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West Nile Virus

West Nile virus (WNV) likely spread to the U.S. in 1999, having been endemic in the Middle East, Eastern Europe, and Africa, and is regularly active along the western Gulf coast, during mosquito high-season. it has now spread to a majority of U.S. states.


WNV most commonly infects birds, horses, and humans (also dogs, cats, rabbits, bats, chipmunks, skunks, and squirrels). Mosquitoes transmit WNV between these reservoirs. Culex mosquitoes, which feed mostly dusk to dawn, are the main WNV vehicle on the Gulf coast. Recently, Asian tiger mosquitoes have also been found infected with WNV in Texas. This may be significant since they feed around the clock and, as opposed to Culex, breed in small areas of standing water (tires, buckets, etc). Even where WNV is endemic, very few mosquitoes are infected. They may also be more resistant to repellents.

Report dead birds to Animal Control and do not handle with bare hands, though there is no evidence that this is a risk for acquiring WNV. Birds can also die from chemical spills, pesticides, drought, severe weather, and other diseases.

There is no evidence of human-human or human-animal spread.


  • Incubation: 3-4 days
  • Average Illness Duration: 3-6 days


Description/Severity Symptoms % of all individuals infected with WNV

(not ill)

No symptoms Approximately 80% of all cases
Mildly-moderately ill Fever, headache, muscle/joint aches, rash, swollen glands; typically lasts 3-6 days Approximately 20% of all cases
Severely Ill (brain inflammation; AKA encephalitis) In addition to those above: acute intellectual and/or behavioral abnormalities, stiff neck, light aversion (photophobia), severe headache, and/or nervous system abnormalities (coma, paralysis, seizures, etc.) Less than 1%

About 1 in 150 cases. Typically in individuals over 50 years old.

3-15% of these cases are fatal (0.3-0.15 of all illness cases)


Test Specimen/Turnaround Turnaround Notes
WNV Antibody Testing (EIA)* Spinal fluid or serum (blood)Usually a second specimen is desirable 2 weeks after the first one 3-5 days Specimen procured by routine medical lab and forwarded to: Bureau of Laboratories, Texas Department of Health (TDH), 1100 West 49th Street, Austin, TX 75756.
WNV Cultures (cell) Spinal fluid or brain tissue (from ill patient) = 10 days Not performed on blood because virus only present there 2-3 days, early in illness
WNV Genetic Polymerase Chain Reaction (PCR)     Detects viral RNA in specimens


  • An initial WNV antibody positive result does not prove active disease or infection; may be antibody from the past (and the individual is now immune). This is the reason for 2 specimens, 2 weeks apart – to look for significant antibody change over the course of illness, suggesting active infection. A single, positive WNV IgM antibody is suggestive of acute, active WNV infection.
  • False antibody positives: can result from recent vaccinations against or illness from other flaviviruses: yellow fever, dengue, Japanese encephalitis.


General medical support, no specific anti-WNV medication.
*NOTE: Important to seek medical evaluation because there are other causes of encephalitis (such as Herpes simplex virus) which are treatable, and potentially fatal or severely damaging without treatment.



  • No vaccine yet (for humans or animals); work in progress (vaccine human trials in 2003).
  • It is currently assumed WNV infected individuals (even if not ill) develop lifelong immunity, but this has not been established.

Mosquito-Bite Avoidance

  • Long-sleeved shirts/pants outdoors whenever possible, especially at dawn and dusk
    • Loose, white clothing at night
    • Minimize outdoor activities at dawn, dusk, and early evening.
  • Chemical barriers
    • DEET Repellents
      • Adults – 35% concentration; greater strength not more effective
      • Children – lower strength (less than 10%) DEET, applied more frequently; avoid under 2 years old and do not put on kids’ hands – may get into their mouth/eyes and cause irritation.

NOTE: Best Brands of DEET by Duration (Hrs)

Brand (Manufacturer) % DEET Duration (Hr)
OFF! Deep Woods (SC Johnson) 23.8% 4.5-5.5
Sawyer Controlled Release (Sawyer) 20% 3.5-4.5
OFF! Skintastic (SC Johnson) 6.65% 2
OFF! Skintastic for Kids (SC Johnson)
4.75% 1-2
  • Permethrin
    • this is an insecticide (actually kills insects that contact it, not just repels): mosquitoes, ticks, lice, bed bugs, others.
    • Clothing is treated with this and it will last 2-3 washings

NOTE: Permethrin excellent for pregnant women and small children, who are unable to use DEET on their skin.

  • Minimize standing water.
    • Old tires, cans, blocked gutters, trash containers, pet bowls, flowerpots, etc.
    • Empty or cover swimming pools not in use.
    • Change birdbath water at least weekly.
  • Living quarters
    • Close and repair windows/doors.
    • Air-condition: cool temperature significantly decreases mosquito activity.


  • Harris County Mosquito Control: (713) 440-4800
  • National Pesticide Information Center: 1-800-858-7378