Q: What are West Nile virus, West Nile fever, and West Nile encephalitis?
A.“West Nile Virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals.

“West Nile fever is a case of mild disease in people, characterized by flu-like symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.

More severe disease due to a person being infected with this virus can be “West Nile encephalitis,” West Nile meningitis or West Nile meningoencephalitis.” Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.

Q. How long has West Nile virus been in the U.S.?
A. It is not known how long it has been in the U.S., but CDC scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999, possibly longer.

Q. Is West Nile virus now established in the Western Hemisphere?
A. The continued expansion of West Nile virus in the United States indicates that it is permanently established in the Western Hemisphere.

Q. Is the disease seasonal in its occurrence?
A. In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), West Nile encephalitis cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile virus can be transmitted year round.

Q. How do people get infected with West Nile virus (WNV)?
A. The principle route of human infection with West Nile virus is through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. The virus eventually finds its way into the mosquito's salivary glands. During subsequent blood meals, the virus may be injected into humans and animals, where it can multiply and possibly cause illness.

Q. If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?
A. No. 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. Besides mosquitoes, can you get West Nile virus directly from other insects or ticks?
A. Infected mosquitoes are the primary source for West Nile virus. Although ticks infected with West Nile virus have been found in Asia and Africa, their role in the transmission and maintenance of the virus is uncertain. However, there is no information to suggest that ticks played any role in the cases identified in the United States.

Q. How many types of animals have been found to be infected with West Nile virus?
A. Although the vast majority of infections have been identified in birds, WN virus has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.

Q. Can you get West Nile virus directly from birds?
A. There is no evidence that a person can get the virus from handling live or dead infected birds. However, persons should avoid bare-handed contact when handling any dead animals and use gloves or double plastic bags to place the carcass in a garbage can.

Q. If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?
A. It is assumed that immunity will be lifelong; however, it may wane in later years.

Q. Has West Nile virus caused severe illness or death in horses?
A. Yes, while data suggest that most horses infected with West Nile virus recover, results of investigations indicate that West Nile virus has caused deaths in horses in the United States.

Q. How do the horses become infected with West Nile virus?
A. The same way humans become infected—by the bite of infectious mosquitoes. The virus is located in the mosquito's salivary glands. When mosquitoes bite or "feed" on the horse, the virus is injected into its blood system. The virus then multiplies and may cause illness. The mosquitoes become infected when they feed on infected birds or other animals.

Q. How does the virus cause severe illness or death in horses?
A. Following transmission by an infected mosquito, West Nile virus multiplies in the horse's blood system, crosses the blood brain barrier, and infects the brain. The virus interferes with normal central nervous system functioning and causes inflammation of the brain.

Q. Can I get infected with West Nile virus by caring for an infected horse?
A. West Nile virus is transmitted by infectious mosquitoes. There is no documented evidence of person-to-person or animal-to-person transmission of West Nile virus. Normal veterinary infection control precautions should be followed when caring for a horse suspected to have this or any viral infection.

Q. Can a horse infected with West Nile virus infect horses in neighboring stalls?
A. No. There is no documented evidence that West Nile virus is transmitted between horses. However, horses with suspected West Nile virus should be isolated from mosquito bites, if at all possible.

Q. My horse is vaccinated against eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE). Will these vaccines protect my horse against West Nile virus infection?
A. No. EEE, WEE, and VEE belong to another family of viruses for which there is no cross-protection.

Q. Can I vaccinate my horse against West Nile virus infection?
A. A West Nile virus vaccine for horses was recently approved, but its effectiveness is unknown.

Q. What is the treatment for a horse infected with West Nile virus? Should it be destroyed?
A. There is no reason to destroy a horse just because it has been infected with West Nile virus. Data suggest that most horses recover from the infection. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent.

What is the health risk to people from the West Nile Virus?

