in Africa, the Middle East and parts of southern Europe. The strain of WNV that has appeared in the U.S. is most closely related to a strain found in Israel in 1998. It is not known how the virus came to the U.S. but, given the location of the first outbreak in New York City, it is likely that it arrived via plane or boat in an infected bird or mosquito.
WNV infection has been documented in the U.S. primarily in wild birds, humans and horses, but a few cases have also occurred in other species such as sheep, bats, squirrels, chipmunks, raccoons, skunks, rabbits, cats, and dogs. There were 15,257 confirmed clinical cases of WNV in horses during the year 2002 from 43 states. In Virginia, there were a total of 45 equine cases in 2002, from 12 different localities. Of those 45 cases, 17 died or were euthanized due to their illness. Horses of all ages, young and old, have been affected.
Transmission: WNV usually lives in wild birds of many different species. Mosquitoes transmit the virus from bird to bird. Occasionally a mosquito that has bitten an infected bird will then bite a human, horse or other mammal and transmit WNV to them. The virus does not multiply enough in horses and humans to be picked up by a mosquito and transmitted to another host so horse-to-horse, horse-to-human or human-to-horse transmission is not likely either. Many types of mosquitoes have been shown to be capable of transmitting WNV. This is important because different species of mosquitoes have different feeding and habitat requirements, some feed during the day and others from dusk to dawn. Some species of mosquitoes prefer brackish or marshy water to breed in, others are "container breeders" which will lay eggs in anything that holds water (even a bottle cap). Regardless of these differences, all mosquitoes require a watery environment of some sort for their eggs to hatch so control of these watery breeding sites is the single best way both to control mosquitoes and prevent them from transmitting diseases such as WNV.
Symptoms: While WNV is found in many tissues in the normal bird host (liver, kidneys, heart, brain), in horses it seems to like the brain most, causing inflammation of the brain (encephalitis) which leads to a loss of coordination, lack of interest in their surroundings and loss of appetite and can cause the horse to go down and be unable to get up without help. Even though the disease is called West Nile Fever, affected horses may not develop a noticeable fever although some will. Over the last several years of the outbreak in North America, approximately 33% of affected horses either die or are euthanized because of the severity of their condition. The good news is though, unlike some other encephalitis viruses such as Eastern Equine Encephalitis (EEE), horses that make it through the 2-3 weeks of WNV encephalitis generally recover fully with no long term problems. There are many other diseases of horses that may look like WNV including Rabies, EEE, Moldy Corn Poisoning and EPM (Equine Protozoal Myelitis) so a licensed veterinarian should always be called to examine any animal with signs of neurologic (brain) problems.
Diagnosis: There is a blood test that can be run on a live horse that is showing the symptoms described above, but, to be accurate, it must be done within the first 7-10 days after the horse becomes ill. Alternatively, if the horse dies or is euthanized, at least the head and preferably the entire carcass should be submitted to the nearest VDACS Regional Animal Health Laboratory for a complete post-mortem examination. There are currently no live-animal tests for WNV in animals other than horses and chickens but testing can be done on any dead animal. Contact the nearest VDACS Regional Animal Health Laboratory for more information.
Treatment: There is no drug available currently to kill WNV. Treatment consists of supportive therapy to prevent the animal from injuring themselves and nurse them through the 2-3 weeks of the disease. Please consult with your veterinarian for treatment tailored to the particular case.
Vaccine: A WNV vaccine for horses has received full approval from the USDA. Owners should contact their private veterinarian for further information on the availability and use of the vaccine.
Prevention: The most effective method for preventing WNV infections is to control the population of mosquitoes on your property. As mentioned above, this is related to eliminating breeding sites for mosquitoes. Suggestions include:
- Empty water containers (troughs, buckets etc.) at least twice a week.
- Look for other places such as open boats, tarps, old tires, wagons, plant saucers, clogged rain gutters etc. where standing water may gather temporarily and empty them.
- For permanent water structures such as ponds, consider stocking with "mosquito fish" that are natural predators of mosquitoes or using "dunks", floating donuts that contain a chemical to kill mosquito larvae (available at many feed/farm supply stores).
- Improve drainage for all low-lying areas that catch and hold water after it rains.
- Consider putting up bat houses or otherwise encouraging mosquito eating birds to visit your property.
- If you are fortunate enough to live in an area that has a Mosquito Control District, ask them to come survey your property and make recommendations for mosquito control.
Protection for your horse includes:
- Keep your horse away from wet or swampy areas where mosquitoes breed.
- Apply mosquito repellents labeled for use on horses according to label directions.
- Apply premises sprays labeled for use in infested areas according to label directions.
- Apply foggers labeled for use in barns and stables according to label directions.
NOTE: If you purchase products through the mail or over the Internet and are unsure if they are registered in Virginia, please inquire with the retailer. Virginia law requires that all products used in Virginia be registered with the Department of Agriculture and Consumer Services.