West Nile Virus
West Nile virus (WNV) is an arthropod-borne virus (arbovirus) most commonly spread by infected mosquitoes that can lead to severe fever, encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). The primary vector in Ohio is the northern house mosquito, Culex pipiens. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread the virus to humans and other animals when they bite.
WNV was first detected in the United States in New York City in 1999 and quickly spread across the country within a few years. In Ohio, WNV was first identified in birds and mosquitoes in 2001. The following year, the first human cases and deaths were reported. By the end of 2002, Ohio had 441 human cases, 31 fatalities and all but one of the state’s 88 counties reported positive humans, mosquitoes, birds or horses. WNV is now established in Ohio where cases occur each year and seasonal epidemics can flare up under certain conditions in the summer and continue into the fall (see current human case map).
Approximately 80 percent of people who are infected with WNV will not show any symptoms at all, but there is no way to know in advance if you will develop an illness or not. Those who do develop symptoms usually do so between three to 14 days after they are bitten by the infected mosquito:
- Serious symptoms in a few people. 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.
- Milder symptoms in some people. Up to 20 percent of people who become infected will have symptoms that can include fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for a few days to as long as several weeks.
There is no specific treatment for WNV infection, and care is based on symptoms.
The easiest and best way to prevent WNV is to prevent mosquito bites:
- When outdoors, use repellents containing DEET, picaridin, IR3535, 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.
Animals associated with West Nile virus in Ohio include:
- Birds: Birds are the natural reservoir for WNV. If a mosquito bites an infected bird and the virus is transmitted to the mosquito, it may then become a host itself. If the same mosquito then bites a human, it can pass the disease to the human.
- Horses: Horses are known as dead-end hosts of WNV, meaning they can become ill with WNV, but they do not maintain sufficient virus in the blood to infect either other mammals (including humans) or mosquitoes. A vaccine is available for horses to prevent WNV.
- Mosquitoes: Mosquitoes become infected with WNV primarily through taking blood meals from infected birds. However, the virus can be transmitted from infected female mosquitoes to their eggs, which results in infected offspring.
West Nile virus resources:
Ohio statistics and maps:
Disease reporting and surveillance:
Mosquito-borne disease prevention literature:
Last updated: 03/05/2015
Zoonotic Disease Program