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Medical Index
First Posted: July 6, 2007
Mar 18, 2015

West Nile Virus

"West Nile Virus is a mosquito-borne zoonotic (affecting humans and specific animal species) arbovirus belonging to the genus Flavivirus in the family Flaviviridae. It is commonly found in temperate and tropical regions of the world. Horses are highly susceptible to the virus, which causes the neurologic disease West Nile encephalitis. AAEP Core Vaccines: Eastern/Western equine encephalitis, Rabies, Tetanus, and West Nile virus. ..." West Nile Virus in US Horses

Note: One of my horses had very bad reactions to Ft. Dodge vaccines. He reacted badly to the West Nile Virus injection and to the Rhino Flu injection. If you have this problem two other manufacturers were given to me. Many vets will not order single dose shots because they get orders of 10 shots at a cheaper rate. They may also feel more comfortable with a vaccine that they are use to using. Whatever the reason, you may find yourself having to address this problem on your own. Suggestions: You can, however, order your own injections from Valley Vet Supply . They will overnight the vaccine to you--it must be refrigerated. Vestra is what they have and is made by a reputable manufacturer. Another brand is Murial . I have located a local vet who uses the murial brand of the West Nile Virus vaccine. I suspect A Patchy will do fine with that different manufacturer. He will be getting his shots the end of March and I will upload the result. I might add that we do give him bute for two days before the injection and one day after the injection. Some people will premedicate with banamine or trihist. Always check with your vet first. West Nile Virus is prevalent in our area and is a deadly disease. This shot is not an option for my horses. I do not give the Rhino Flu to my reactive horse anymore. The barn is stable, no horses coming and going. Also the three horses are all geldings. I have opted to not plague him with this shot. If there is an outbreak in our area I will, of course, give him the shot along with premedications to reduce his reactions. Hopefully he will not get this disease, but if he does this disease it is not deadly and is treatable. I do titers on him to see where his immunity level stands. If the titers indicate that another shot is necessary I will have it given to him, of course.


I took the above pictures of A Patchy after his injection with Rhino Flu vaccine made by Fort Dodge. He had been premedicated with bute, as stated above. A Patchy had the same type of reaction, but more severe, the next time he was inoculated with a Ft. Dodge West Nile Virus vaccine. I no longer give him any Ft. Dodge injections. He seems to be sensitive to whatever adjuvants Ft. Dodge uses. (Adjuvants are pharmacological or immunological agents that modify the effect of other agents {e.g., drugs, vaccines} while having few if any direct effects).

Different West Nile Virus Genetic Lineage Evolving?

Referenced from Ohio State University
What Horse Owners Should Know About West Nile Virus

Updated Information

"West Nile-Innovator DNA, a vaccine for horses to aid in the prevention of viremia caused by the potentially deadly West Nile virus, was launched by Fort Dodge Animal Health, a division of Wyeth, Dec. 4. Developed in collaboration with the U.S. Centers for Disease Control and Prevention (CDC), this represents a tremendous milestone in DNA science and vaccine technology, as it is the first DNA vaccine of its kind ever registered and the first DNA vaccine fully licensed by the USDA.

'Fort Dodge, which has a long-standing commitment to innovative research and product development, recognized the significant value of DNA in the prevention of infectious diseases in animal populations, as well as humans, and worked with the CDC to develop this product," said Steve Chu, DVM, PhD, executive vice president of Animal Health Research and Development, Fort Dodge Animal Health. "During this process, Fort Dodge made some groundbreaking discoveries that improved the preparation and formulation of the vaccine, enabling the successful development, testing and registration of the first product of its kind.'" West Nile Virus DNA Vaccine for Horses Introduced

Recent epidemiological data suggests that vaccinated horses are three to 16 times less likely to die from West Nile Virus infections than are non-vaccinated horses. Few adverse reactions have been reported from the vaccine.

West Nile Virus vaccine requires annual boosters. It is recommended that horses receive their boosters in May, to coincide with the onset of mosquito season.

If horses have not previously been vaccinated, a two-dose schedule is recommended. Different vaccine products will vary slightly in the interval between vaccinations, so label directions should be closely followed. Foals experiencing their first mosquito season in South Dakota should get a series of two or three vaccine doses, generally beginning when they are three to four months of age. Epidemiological data suggests that vaccinated horses are three to 16 times less likely to die from West Nile Virus infections than are non-vaccinated horses. Few adverse reactions have been reported from the vaccine. West Nile Virus vaccine requires annual boosters. It is recommended that horses receive their boosters in May, to coincide with the onset of mosquito season.

West Nile Virus (WNV) is a viral disease that can cause encephalitis or meningitis. (Infection of the brain or spinal cord or their protective coverings) It originated in Africa, and spread to Asia, and Southern Europe. WNV is now in the United States.

How Does A Horse Contract West Nile Virus?