SYMPTOMS of SERIOUS ILLNESS: fever, disorientation, muscle weakness, neck stiffness, headache, nausea
RISK of SERIOUS ILLNESS: greater for older people and the immuno-compromised
CHILDREN ARE NOT PARTICULARLY VULNERABLE (few if any cases of serious illness have involved children)
INCUBATION PERIOD: 3-14 days after being bitten by an infected mosquito
CALL a DOCTOR if you or someone you care about shows symptoms of serious illness, whether or not caused by WNV
TREAT the SYMPTOMS -- there is no vaccine or medication specific to WNV
AUGUST & SEPTEMBER are when WNV has been most likely to bridge into the human population in the Northeast US. Human illness has occurred earlier in the Southeast and Gulf Coast states of the US in 2002
"FLU-LIKE" SYMPTOMS have been reported by about 30% of infected people, but most of those infected do not get sick
In US outbreaks, about 1 infected person in 150 has become seriously ill with central nervous system infection (encephalitis &/or meningitis)
About 12% of hospitalized cases have been fatal (7 of 62 hospitalized in 1999, 2 of 20 in 2000, 9 of 64 in 2001, )
Most mosquitoes bites will not lead to a WNV infection
BIRDS are far more likely than people to become infected & sickened by West Nile Virus

In some areas where West Nile Virus is showing up for the third and fourth year, there is some complacency about taking precautionary and preventative measures. Although fortunately WNV has not caused widespread human mortality--as was initially feared by some--it can cause unpleasant (flu-like) and sometimes long-lasting symptoms in people.

How can I reduce my personal risk and my community's risk from the West Nile virus?

"Mosquito hygiene" includes source reduction of mosquito breeding sites and avoidance of biting mosquitoes--Both are key to reducing risk from WNV. Precautions should continue for the duration of the mosquito season. In the North this is into the Fall until after there have been two hard frosts.

Mosquitoes breed in wet areas, and Culex are found particularly where there is decaying organic matter (e.g., leaves, grass clippings, animal wastes). There does not have to be much water and the water does not have to be left standing for very long - Some species can reproduce within a week! Check for standing water especially after each rainstorm, drizzle, watering of the garden or washing the car). Reduce mosquito breeding opportunites by:

Eliminate or empty the "artificial water-collecting containers" that are prime breeding spots for the mosquito species implicated in transmission of West Nile Virus
Clean out rain gutters
Aerate swimming pools and ponds (and perhaps stock with mosquito-eating fish)
Empty unused buckets, water troughs, etc.
Keep unused tires under cover so they do not collect water
Drill drainage holes in tires and other containers used in construction sites, farms, gardens and play areas
Clean bird baths and animal water bowls at least once a week

Avoid mosquito bites by wearing long clothes and/or by using insect repellent when out after dusk or in shaded areas (such as woods) during the daytime. This is when and where most vector species are more likely to bite. People should be especially careful when in "mass gatherings" where the CO₂ given off by the crowd attracts more mosquitoes from a greater distance.

Should I (or someone in my family) be tested for West Nile Virus if bitten by a mosquito?

No. Testing for West Nile Virus is a complex process. It takes a long time to get test results and only a very small percentage of the mosquitoes that might bite you are infected. While test results are important to scientists and medical doctors studying the disease, they would not be very useful to you as an individual. There has not been* a particular cure for West Nile Virus--so a sick person does not become better off by being tested for WNV infection. If you become seriously ill, you should see your doctor, no matter what the cause or name of the illness. If you show signs of WNV, you would be treated for relief of your symptoms. (* In late Aug 2002, the US Food and Drug Administration approved clinical trials to see how useful the drug interferon might be in treating WNV patients, but this drug is not widely available for this purpose and its efficacy in treating WNV is unknown.

Can I be vaccinated against West Nile Virus? Is there a vaccine for horses? For birds? For other animals?

People: Two independent research teams have taken similar promising pathways to develop a WNV vaccine for people. Both efforts involve creating a hybrid "modified live virus" that include genes from the West Nile virus spliced into the "backbone" genome of a distantly related virus. When a vaccine is made from a modified life virus it has to replicate within the vaccinated person or animal. A government research group is splicing two genes for West Nile coat proteins into a dengue virus backbone that has been stripped of its own corresponding genes. The other research group, from the Acambis Company, are swapping the same two WNV genes into the backbone genome of an attenuated yellow fever virus. Acambis has already demonstrated that its hybrid vaccine protects horses and primates from WNV infection. They hope to start human clinical trials by summer 2002.

Top US health officials from several agencies (CDC, NIH, FDA) reported at a Sept 24, 2002, Senate Committee Hearing that a nucleic acid blood test may be ready by summer 2003 and a vaccine based on the yellow fever vaccine may be ready in three years.

While the vaccines may be developed rapidly, it will likely take 3-4 years for them to be approved for human use. The use of a vaccine that itself poses risk is controversial, especially given the typically very low risk to people of serious illness from WNV.

Texas Update: West Nile for past year.