Prevention Tests and Treatment of West Nile Virus

Prevention Tests and Treatment of West Nile Virus

By: Katherine Robinson

Prevention of West Nile Virus

            The prevention of West Nile virus infection of people and horses is very similar. The best way to prevent West Nile virus is to prevent the mosquitoes from biting. This can be done in many different ways which include reducing the mosquito population, using insect repellent in many different forms, and being conscious of the hours that they are out and ready to bite. The mosquitoes feed the most at night on hot days so horses in endemic areas should be kept in the barns at night time. All stagnant water should be dumped to prevent a site for mosquitoes to replicate. Frequently removing feces and keeping fans on around horses to keep the air moving will help reduce mosquitoes in the barns. Mosquito repellents that are approved for the horse should be applied to reduce the possibility of bites. In horses, vaccination is the primary method to decrease the risk of infection, but doesn’t fully protect from clinical disease.  Dead birds can indicate WNV is in the area so dead birds should be reported to the local authorities to help monitor for presence of WNV.

            Since risk of exposure varies, and West Nile virus is such a serious disease, it is recommended by the American Association of Equine Practitioners (AAEP), that all horses in North America be vaccinated for the virus. There is an inactivated vaccine and live canarypox vaccine. The vaccine has been shown to prevent viremia in horses for 12 months after the vaccine has been given, but should be given more frequently, every 6 months, in West Nile virus endemic regions. Vaccination is best in the spring before the vector season reaches its prime. Mares should be vaccinated before they are bred because neither of the vaccines is licensed for pregnant mares, although it has become an accepted practice to vaccinate horses while pregnant because the risks of infection outweigh the risks of adverse vaccine effects. However, it should be noted that modified, live vaccines should not be used in pregnant animals. Mares are also vaccinated 4-6 weeks before foaling to pass viral immunity along to their foals through colostrum. Foals should receive a vaccine of their own at 3-4 months, after the colostral immunity has worn off, then again at 1 year.

Testing for West Nile Virus

            Identifying infected horses should be done with an antibody test and should use horse serum or CSF. The horses that are tested are ones who show clinical signs of WNV, as this is not a routine test done on all horses. IgM capture ELISA, or plaque-reduction neutralization are the available tests for veterinarian diagnosis. IgM antibodies are expressed earlier and at a higher concentration then IgG in WNV infected horses. Tests should detect IgM rather then IgG because it is a better sign of an active infection. IgG tests can be done to test for previous infection of WNV or secondary infection. When testing for IgM it is important to test early in infection because the antibodies do start to decrease around 4 days after clinical sign onset. Tissues from dead horses should be analyzed with real time PCR.

West Nile Virus Treatment

            Treatment for horses that are infected is supportive, like treatment for most other viral infection. Horses should receive fluids and appropriate nutrients.  If they are not eating they should be tube fed. There is no need for the horse to be euthanized.  There is no evidence of spread from infected horses to other horses or people because of the low level of WNV in their blood. Infected birds, however, have high levels of WNV in blood and can be a source of infection to humans and horses.  Virus is transmitted from infected birds to humans and horses by mosquitoes. 

Questions to test your knowledge

1. Which is not a way to prevent and monitor infection with West Nile virus?

1. Empty stagnant water that is sitting around.
2. Report dead birds in your area.
3. Apply mosquito repellant to you or your horse
4. Stay indoors during the night when the mosquitoes are out
5. All of the above prevent infection

2. Pregnant mares in West Nile virus endemic areas should be vaccinated even though the vaccine is not licensed for pregnant animals and there can be adverse effects. (True/ False)

3.  Horses should be euthanized if they are infected with WNV because of viremia spread.     (True/ False)

4. Why should foals receive vaccinations after 3-4 months?

A. Foals are not susceptible to the virus when they are younger then 4 months

B. The colostral immunity wears off at 3-4 months

C. The placental immunity wears off at 3-4 months

D. All of the above

5. It is important to keep your horse away from others if it is infected with WNV, so mosquitoes cannot transfer the virus to other horses. (True/ False)

Answers:

1. E.
2. True
3. False
4. B.
5. False – horses don’t have viremia at high enough levels or for long enough to worry about transfer through mosquitoes

Resources;

California. West Nile Virus. Web.

            <http://westnile.ca.gov/prevention.htm>.

United States. CDC. West Nile Virus. Web. <http://www.cdc.gov/ncidod/dvbid/westnile/>.

"West Nile Virus Vaccination Guidelines." . AAEP, 2005.

            Web. 15 Apr 2012.

            <http://www.aaep.org/pdfs/AAEP_WNV_Guidelines_2005.pdf>.