West Nile Virus. Symptoms, Odds and How to Prevent It

West Nile most commonly spreads through a mosquito bite. Mosquitoes get infected by feeding on infected birds.¹ It can also spread through blood transfusion and organ transplant.¹

In Ohio, the principal transmitter is the northern house mosquito, Culex pipiens.⁵ It is 3 to 7 mm, dark brown to tan with brown and white abdomen stripes, and it is more active at twilight.⁴ It has been reported in every Ohio county.⁴

In California, UC IPM says Culex mosquitoes are most active at dawn and dusk.⁶ Compare it with the black and white striped tiger mosquito in the Aedes guide. See the mosquito hub for all species.

The northern house mosquito lays a raft of about 150 eggs glued together, usually three to five days after a blood meal.⁴ Larvae hatch one to three days later.⁴

The larval stage lasts 5 to 14 days, and the pupal stage about two days.⁴ Adults live several days to a month in spring and summer.⁴

Females travel at most three fourths of a mile from where they hatched.⁴ That makes your yard and the next few lots the real risk zone.

It also explains the virus cycle. Birds carry it, mosquitoes bite birds and then bite people.¹ Dead birds are a warning sign.³

Most people (80%) have no symptoms. About 20% have mild illness with fever, headache, body aches, joint pain, vomiting, diarrhea or rash.^1,2 Less than 1% develop severe illness that affects the central nervous system, needs hospital care or ends in death.²

Severe signs include high fever, headache, neck stiffness, stupor, disorientation and muscle weakness.¹ CDC says to get care right away for high fever, neck stiffness, muscle weakness, confusion or tremors.²

Symptoms usually start 2 to 6 days after the bite. They can start anywhere from 2 to 14 days.² People with weak immune systems may take longer.²

For most people, not very. For some, very. About 10% of people who develop severe nervous system illness die.² CDC says about 2,000 people in the United States are diagnosed each year, which is an underestimate. More than 1,300 get severe illness, and more than 130 die.¹

Ohio's numbers are lower but real. West Nile is the most common mosquito borne pathogen in Ohio. The state averages 58 confirmed cases and four deaths a year.⁴ It has been confirmed in Ohio every year since 2001.⁵

Horses account for 96.9% of reported nonhuman mammal cases in Ohio.⁵ Birds, horses and humans have all been found infected.⁵

Recovery from mild illness is usually complete, though fatigue and weakness can last weeks or months. Severe illness can take months and some effects may be permanent.²

No medicines treat West Nile, and antibiotics do not work on viruses.¹ For mild illness, CDC says to take over the counter acetaminophen, drink fluids and rest.¹

People with severe illness usually need hospital care with fluids, pain medicine and nursing support.¹

There are no licensed vaccines or medicines to prevent it in people. The best protection is preventing mosquito bites.¹

Call a doctor if you have a fever and headache after mosquito season bites, and go to the ER for neck stiffness, confusion, tremors or muscle weakness.²

If you find a dead bird, report it. California accepts dead bird reports year round, and they help track where the virus is spreading.³

The virus itself has no treatment, so your money goes into stopping bites. A one time yard treatment averages $135 to $251. A season plan runs $373 to $694.⁹

A yard up to 1/2 acre runs $300 to $500.⁹ Yard size, treatment type, infestation size and season length move the price.⁹

Start free. Dump water from pet dishes, flower pots and kiddie pools.³ Use Bti dunks in water you cannot drain.⁸ Your local mosquito and vector control district or health department may already spray. Ask before you pay.^8,7

• Dump and drain standing water around your home, including pet dishes, flower pots and kiddie pools.³
• Clean bird baths at least weekly. Aerate backyard ponds or stock them with mosquito eating fish.⁵
• Use repellent outdoors, especially at dawn and dusk.³
• Use DEET or picaridin, and spray permethrin on clothes, never on skin.^6,8
• Wear long sleeves and pants, and fix screens.^6,8
• Skip bug zappers. Ohio State says they do not help control mosquitoes.⁵

Reduce outdoor time at dawn and dusk when Culex are most active.⁶ Ohio State says most mosquitoes are most active then, and repellents typically last one to five hours.⁸

CDC says mosquito season is typically June through October, and diagnoses usually peak from late August to early September.¹

In Ohio, health agencies monitor mosquitoes from May to September. Human risk is highest from June to mid October.⁴

California can run later. As of October 30, 2026, the state's West Nile site listed 502 human cases and 31 deaths for the year. It also listed 772 dead birds and 4,098 positive mosquito samples.³ CDPH called it the highest activity in five years.³

West Nile has been confirmed in Ohio every year since 2001. Ohio's case table shows 441 cases and 31 deaths in 2002.⁵ The northern house mosquito is the principal transmitter.⁵

Ohio averages 58 confirmed cases and four deaths a year, including cases acquired elsewhere.⁴ Health agencies monitor mosquitoes from May to September, and response takes about two to three weeks from surveillance to treatment.⁴

Ohio municipalities often monitor and spray in summer, so check with your local health department.⁸ Ohio State says to clean bird baths at least weekly.⁵

California's first year in the state record is 2003, with 3 human cases and 96 dead birds.³ From 2003 to 2026 there have been 8,867 human cases and 417 deaths.³

In 2026, as of October 30, there were 502 human cases and 31 deaths across 9 jurisdictions.³ CDPH reports the highest activity in five years.³

The state's advice is simple. Drain standing water, use repellent at dawn and dusk and report dead birds online.³ California has more than 60 mosquito and vector control districts, and MVCAC says to contact your local one.^6,7