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West Nile Virus FAQS

West Nile Virus FAQS  


Eric T. Natwick

Q: What is WNV and where did it come from?

A. West Nile Virus (WNV) is a virus transmitted by mosquitoes that can cause flu-like symptoms, encephalitis, or death. WNV is commonly found in Africa, Eastern Europe, West Asia, and the Middle East, but until 1999 had not been documented in the U.S. and was found in Imperial County this summer. WNV viruses infect humans, birds, mosquitoes, horses and some other mammals.

Q: What is West Nile fever and West Nile encephalitis?

A. West Nile fever is an infection from WNV resulting in mild flu-like symptoms, typically lasting only a few days and does not cause long-term health problems. West Nile encephalitis is a severe illness with symptoms including headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.

Q. Who is at risk of severe illness due to WNV? Who is at risk of death?

A. Most people infected with WNV have no symptoms whatsoever. About 20% of infected develop West Nile fever including mild symptoms of fever, headache, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands lasting a few days. About 1 in 150 persons infected with WNV develop a more severe form of disease resulting in encephalitis, meningitis, or meningoencephalitis, requiring hospitalization. Symptoms of severe disease may last several weeks, although neurological effects may be permanent. More than 4,000 cases of WNV were confirmed in the U.S. in 2002 and 6% of those people died. People over the age of 50 are most susceptible to severe symptoms requiring hospitalization.

Q. What should I do if I think I have symptoms of WNV infection?

A. See your doctor immediately if you or a family member develops symptoms of high fever, confusion, muscle weakness, and severe headaches. WNV is diagnosed by laboratory tests on blood serum of cerebrospinal fluid. Medical attention is not needed for mosquito bites without symptoms.

Q. How do people get infected with WNV?

A. People become infected by the bite of a mosquito infected with WNV. Mosquitoes get WNV from birds. Most mosquito bites will not transmit the disease, but it only takes the bite of one mosquito carrying the WNV to potentially cause an infection. There is an incubation time of 5-15 days between the time a person is bitten and the onset of illness.

Q: Where do the mosquitoes come from and is there any danger in being bitten?

A. Mosquitoes breed in pools of water from home landscapes, in neglected birdbaths or swimming pools, water puddles from rain, and in tail water pools from irrigated crops. The two mosquito species most likely to spread WNV in Imperial County, Culex tarsalis and Culex pipiens. These mosquitoes actively seeking a blood meal from dusk until dawn.

Q. How can I reduce my risk of becoming infected with WNV?

 A. There are no human vaccines to prevent WNV infection at this time, but several companies are working towards developing a vaccine.

Avoid spending time outside when mosquitoes are most active, from dusk until dawn. Wear loose fitting, long-sleeved shirts and long pants when outdoors. Mosquitoes can bite through thin clothing tight fitting clothing. Treat clothing with repellents containing permethrin or DEET. DO NOT apply permethrin to your exposed skin!

Apply only enough DEET to lightly cover skin and clothing, following label instructions. DO NOT apply DEET liberally. MORE IS NOT BETTER! Do not spray DEET indoors. DEET doesn’t kill mosquitoes; it only repels them within a few inches of a treated surface.

DO NOT spray DEET toward your face or on children. Use a DEET wipe formulation or apply DEET to your hand and wipe on children or to your facial skin including behind the ears.

DEET only prevents mosquitoes from landing on treated surfaces; coverage is critical. An application of DEET usually lasts from 2 to 4 hours before reapplication is needs. Flying mosquitoes don’t bite. Reapply when mosquitoes begin to land on skin or clothing.

DEET has been safely used for over 50 years. A very low percentage of people may develop a mild rash. The rash may be due to the formulation and not to the DEET. If you get a rash from DEET sprays, DEET wipes, or clear liquid formulations, you may be reacting to alcohols or other carriers in the formulation. Try a DEET cream formulation that does not contain alcohol.

Mosquito netting can be placed over infant carriers when outdoors. Window and door should have tight fitting screens, kept in good repair. Repair or replace screens with tears or holes. Caulk around windows and doors. Install door sweeps or thresholds at the base of exterior doors.

