How do people get West Nile virus?
Most often, WNV is spread by the bite of an infected mosquito. Viruses that are spread by mosquitoes are called “arboviruses.” 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.
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 breast-feeding is extremely rare.
West Nile virus 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 three to 14 days after they are bitten by an infected mosquito.
What are the symptoms of West Nile virus?
WNV affects the central nervous system. However, symptoms may vary. Approximately 80 percent of people (about four out of 5) who are infected with WNV will not show any symptoms.
Up to 20 percent (about one 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.
Less than 1 percent (one 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, and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. West Nile virus infection can be fatal.
Who is at greatest risk of getting severely ill from West Nile virus?
People over the age of 50 and individuals with compromised immune systems 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 (wear repellent) to avoid mosquito bites if you spend a lot of time outside.
How is West Nile virus 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 West Nile virus, 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 from West Nile virus?
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, scrub jays, ravens and magpies. In Kern County, mockingbirds, scrub jays, crows, house finches and house sparrows have tested positive for WNV in previous years.
Like people, most horses bitten by infected 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. Vaccine is available to prevent West Nile virus in horses and horse-owners should consult with a veterinarian about WNV vaccine and other vaccines that protect against other mosquito-borne viruses, such as western equine encephalitis.
For more information on West Nile virus and horses, please visit the California Department of Food and Agriculture website at www.cdfa.ca.org
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.