West Nile virus is an arbovirus (short for arthropod borne virus) that causes encephalitis (inflammation of the brain). Arboviruses are transmitted by blood-feeding insects such as mosquitoes. Most infections with West Nile virus have been identified in wild birds, horses and humans, but the virus can also infect various other wild and domestic animals.
WNV is transmitted by mosquitoes
WNV is a mosquito-borne infectious disease that was first identified in the West Nile subregion of Uganda in 1937. It is maintained in a cycle between mosquitoes and birds. Birds are the natural hosts of WNV. Humans and horses are incidental dead-end hosts through the bite of an infected mosquito, usually of the genus Culex. Infected humans and horses do not seem to spread the virus to other mammals. Direct person-to-person transmission has not been reported. The virus can be transmitted from person to person through blood transfusion or organ transplantation.
Culex mosquitoes appear to be the most important maintenance vectors for WNV, but mosquito species from other genera are also susceptible to infection. Of the two species of Culex mosquitoes that transmit the virus, Culex pipiens is distributed widely in the European Region, while Culex modestus is confined to the wetland and fishpond areas of southern and central Europe and lives only in reedbelt habitats.
What Can I Do to Prevent WNV?
The easiest and best way to avoid WNV is to prevent mosquito bites.
▪ When outdoors, use repellents containing DEET, picaridin, IR3535, some oil of lemon eucalyptus or para-menthane-diol. Follow the directions on the package.
▪ Many mosquitoes are most active from dusk to dawn. Be sure to use insect repellent and wear long sleeves and pants at these times or consider staying indoors during these hours.
▪ 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.
Humans— most 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 nodes. 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.
Horses— Horses that develop clinical West Nile virus infection may exhibit incoordination, stumbling, weakness, muscle twitching, depression, or fearfulness. A fever is not commonly observed. Severe cases may become recumbent. The death rate in horses is approximately 30%, and is highest in recumbent horses. The number of cases of equine WNV has decreased annually since 2002, and may be due to vaccination and/or increased development of naturally acquired immunity.
Other animals—Wild 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. Clinical signs associated with West Nile virus infection in dogs, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and domestic birds have not been well described. It appears that, although they may be infected with the virus, many members of these latter species rarely develop clinical signs of disease.
There is no specific treatment or cure for infected horses. Veterinary care includes administration of anti-inflammatory drugs and intravenous fluids (if necessary). Supportive care is exceedingly important for infected horses to ensure adequate food and water intake, protect the safety of the horse (to prevent injuries in ataxic horses), and to prevent pressure sores in recumbent horses. Some veterinarians have attempted treating horses with antiviral drugs such as interferon and passive antibody products for WNV, but published clinical trials demonstrating efficacy or safety of this approach are lacking at present.
There is no specific treatment for WNV infection in humans. In cases with milder symptoms, people experience symptoms such as fever and aches that pass on their own, although illness may last weeks to months. In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing, and nursing care.
What Should I Do if I Think I Have WNV?
Milder WNV illness improves on its own, and people do not 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. Severe WNV illness usually requires hospitalization. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.
What Is the Risk of Getting Sick from WNV?
▪ People over 50 at higher risk to get severe illness.
People over the age of 50 are more likely to develop serious symptoms of WNV if they do get sick and should take special care to avoid mosquito bites.
▪ Being outside means you’re at risk.
The more time you’re outdoors, the more time you could be bitten by an infected mosquito. Pay attention to avoiding mosquito bites if you spend time outside, either working or playing.
▪ Risk 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. If you have concerns, talk to your doctor.