West Nile - Frequently Asked Questions

  • Q: Where did West Nile come from

    A: Outbreaks of WNV disease have occurred in Africa, Egypt, Israel, Asia, Romania, Russia, and France. Before 1999, however, WNV had never before been found in the Americas. The most likely explanation for its appearance here is that the virus was introduced by an infected bird that was imported or an infected person returning from a country where the virus is common. 1

  • Q: Besides mosquitoes, can you get West Nile Virus from other insects or ticks?

    A: Infected mosquitoes are the primary source of WNV. There is no evidence to suggest that ticks or other insects transmit WNV.1

  • Q: Can dogs, cats and other pets get the West Nile virus?

    A. Yes. But they rarely, if ever, get sick. No cases of West Nile disease have been confirmed in dogs and cats. The virus can infect many species of animals, but few actually get the disease. Most infections have been identified in birds, but West Nile virus has been shown to infect dogs, cats, horses, and domestic rabbits, as well as bats, chipmunks, skunks, and squirrels. 2

  • Q. What is the risk of someone becoming infected with West Nile?

    A. The risk is very low. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one mosquito bite are extremely small.

  • Q. I've heard of 'suspect cases', 'positive cases', 'confirmed cases', and 'probable cases'. What does it all mean?

    A. Infections with West Nile virus may, or may not, produce illness. A few people who get ill may seek medical care. Those with severe enough disease that has the appearance of West Nile and similar viruses may have blood or spinal fluid sampled for testing. 2

    Testing of samples has two main components:

    1. Samples may be tested for antibodies to West Nile and other viruses. A positive result indicates that the patient may have been exposed to the virus, but does not prove that the virus is still in the patient.
    2. Samples may also be tested to find the actual virus. This type of test is more difficult, takes much longer, and may not be successful. A negative test does not prove that the virus did not cause the illness.

    Here are definitions of the terms you asked about:

    Suspect: A patient with symptoms similar to West Nile might be considered a suspect by their physician. Since they would also be suspect for a multitude of others diseases with similar signs, they are not counted or reported by Texas.

    Positive case: Many laboratories can test for West Nile and may find positive results. These results are often cross-reactions with other conditions and may not represent a true West Nile positive. Only those results that meet the case definitions for "Confirmed" or "Probable" will be reported as "cases."

    Confirmed: A febrile illness associated with neurologic manifestations ranging from headache to aseptic meningitis or encephalitis, plus at least one of the following:

    • Isolation of WN virus from or demonstration of WN antigen or genomic sequences in tissue, blood, cerebrospinal fluid, or other body fluid;
    • Demonstration of IgM antibody to WN virus in cerebrospinal fluid by IgM-capture enzyme-linked immunoassay (EIA);
    • A > 4-fold serial change in plaque-reduction neutralizing test (PRNT) antibody titer to WN virus in paired, appropriately timed serum or cerebrospinal fluid samples;
    • Demonstration of both WN virus-specific IgM (by EIA) and IgG (screened by EIA or HI and confirmed by PRNT) antibody in a single serum specimen.

    Probable case: Symptoms identical to that for a confirmed case plus one or more of the following:

    • Demonstration of serum IgM antibody against WN virus (by EIA);
    • Demonstration of an elevated titer of WN virus-specific IgG antibody in convalescent-phase serum (screened by EIA or hemagglutination inhibition (HI) and confirmed by PRNT).
  • Q: How do people get West Nile virus?

    Infected Mosquitoes
    Humans primarily get West Nile virus through mosquito bites. WNV is spread to humans by the bite of an infected mosquito. A mosquito gets infected by biting a bird that is infected with the virus. An uninfected mosquito cannot bite an infected human and become infected. Mosquitoes also do not transmit the virus between people.

    Transfusions, Transplants, Mother-to-Child
    Less often, the virus has been spread by blood transfusion, organ transplant, breastfeeding, and from mother to child during pregnancy. Donated blood is check for WNV before being used.

    West Nile virus is NOT spread by casual contact such as touching, kissing, or caring for someone who is infected. 3

  • Q: What are the symptoms of West Nile virus infection?

    Symptoms generally occur 3-14 days following the bite of an infected mosquito.

    Asymptomatic West Nile (most people)
    Most people - 80 percent- of the people who are infected with West Nile have no symptoms or may experience mild illness, then recover fully. Often, these people only know they have West Nile because the virus is detected when they have blood work done for an unrelated reason, such as blood donation or routine lab work.

    West Nile Fever (some people)
    Up to 20 percent of people who are infected with West Nile contract West Nile Fever, a flu-like illness that causes symptoms such as fever, headache, body aches, nausea, and, less frequently, skin rash on chest, stomach and back, or sometimes swollen lymph glands or eye pain.

    West Nile Neuroinvasive Disease (few people)
    Fewer than 1 percent of individuals infected with West Nile can develop West Nile neuroinvasive disease, a serious form of the virus that affects the nervous system. Symptoms include severe headache, high fever, stiff neck, disorientation, muscle weakness, numbness, tremors, vision loss, coma, or paralysis. These symptoms last several weeks, and can cause death or permanent brain damage. The elderly are most at risk for this form of West Nile, but anyone who contracts West Nile has a chance of developing this most severe form. 3

  • Q: Is there a treatment or a vaccine for West Nile virus infection?

    There is no specific treatment, medication, cure, or vaccine. Most people who get this illness recover from it on their own. In more severe cases, people may need to go to the hospital where they can receive supportive care to treat their symptoms.3

  • Q: Can a human get West Nile virus twice?

    Medical professionals don't believe so. Like many other viruses, medical tests indicate that once a person has been infected with West Nile, he/she develops a natural immunity to future infection by the virus for life. However, this immunity may decrease over time or with further health conditions that compromise the immune system. Because West Nile has not been active in the United States for many years, future tests may tell us more about the long-term health implications of West Nile. 3