West Nile virus (WNV) is an arthropod-borne virus (arbovirus) spread by the bite of infected mosquitoes.

Most people are infected in Ohio by 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.

West Nile virus was first detected in the United States in New York City in 1999 and quickly spread across the country within a few years.  West Nile virus was first identified in Ohio birds and mosquitoes in 2001.  The following year, the first human cases and deaths were reported.  By the end of 2002, all but one of the state's 88 counties reported positive humans (441 total human cases), mosquitoes, birds, or horses.  West Nile virus 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.

The best way to prevent West Nile virus disease is to prevent mosquito bites.

Where does West Nile virus disease occur in Ohio?

Northern house mosquitoes that can carry West Nile virus are found throughout Ohio wherever suitable habitats for breeding are found.  However, the majority of West Nile virus disease human cases reported in Ohio are in the northern and western parts of the state.

West Nile Virus Disease in Ohio
2021* Cases Compared to Incidence 2002-2020

Source: Ohio Department of Health
* Data as of Nov. 23, 2021
County-level data are based on the county of residence of the case

What are the signs and symptoms of West Nile virus disease?

Approximately 80% of people who are infected with West Nile virus 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 two to 14 days after they are bitten by the infected mosquito.

Up to 20% of people who become infected will have symptoms that can last for a few days to as long as several weeks and include:

• Fever.
• Headache.
• Body aches.
• Nausea.
• Vomiting.
• Swollen lymph glands.
• Rash on chest, stomach or back.

About one in 150 people infected with West Nile virus will develop severe illness.  The severe symptoms may last several weeks, and neurologic effects may be permanent.  Symptoms of severe illness can include:

• High fever.
• Headache.
• Neck stiffness.
• Stupor.
• Disorientation.
• Coma.
• Tremors.
• Convulsions.
• Muscle weakness.
• Vision loss.
• Numbness.
• Paralysis.

Death from infection with West Nile virus occurs in 10% of those diagnosed with severe illness, but is much higher for patients diagnosed with West Nile virus encephalitis and acute flaccid paralysis.

For more information, see the Centers for Disease Control and Prevention (CDC)'s website for West Nile virus disease symptoms.

How is West Nile virus disease diagnosed?

West Nile virus infection can only be diagnosed by a healthcare provider.  A blood or cerebrospinal fluid sample may be collected for laboratory testing.  Please visit the CDC's website for information on diagnosis and testing.

What is the treatment for West Nile virus disease?

There is no specific treatment for West Nile virus infection, and care is based on symptoms.

Who is at risk in Ohio?

Anyone who spends time outdoors can be at risk for West Nile virus infection.  The mosquito that transmits West Nile virus, the northern house mosquito, breeds in catch basins, stagnant water in ditches, and containers of water with high organic matter (e.g., flowerpot saucers, clogged rain gutters) so people who live or recreate near these breeding habitats are at increased risk.

Ohioans of all ages can get sick with West Nile virus, but adults greater than 50 years of age are more at risk for severe disease.  Most cases of West Nile virus reported in Ohio are in adults aged 70-79 years, particularly men.

West Nile Virus Disease by Age and Sex, Ohio, 2012-2021

Source: Ohio Department of Health

What time of year are Ohioans at risk for contracting West Nile virus disease?

In Ohio, West Nile virus infection can occur anytime during mosquito season, which typically runs from May through October.  Most human cases are reported in July through October.

It can take anywhere from two to 14 days from when the mosquito bite occurs to when symptoms of West Nile virus disease appear.  Since most human cases become ill in late July through October, that means most are bitten by an infected northern house mosquito between early July and mid-September.  Therefore, summer through early fall is the time of year when Ohioans are most at risk for contracting West Nile virus disease.

West Nile Virus Disease by Week of Illness Onset, Ohio, 2012-2021

Source: Ohio Department of Health

What are the trends over time?

Ohio has tracked human, mosquito, and veterinary cases of West Nile virus infection since 2001 when it was first detected here.  An average of 58 human cases are reported each year in Ohio.  However, epidemics can flare up under certain environmental conditions in the summer and continue in the fall as was seen in Ohio during 2002 and again in 2012.

* Minimum Infection Rate (MIR) = (Number of positive mosquito samples / Number of mosquitoes tested) X 1000
** Includes counties with any human cases, veterinary cases, positive birds, or positive mosquitoes

How can I reduce my risk of West Nile virus infection?

Steps to prevent West Nile virus infection include avoiding mosquitoes and mosquito bites, planning ahead when traveling to areas at risk for West Nile virus infection, and stopping mosquitoes from breeding in and around your home.

What are the roles of other animals in West Nile virus transmission?

	Birds are the natural reservoir for West Nile virus.  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 are known as dead-end hosts of West Nile virus, meaning they can become ill from West Nile virus, 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 West Nile virus.
	Mosquitoes become infected with West Nile virus 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.