Draining standing water where mosquitoes can breed. Empty water from flowerpots, pet water dishes, birdbaths, swimming pool covers, buckets, barrels, and cans weekly. Clean out clogged rain gutters and remove discarded tires, and other items that collect water.

If you are experiencing high mosquito populations in areas where you live, work or go to school, contact the Imperial County Health Department Vector Control office.


There are an estimated one million horses in California and approximately 1,500 horses in Imperial County. West Nile Virus (WNV) is now in Imperial County and horses are at risk of becoming infected. There is no reason to destroy a horse just because it has been infected with West Nile virus. Most horses infected with WNV virus recover, but WNV has caused deaths in about 10 to 30 percent of the horses infected.

Q. How do I know if my horse has West Nile Virus?

A. A positive diagnosis can only be determined through laboratory analysis from blood serum or cerebrospinal fluid extracted by a veterinarian. Horses show symptoms of West Nile encephalitis or myelitis when the virus crosses the blood-brain barrier and causes inflammation of the brain and/or the spinal cord. Symptoms of WNV infection include depression and appetite loss or inability to swallow, along with one of more of the following: stumbling or lack of coordination, weakness of limbs, partial paralysis, muscle twitching, fever, muzzle twitching, impaired vision, head pressing, circling, aimless wandering, convulsions, and occasionally coma and death. If you suspect that a horse is infected with WNV, contact your veterinarian. It is important to note that not all horses with clinical signs of encephalitis have West Nile encephalitis.

Q. How do the horses become infected with West Nile virus?

A. Horses become infected with WNV by the bites of infectious mosquitoes. Mosquitoes become infected when they feed on infected birds. When an infected mosquitoes feeds on a horse, the virus is injected through salivary fluids into the blood system. The virus multiplies in the horse and may cause illness in 5 to 15 days.

Q. Can my horse transmit West Nile virus to me or to other horses? A.

There is no evidence that horses can transmit WNV to other horses, birds, or people. However, follow veterinary infection control precautions when caring for a horse suspected to have this or any viral infection.

Q. Will my horse be protected from West Nile virus if it is vaccinated for other diseases?

A. There is no cross-protection for WNV from vaccines for other mosquito transmitted encephalitis viruses such as eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE).

Q. Is there a vaccine to protect my horse against West Nile virus infection?

A. Fort Dodge Animal Health developed a WNV horse vaccine granted full approval U.S. Department of Agriculture. The vaccine is effective in preventing West Nile encephalitis in 95 percent of vaccinated, according to the company. It is important that horses are vaccinated twice, 3 to 6 weeks apart with the WNV vaccine. One to two boosters are recommended after the initial series. Avoid vaccinating pregnant mares in the first trimester of gestation. Talk to your veterinarian about vaccinating pregnant mares after the first trimester.

Q. How can I care for my horse if it is infected with West Nile virus? Is there a treatment?

A. There is no specific treatment. Supportive care consistent with standard veterinary practices for animals infected with a viral agent is recommended.

Q. How can I protect my horse from mosquito bites?

A. You can I prevent mosquitoes from affecting horses by following these recommendations:

  • Keep horses indoors during peak hours of mosquito activity from dusk to dawn.
  • Mosquitoes are attracted to incandescent light bulbs, so avoid turning on lights inside the stable during the evening and overnight. Ultraviolet light (black light) is not as attractive to mosquitoes. Incandescent bulbs may be placed around the perimeter of the stable to attract mosquitoes away from the horses.
  • Enclose stables with a fine mesh screen to exclude mosquitoes.
  • Keep birds, including chickens, away from stables.
  • Eliminate standing water on your property, including, used tires, manure storage pits and drainage areas with stagnant water.
  • Topical mosquito repellents are available for horses. Read and follow the product label instructions before using.
  • Use of strong blowers or fans on the horses while in the stable can help deter mosquitoes.
  • Fog stable premises with a pesticide in the evening to reduce mosquitoes carefully following label directions. For help in assessing mosquito exposure risks on your property and for suggested control practices, please contact Imperial County Department Health Department, Vector Control.