West Nile Virus (WNV)
What is WNV?
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.
How do people contract WNV?
- Infected mosquitoes: Most often, 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.
- Transfusions, transplants, and mother-to-child: All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. Transmission during pregnancy from mother to baby or transmission to an infant via breastfeeding is extremely rare.
- Not through touching: WNV is not spread through casual contact such as touching or kissing a person with the virus, or by breathing in the virus.
How soon do infected people get sick?
People typically develop symptoms from 3 to14 days after they are bitten by an infected mosquito.
What are the symptoms of WNV?
- 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, paralysis, and death. These symptoms may last several weeks, and neurological effects may be permanent.
- 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.
Who is at the greatest risk of getting severely ill from WNV?
- People over the age of 50 have a higher chance of getting sick and are more likely to develop serious symptoms when infected with WNV.
- 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.
- Risk of transmission through medical procedures is very low. All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it.
How is WNV infection treated?
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.
If you have had WNV, are you immune to further infections?
It is thought that once a person has recovered from WNV, they are immune for life to future infections with WNV. This immunity may decrease over time or with health conditions that compromise the immune system.
Can animals get sick with 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.
- Squirrels & Rabbits 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 vaccinations for WNV and other mosquito-borne viruses, such as western equine encephalomyelitis. For more information on West Nile Virus and horses, please visit the California Department of Food and Agriculture.
- 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.