Horses become infected with WNV after being bitten by an infected mosquito. There is no evidence that horses can transmit WNV to other horses, birds, or people. Some symptoms of West Nile Virus include neurological symptoms: paralysis in the hind limbs, weakness in the hind limbs, and ataxia (incoordination). There may also be loss of appetite, lethargy, fever, twitching in the muzzle, vision loss, convulsions, difficulty swallowing, circling, wandering, and extreme excitability. Sometimes horses go into a coma. Other diseases also cause symptoms as described above including rabies, botulism, EPM, and other mosquito borne viral encephalitic diseases in horses. Contact your vet. There is no treatment for WNV.


Currently, there are four vaccines available against West Nile Virus. It is imperative that horses are vaccinated according to the label on the vaccine. At least one yearly booster is recommended after the initial series. Horses that are stressed, such as show and race horses, should have two boosters annually, in April and late July. Horses vaccinated against Eastern, Western, and Venezuelan equine encephalitis are not protected against West Nile Virus.

Protect Your Horse From Mosquitos
  • House horses indoors during peak periods of mosquito activity (dusk and dawn).
  • Avoid turning on lights inside the stable during the evening and overnight. Mosquitoes are attracted to lights.
  • Place incandescent bulbs around the perimeter of the stable to attract mosquitoes away from the horses. Black lights do not attract mosquitoes well.
  • Remove all birds, including chickens, that are in or close to the stable.
  • Look around the property periodically for dead birds, such as crows. Any dead birds should be reported to the local health department. Use rubber gloves to handle dead birds or use an implement, such as a shovel.
  • Eliminate areas of standing water on your property. Shallow standing water, used tires, manure storage pits, and drainage areas with stagnant water are ideal mosquito breeding places.
  • Topical preparations containing mosquito repellents are available for horses. Read the product label before using and follow all instructions.
  • Use fans on the horses while in the stable to help deter mosquitoes.
  • Fog stable premises with a pesticide in the evening to reduce mosquitoes. Read directions carefully before using.
  • For help in assessing mosquito exposure risks on your property and for suggested control practices, please contact your county Extension office, county Department of Environmental Protection, local Department of Health, local veterinarian, or a mosquito and pest control company.
Mosquito Reduction

You can reduce the number of mosquitoes around your home and neighborhood by reducing the amount of standing water available for mosquito breeding. Here are some simple steps you can take.

  • Dispose of tin cans, plastic containers, ceramic pots, or similar water-holding containers on your property.
  • Pay special attention to discarded tires. That's where lots of mosquitoes breed.
  • Clean clogged roof gutters every year, particularly if the leaves from surrounding trees have a tendency to plug up the drains. Millions of mosquitoes can breed in roof gutters each season.
  • Turn over plastic wading pools when not in use. A wading pool becomes a place for mosquitoes to breed.
  • Turn over wheelbarrows and do not let water stagnate in birdbaths. Both provide breeding habitats for domestic mosquitoes.
  • Aerate ornamental pools or stock them with fish. Water gardens can become major mosquito producers if they are allowed to stagnate.
  • Clean and chlorinate swimming pools when not in use. A swimming pool left untended by a family on vacation for a month can produce enough mosquitoes to infest an entire neighborhood. Mosquitoes may even breed in the water that collects on pool covers.
  • Use landscaping to eliminate standing water that collects on your property. Mosquitoes may breed in any puddle that lasts for more than four days.
  • Can a horse with West Nile Virus infect horses in neighboring stalls?

No. There is no documented evidence that West Nile Virus is transmitted from horse-to-horse. However, if at all possible, horses with suspected West Nile Virus should be isolated from mosquitoes and tested for the virus.


Clinical signs for WNV include flulike signs, anorexic, depressed; skin and muscle twitching, mental changes, daydreaming, drowsiness, walking forward, often without control, and "spinal" signs, including asymmetrical weakness, and symmetrical ataxia (incoordination on one or both sides, respectively). Equine mortality rate can be as high as 30-40%.

Incubation Period

The incubation period for a West Nile Virus infection is usually five to 15 days.

Newest Research Results

Coat Color and West Nile Virus

The newest research from Western College of Veterinary Medicine in Saskatchewan, Canada has found that it appears that light colored horses diagnosed with WNV may have a higher mortality rate than darker colored horses. Horses were divided into three groups:

  • Dark - Chestnut, black, bay, brown, sorrel
  • Light - Gray, white, palomino, buckskin, dun
  • Multicolored - Roan, paint, pinto, appaloosa

Here are the results of the study: There were a total of 124 horses in the study group diagnosed with WNV.

  • Dark colored with the most infected with WNV, 89 of the 124 total cases. However, the dark colored horses had the smallest mortality rate. (36%)
  • Light colored or multicolored horses had a mortality rate of 74%, although the numbers in the study group were less. The study found that this group was "4.4 times more likely to die or be euthanized than dark colored horses." Canadian Veterinary Journal in November 2007

How the immune system responds may have something to do with a genetic factor and coat color.

For More Information:

West Nile Virus/theHorse
West Nile Virus (Excellent)

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