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West Nile Virus

Northern house mosquito

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.

Map: West Nile virus in Ohio

West Nile Virus Disease in Ohio
2022* Cases Compared to Incidence 2002-2021

County Rate 2002-2021
(per 100,000 per year)
2022 Cases*
 Adams 0.4 0
 Allen 0.7 0
 Ashland 0.1 0
 Ashtabula 0.3 0
 Athens 0.2 0
 Auglaize 1.1 0
 Belmont 0.1 0
 Brown 0.2 0
 Butler 0.3 0
 Carroll 0.2 0
 Champaign 0.4 0
 Clark 1.3 0
 Clermont 0.5 0
 Clinton 0.6 0
 Columbiana 0.1 0
 Coshocton 0.1 0
 Crawford 0.1 0
 Cuyahoga 1.4 2
 Darke 0.8 0
 Defiance 1.4 0
 Delaware 0.0 0
 Erie 0.2 0
 Fairfield 0.1 0
 Fayette 0.0 0
 Franklin 0.2 2
 Fulton 1.2 0
 Gallia 0.0 0
 Geauga 0.3 0
 Greene 0.2 0
 Guernsey 0.3 0
 Hamilton 0.4 0
 Hancock 0.9 0
 Hardin 0.8 0
 Harrison 0.0 0
 Henry 0.4 0
 Highland 0.1 0
 Hocking 0.0 0
 Holmes 1.2 0
 Huron 0.3 0
 Jackson 0.3 0
 Jefferson 0.1 0
 Knox 0.3 0
 Lake 0.3 0
 Lawrence 0.1 0
 Licking 0.2 0
 Logan 0.3 0
 Lorain 0.5 0
 Lucas 0.6 0
 Madison 0.4 0
 Mahoning 0.2 0
 Marion 0.5 0
 Medina 0.1 0
 Meigs 0.2 0
 Mercer 1.1 0
 Miami 0.2 0
 Monroe 0.0 0
 Montgomery 0.3 1
 Morgan 0.3 0
 Morrow 0.0 0
 Muskingum 0.1 0
 Noble 0.3 0
 Ottawa 0.4 0
 Paulding 1.0 0
 Perry 0.1 0
 Pickaway 0.2 0
 Pike 0.0 0
 Portage 0.1 0
 Preble 0.7 0
 Putnam 2.2 0
 Richland 0.2 0
 Ross 0.3 0
 Sandusky 0.2 0
 Scioto 0.1 0
 Seneca 0.2 0
 Shelby 0.2 0
 Stark 0.3 0
 Summit 0.1 0
 Trumbull 0.4 0
 Tuscarawas 0.1 0
 Union 0.4 0
 Van Wert 1.0 0
 Vinton 0.0 0
 Warren 0.2 0
 Washington 0.1 0
 Wayne 0.7 0
 Williams 1.1 0
 Wood 0.7 0
 Wyandot 1.8 0
TOTAL 0.5 5

Source: Ohio Department of Health.
* Data as of September 27, 2022.
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.

Graph: West Nile virus disease by age and sex in Ohio

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

Age Female Male
 0-9 years 0 4
 10-19 years 4 1
 20-29 years 10 10
 30-39 years 15 13
 40-49 years 20 14
 50-59 years 27 36
 60-69 years 27 39
 70-79 years 24 52
 80+ years 12 19

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.

Graph: West Nile virus disease by week of illness onset in Ohio

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

Month Week of Illness Onset Cases 2012-2021
 January 1 0
  2 0
  3 0
  4 0
 February 5 0
  6 0
  7 0
  8 0
 March 9 0
  10 0
  11 0
  12 0
  13 0
 April 14 0
  15 0
  16 0
  17 0
 May 18 0
  19 0
  20 0
  21 0
 June 22 0
  23 0
  24 0
  25 1
  26 1
 July 27 2
  28 5
  29 3
  30 12
 August 31 19
  32 49
  33 46
  34 29
 September 35 40
  36 35
  37 29
  38 23
  39 15
 October 40 8
  41 4
  42 4
  43 2
 November 44 0
  45 0
  46 0
  47 0
 December 48 0
  49 0
  50 0
  51 0
  52 0

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.

Ohio West Nile Virus Disease Annual Human Case Statistics
Year Human Cases Deaths Median Age (Years) Age Range of Cases (Years) Earliest Date of Symptom Onset Asymptomatic Blood Donors
2001 0 0 n/a n/a n/a n/a
2002 441 31 61 2 – 98 n/a n/a
2003 108 8 49 11 – 90 n/a 6
2004 12 2 49.5 12 – 87 Jul 5 1
2005 61 2 53 22 – 96 Jun 14 14
2006 48 4 57.5 2 – 86 Aug 1 10
2007 23 3 52 11 – 86 Jul 12 9
2008 15 1 57 20 – 86 Jul 9 1
2009 2 0 36.5 11 – 62 Aug 27 0
2010 5 0 46 4 – 74 Jul 9 0
2011 21 1 55 14 – 83 Aug 1 6
2012 122 7 57.5 4 – 91 Jul 10 13
2013 24 4 71.5 38 – 82 Jul 29 4
2014 11 1 65 19 – 79 Jul 27 0
2015 35 2 65 14 – 91 Jul 9 10
2016 17 4 66 4 – 84 Jul 28 4
2017 34 5 59 6 – 82 Jul 24 8
2018 65 6 61 5 – 89 Jun 23 16
2019 3 1 68 59 – 68 Sep 7 0
2020 3 0 62 41 – 68 Aug 7 0
2021 13 1 68 30 – 80 Jul 8 1
 AVERAGE  53 4 57 n/a n/a 5
TOTAL 1.063 83 n/a n/a n/a 103

 

Ohio West Nile Virus Annual Mosquito, Bird, and Veterinary Case Statistics
Year Mosquitoes Tested Positive Mosquito Samples MIR* Positive Birds Positive Horses & Other Mammals Counties with Reported WNV Activity**
2001 90,948 26 0.3 286 0 29
2002 185,372 1,976 10.7 1,002 645 88
2003 488,033 799 1.6 249 106 78
2004 398,832 875 2.2 80 9 56
2005 390,010 1,373 3.5 84 15 44
2006 443,937 913 2.1 126 12 50
2007 252,195 325 1.3 26 2 36
2008 388,149 381 1.0 14 0 24
2009 278,677 243 0.9 n/a 0 9
2010 316,623 260 0.8 n/a 5 15
2011 290,840 586 2.0 n/a 4 20
2012 180,252 1,218 6.8 n/a 12 45
2013 n/a n/a n/a n/a n/a n/a
2014 192,929 369 1.9 n/a 1 16
2015 413,109 544 1.3 n/a 4 38
2016 379,932 491 1.3 n/a 6 34
2017 447,079 2,398 5.2 n/a 14 51
2018 501,366 3,281 6.5 n/a 50 68
2019 407,718 263 0.7 n/a 3 28
2020 211,365 666 3.1 n/a 0 14
2021 544,582 1,286 2.4 n/a 1 32
 AVERAGE  340,097 914 n/a 233 44 39
TOTAL 6,801,948 18,273 n/a 1,867 889 n/a

* 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 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 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 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.