West Nile Virus Q&A

Introduction: West Nile (WN) virus has emerged in recent years in temperate regions of Europe and North America, presenting a threat to public, equine, and animal health. The most serious manifestation of WN virus infection is fatal encephalitis (inflammation of the brain) in humans and horses, as well as mortality in certain domestic and wild birds.
History: West Nile virus was first isolated from a febrile adult woman in the West Nile District of Uganda in 1937. The ecology was characterized in Egypt in the 1950s. The virus became recognized as a cause of severe human meningoencephalitis (inflammation of the spinal cord and brain) in elderly patients during an outbreak in Israel in 1957. Equine disease was first noted in Egypt and France in the early 1960s. The appearance of WN virus in North America in 1999, with encephalitis reported in humans and horses, may be an important milestone in the evolving history of this virus.

Transmission of West Nile Virus

Q. How do people get West Nile encephalitis?
By the bite of mosquitoes infected with West Nile virus.
Q. What is the basic transmission cycle?
Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing 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?
No. Even in areas where mosquitoes do carry the virus, very few mosquitoes—much less than 1%—are infected. 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. Can you get West Nile encephalitis from another person?
No. West Nile encephalitis is NOT transmitted from person-to-person. For example, you cannot get West Nile virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.
Q. Is a woman's pregnancy at risk if she gets West Nile encephalitis?
There is no documented evidence that a pregnancy is at risk due to infection with West Nile virus.
Q. Besides mosquitoes, can you get West Nile virus directly from other insects or ticks?
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?  
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?
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. Can I get infected with West Nile virus by caring for an infected horse?
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. How does West Nile virus actually cause severe illness and death in humans?
Following transmission by an infected mosquito, West Nile virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.
Q. What proportion of people with severe illness due to West Nile virus die?
Among those with severe illness due to West Nile virus, case-fatality rates range from 3% to 15% and are highest among the elderly. Less than 1% of those infected with West Nile virus will develop severe illness.
Q. If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?
It is assumed that immunity will be lifelong; however, it may wane in later years.

Symptoms of West Nile Virus

Q. Who is at risk for getting West Nile encephalitis?
All residents of areas where virus activity has been identified are at risk of getting West Nile encephalitis; persons older than 50 years have the highest risk of severe disease.
Q. What are the symptoms of West Nile encephalitis?
Most infections are mild, and symptoms include fever, headache, and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death.
Q. What is the incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile encephalitis?
Usually 3 to 15 days.

Prevention of West Nile Virus

Q. What can be done to prevent outbreaks of West Nile virus?
Prevention and control of West Nile virus and other arboviral diseases is most effectively accomplished through integrated vector management programs. These programs should include surveillance for West Nile virus activity in mosquito vectors, birds, horses, other animals, and humans, and implementation of appropriate mosquito control measures to reduce mosquito populations when necessary. Additionally, when virus activity is detected in an area, residents should be alerted and advised to increase measures to reduce contact with mosquitoes. Details about effective prevention and control of West Nile virus can be found in CDC's Guidelines for Surveillance, Prevention, and ControlAbout PDF (286 KB, 111 pages).

Q. Is there a vaccine against West Nile encephalitis?
No, but several companies are working towards developing a vaccine.

Q. What can I do to reduce my risk of becoming infected with West Nile virus?
  • Stay indoors at dawn, dusk, and in the early evening.
  • Wear long-sleeved shirts and long pants whenever you are outdoors.
  • Spray clothing with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing.
  • Apply insect repellent sparingly to exposed skin. An effective repellent will contain 35% DEET (N,N-diethyl-meta-toluamide). DEET in high concentrations (greater than 35%) provides no additional protection.
  • Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children.
  • Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product.
  • Note: Vitamin B and "ultrasonic" devices are NOT effective in preventing mosquito bites.

Q. Where can I get more information on mosquito repellents?
Visit the American College of Physicians website: "Mosquitoes and mosquito repellents: A clinician's guide" (Mark S. Fradin, MD. Annals of Internal Medicine. June 1, 1998;128:931-940). You can also find information on insect repellents containing DEET
disclaimer at the Environmental Protection Agency (EPA) website.

Q. Where can I get information about the use of pesticide sprays that are being used for mosquito control?
The federal agency responsible for pesticide evaluation is the Environmental Protection Agency (EPA). See the EPA website
disclaimer for detailed answers to the questions about pesticides used for mosquito control.

Testing and Treating West Nile Encephalitis in Humans

Q. I think I have symptoms of West Nile virus. What should I do?
Contact your health care provider if you have concerns about your health. If you or your family members develop symptoms such as high fever, confusion, muscle weakness, and severe headaches, you should see your doctor immediately.

Q. How do health care providers test for West Nile virus?
Your physician will first take a medical history to assess your risk for West Nile virus. People who live in or traveled to areas where West Nile virus activity has been identified are at risk of getting West Nile encephalitis; persons older than 50 years of age have the highest risk of severe disease. If you are determined to be at high risk and have symptoms of West Nile encephalitis, your provider will draw a blood sample and send it to a commercial or public health laboratory for confirmation.

Q. How is West Nile encephalitis treated?
There is no specific therapy. In more severe cases, intensive supportive therapy is indicated, often involving hospitalization, intravenous fluids, airway management, respiratory support (ventilator), prevention of secondary infections (pneumonia, urinary tract, etc.), and good nursing care.

These are excerpts from the Centers for Disease Control web site. For more information visit their web site at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm