Frequently Asked Questions (FAQs)

West Nile Virus (WNV) is transmitted by mosquitoes that feed off birds infected with the virus. After mosquitoes have been infected with WNV, they may then infect humans, birds, horses, and other animals by biting for blood. Although the majority of people and animals infected with WNV do not show any symptoms, it has the potential to cause mild to severe illness. The virus may cause illness known as West Nile encephalitis (inflammation of the brain).

Historically, the WNV has been commonly found in Africa, Eastern Europe, West Asia, and the Middle East. Until 1999, WNV was not known to be present in the United States. Though scientists are not sure when the virus came to the U.S., they estimate that it was around summer of 1999 on the East Coast.

WNV case information changes daily. Check the CDC website for updated information.

West Nile Virus is transmitted by mosquitoes that have become infected by feeding on infectious birds. Mosquitoes may then infect humans and animals while biting for blood. Most people infected with WNV do not have any symptoms. Some people may develop mild or moderate flu-like symptoms such as headache, fever, and body aches. Severe symptoms include high fever, neck stiffness, and inflammation of the brain (encephalitis). People age 50 and older are at highest risk for serious illness.

After West Nile virus has been transmitted to humans/animals, the virus multiplies in the blood system and travels to the brain. This may cause inflammation of the brain tissue (encephalitis). Encephalitis may lead to death in some cases.

Less than 1 percent of the people infected with West Nile virus will develop severe illness as a result. Fatality rates for those with severe illness caused by West Nile virus are highest in people age 50 and over.

West Nile virus is transmitted by mosquitoes. There is no evidence of person-to-person or animal-to-person transmission of West Nile virus. There is a potential for donated blood or organs to carry the virus. Lab testing is routinely done to minimize the chances of this happening.

Any resident of an area where West Nile virus has been documented is at risk of getting West Nile virus; however, precautions can be taken to avoid mosquito bites (see recommendations below). Most people who do become infected will have either no symptoms, or only mild ones. Persons age 50 years or older have the highest risk of severe disease caused by the virus.

Because West Nile virus is spread by mosquitoes, the best way to prevent infection is to prevent mosquito bites.

• Use insect repellent containing DEET (N, N-diethyl-meta-toluamide). Repellents should contain 30-35% concentration for adults, and 15% for children. Do not use any repellent on infants. Repellent containing concentrations of more than 35% DEET provides no additional protection. Apply repellent sparingly. Picardin is a similar repellent recently approved for use. Note: whenever insecticide or insect repellent is used, be sure to read and follow the manufacturer’s DIRECTIONS FOR USE, as printed on the product.
• Vitamin B and “ultrasonic” devices are NOT known to be effective in preventing mosquito bites.
• Avoid being outdoors between dusk and dawn when mosquitoes are most active and likely to bite.
• Wear loose-fitting, long-sleeved shirts and long pants when outdoors.
• Since mosquitoes may bite through thin clothing, spray clothing with insect repellents containing DEET.
• Install or repair window and door screens so that mosquitoes cannot get indoors. Avoid leaving doors or windows without screens open for long periods of time.
• Eliminate from around your home standing water sources such as blocked or clogged gutters, leaky pipes, air conditioner condensate, evaporative cooler water, bottles, buckets, old tires, and other objects that may hold water. Standing water is a breeding ground for mosquitoes. Swimming pools and fountains do not pose a threat as long as water is kept circulating. Swimming pools that are not maintained need to be covered at all times (covers should be drained of pooled water frequently).
• Change water in birdbaths, planters, and animal watering containers at least twice weekly.
• Control irrigation. Fill low spots and level yards/pastures. Water should never stand more than three days. Check drip irrigation systems for ponding water.
• Mow the lawn regularly. Overgrown yards, especially if they contain tree holes or stumps, can also be attractive breeding sites for mosquitoes.
• Share mosquito control measures among neighbors. Knowledge helps to prevent potential problems.

Since the fall of 2003, West Nile virus has been found in mosquitoes, birds, horses and humans throughout Arizona. In Yavapai County, West Nile virus has been confirmed in most communities. Check the CDC case information maps for the latest updates on the affected states.

No. At this time, the best way to prevent infection of WNV is to avoid mosquito bites.

Arizona has a surveillance program in place to detect virus activity in mosquitoes, chicken flocks, dead birds, sick horses and humans. The surveillance program will aid in detection of and response to the WNV in Arizona.

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