News & Information
West Nile Virus

Edward R. Rensimer, MD, FACP - Last Updated 6/1/2003

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 currently active along the western Gulf coast, during mosquito high-season.

I. SPREAD

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.

II. DISEASE

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

Description/Severity

Description/Severity	Symptoms	% of all Individuals infected with WNV
Asymptomatic (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)

III. DIAGNOSTIC TESTING

Test	Specimen/Turnaround	Turn-around	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

* NOTE:

• 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.
• False antibody positives: can result from recent vaccinations against or illness from other flavivirus: yellow fever, dengue, Japanese encephalitis.

IV. TREATMENT

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.
 

V. PREVENTION

Immunity

• No vaccine yet (for humans or animal); work in progress (vaccine human trials in 2003).
• It is currently assumed WNV infected individuals (even if not ill) develop lifelong immunity.

Mosquito-Bite Avoidance

1. 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.

2. Chemical barriers

1) 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)	20.0%	3.5-4.5
OFF! Skintastic (SC Johnson)	6.65%	2
OFF! Skintastic for Kids (SC Johnson)	4.75%	1-2

2) Permethrin

• this is an insecticide (actually kills insects that contact, 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.

3) 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.

4) Living quarters

• Close and repair windows/doors.
• Air-condition: cool temperature significantly decreases mosquito activity.

VI. OTHER SOURCES OF INFORMATION

1. Harris County Mosquito Control: (713) 440-4800
2. www.harriscountyhealth.com
3. www.cdc.gov
4. National Pesticide Information Center: 1-800-858-7378
5. www.tdh.state.tx.us/zoonosis/diseases/arboviral/westnile/westnile.asp
6. www.westnilefever.com