What is West Nile Virus?

West Nile virus (WNV) is a potentially serious illness. Experts  believe WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall.

Where did West Nile virus come from?

West Nile virus was first identified in the West Nile district of  Uganda in 1937, and has since been found in other parts  of Africa, Eastern Europe, West Asia, the Middle East, and  the United States. The strain of virus found in the United  States most closely resembles that found in the Mediterranean and Middle East.

What are the symptoms? 

About one in 150 people infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks and neurological effects may be permanent.

Mosquitoes Most Active August -September

West Nile virus activity peaks in August and September so make sure you're protected against mosquito bites. Even one bite can transmit West Nile virus or other diseases. Enjoy the outdoors, but remember:

· Use Mosquito Repellent
· Eliminate Mosquito Breeding Sites
· Install or Repair Screens
· Support Community-Based Mosquito Control Programs

West Nile Virus

Up to 20 percent of the people who become infected will display symptoms which can include fever, headache, and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for as short as a few days, though even healthy people have been sick for several weeks.  Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms at all.

How does it spread?

Most often, WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite. Not through touching. WNV is not spread through casual contact such as touching or kissing a person with the virus.

What is the risk of a person contracting West Nile virus?

The risk of becoming ill from a single mosquito bite is extremely low. In areas where mosquitoes carry the virus, less than 1% are actually infected. Even if mosquitoes are infected, less than 1% of people bitten and infected by those mosquitoes become severely ill.

What clinical signs are associated with West Nile virus infection?

HumansóMost humans infected with the virus are not aware that they have contracted it. If a person does become ill, clinical signs are usually mild and include fever, headache, body aches and, in some cases, skin rash and swollen lymph glands. Signs of more severe infection include high fever, neck stiffness, muscle weakness, convulsions and paralysis. Death rates associated with severe infection range from 3% to 15% and are highest among the elderly.

HorsesóThe most common sign is weakness, usually in the hindquarters. Weakness may be indicated by a widened stance, stumbling, leaning to one side, and toe dragging. In extreme cases, paralysis may follow. Fever is sometimes evident, as are depression and fearfulness. Approximately 33% of cases of West Nile viral encephalitis in horses proved fatal during the 2001 outbreak in the United States.

Other animalsóWild birds infected with West Nile virus in the United States are most often found dead; therefore, descriptions of clinical signs in wild birds are not readily available. Nor have clinical signs associated with West Nile virus infection in dogs, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and domestic birds been well described. It appears that, although they may be infected, many members of these latter species rarely develop clinical signs of disease.

How is West Nile viral encephalitis diagnosed and treated?

Diagnosis of West Nile viral encephalitis is based on a history of exposure, clinical signs, and results of diagnostic tests.
As for all viral diseases, treatment consists of providing support (e.g., hospitalization, intravenous fluids, respiratory support, prevention of secondary infections, and good nursing care) while the affected individual's immune system responds to the infection.

Can West Nile viral encephalitis be prevented?

A vaccine is now available for horses. For other species, including humans, limiting exposure to mosquitoes is considered effective prevention. The following actions may reduce the risk of mosquito bites and possible exposure to West Nile virus:

  • Check the integrity of screens around your home, porch and patio.
  • During warm months, avoid outdoor activities at dusk and dawn.
  • If you must be outdoors during hours when mosquitoes are most active, cover up with shoes, socks, long pants and long-sleeved    shirts.
  • Use mosquito repellant on exposed skin and spray clothing with repellents containing permethrin or DEET  
  • (N,N-diethyl-meta-toluamide) since mosquitoes may bite through thin clothing. When using insecticides or insect repellants, be 
sure to read and follow the manufacturer's directions for use. Products containing DEET should not be sprayed on dogs or cats.   
  • Consult your veterinarian about the best ways to protect your pet from exposure.
  • Eliminate standing water from any receptacles in which mosquitoes might breed.
West Nile viral encephalitis is an emerging disease and new information continues to become available. Visit the American Veterinary Medical Association home  page, www.avma.org for more information on this and other animal and public health issues.

What can I do to prevent West Nile Virus?

The easiest and best way to avoid WNV is to prevent mosquito bites. When you are outdoors, use insect repellents containing DEET (N, N-diethyl-meta-toluamide). Follow the directions on the package. Many mosquitoes are most active at dusk and dawn. Be sure to use insect repellent and wear long sleeves and pants at these times or consider staying indoors during these hours. Light-colored clothing can help you see mosquitoes that land on you.

Make sure you have good screens on your windows and doors to keep mosquitoes out. Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.


  • West Nile Virus is spread by mosquito bites.
  • Most people who become infected experience no symptoms at all, or mild flu-like symptoms.
  • West Nile Virus can be fatal in a very small percentage of cases.
  • Those over 50 are at greater risk.
  • Call your doctor if you think you might have West Nile Virus.
  • Avoid going out in the early morning and early evening hours.
  • If you do go out, wear long pants and long sleeve shirts.
  • Use an effective mosquito repellent with DEET, Picaridin,or lemon oil of Eucalyptus.
  • Make sure screens on doors and windows are tight fitting, and in good repair with no holes or tears.

Breeding locations...

  • Green pools breed mosquitoes! Maintain your pools.
  • Stock ponds with mosquito-eating fish.
  • Get rid of any standing water and puddles.
  • Do not overwater your lawn or garden.
  • Empty water out of buckets, old tires, flower pots, and toys.
  • Clean and hose out birdbaths weekly.
  • Change water every few days from pet bowls.
  • Cover containers or turn empty containers upside down so they do not hold water.
  • Cover trash cans and clean weekly.

More Information

The Centers for Disease Control and Prevention website has detailed information on this subject. Website

(Information courtesy of San Joaquin County Mosquito and Vector Control District, The American Veterinary Medical Association, The Centers for Disease Control and Prevention. )