BUREAU OF ENVIRONMENTAL HEALTH
West Nile virus (WNV) is a mosquito-borne disease that was originally found in Africa. In 1999, it was detected in the eastern United States; since then the virus has spread throughout the United States and is well established in most states, including California.
WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers ("vectors") that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite. WNV is not spread through casual contact such as touching or kissing a person with the virus, or by breathing in the virus.
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 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 an one mosquito bite are extremely small.
People typically develop symptoms from 3 to14 days after they are bitten by an infected mosquito.
WNV affects the central nervous system. However, symptoms vary:
- Serious Symptoms in a Few People. Less than one percent (about 1 in 150 people) of individuals 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. WN virus infection can be fatal.
- Milder Symptoms in Some People. Up to 20 percent (about 1 in 5) of the people who become infected will display symptoms which can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms generally last for just a few days, although even previously healthy people have been sick for several weeks.
- No Symptoms in Most People. Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms.
People over the age of 50 have a higher chance of getting sick and are more likely to develop serious symptoms when infected with West Nile virus. Being outside, especially at dawn or at dusk, increases your risk of being bitten by an infected mosquito. Take precautions to avoid mosquito bites if you spend a lot of time outside, either working or playing.
There is no specific treatment for WNV infection. In cases with milder symptoms, people experience fever and aches that pass on their own. In more severe cases, people may need to go to the hospital where they can receive supportive care including intravenous fluids, help with breathing, and nursing care.
Milder WNV illness improves without treatment, and people do not necessarily need to seek medical attention for this infection, though they may choose to do so. If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek medical attention immediately. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.
An infected mosquito can bite any animal, but not all animals will become sick. The disease most often affects birds but may occasionally cause disease in other animals.
Wild birds serve as the main source of virus for mosquitoes. Infection has been reported in more than 225 bird species. Although many birds that are infected with WNV will not appear ill, WNV infection can cause serious illness and death in some birds. The most severe illnesses are seen among the corvid birds which include crows, jays, ravens, and magpies.
Tree squirrels with West Nile virus can develop neurological symptoms such as uncoordinated movement, paralysis, shaking, or circling and may die.
Like people, most horses bitten by mosquitoes will not become sick with WNV. However, of those that do, clinical signs may include stumbling, circling, hind leg weakness, inability to stand, muscle tremors, and death. A vaccine to prevent West Nile virus is available for horses and horse-owners should consult with a veterinarian about WNV vaccine and other vaccines against mosquito-borne viruses, such as western equine encephalitis.
Dogs and cats can be exposed to WNV in the same way as humans. However, these animals are very resistant to WNV and rarely become ill. Concerned pet owners should consult with a veterinarian.