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La Crosse Virus

Eastern treehole mosquito

La Crosse virus (LACV) is an arthropod-borne virus (arbovirus) in the California group of viruses spread by the bite of infected mosquitoes.

Most people infected in Ohio are bitten by the eastern treehole mosquito, Aedes triseriatus, an aggressive daytime biting mosquito commonly found in wooded areas.  La Crosse virus is endemic in Ohio, and Ohio has reported more human cases than any other state in the United States, averaging about 20 cases per year.

The best way to prevent La Crosse virus is to prevent mosquito bites.

Where does La Crosse virus disease occur in Ohio?

Eastern treehole mosquitoes that can carry La Crosse virus are most commonly found in the eastern and southern areas of the state where much of the silver maple, oak, and beech tree forest habitats appropriate for breeding are found.  However, treehole mosquitoes can be found in suitable wooded habitat throughout most or all of Ohio.

Map: La Crosse Virus Disease in Ohio

La Crosse Virus Disease in Ohio
2022* Cases Compared to Incidence 1963-2021

County Rate 1963-2021
(per 100,000 per year)
2022 Cases*
 Adams 0.2 0
 Allen 0.5 0
 Ashland 0.4 0
 Ashtabula 0.1 0
 Athens 0.4 0
 Auglaize 0.4 0
 Belmont 0.0 0
 Brown 0.0 0
 Butler 0.1 0
 Carroll 0.1 0
 Champaign 0.2 0
 Clark 0.1 0
 Clermont 0.0 0
 Clinton 0.1 0
 Columbiana 0.2 0
 Coshocton 0.7 2
 Crawford 0.4 0
 Cuyahoga 0.1 0
 Darke 0.3 0
 Defiance 1.0 0
 Delaware 0.5 0
 Erie 0.0 0
 Fairfield 0.5 0
 Fayette 0.0 0
 Franklin 0.1 1
 Fulton 0.0 0
 Gallia 0.0 0
 Geauga 0.4 0
 Greene 0.0 0
 Guernsey 0.5 0
 Hamilton 0.0 0
 Hancock 0.3 0
 Hardin 0.7 0
 Harrison 0.3 0
 Henry 0.1 0
 Highland 0.1 0
 Hocking 1.7 0
 Holmes 1.8 0
 Huron 0.1 0
 Jackson 0.3 0
 Jefferson 0.2 0
 Knox 2.6 1
 Lake 0.1 0
 Lawrence 0.1 0
 Licking 0.8 0
 Logan 0.7 0
 Lorain 0.2 0
 Lucas 0.0 0
 Madison 0.0 0
 Mahoning 0.1 0
 Marion 0.4 0
 Medina 0.2 0
 Meigs 0.1 0
 Mercer 0.4 0
 Miami 0.2 0
 Monroe 0.0 0
 Montgomery 0.0 0
 Morgan 0.6 0
 Morrow 1.3 0
 Muskingum 0.7 1
 Noble 0.0 0
 Ottawa 0.0 0
 Paulding 0.3 0
 Perry 1.3 1
 Pickaway 0.1 0
 Pike 1.3 0
 Portage 0.3 0
 Preble 0.3 0
 Putnam 0.6 0
 Richland 0.5 1
 Ross 0.8 1
 Sandusky 0.2 0
 Scioto 0.7 0
 Seneca 0.2 0
 Shelby 0.4 0
 Stark 0.1 0
 Summit 0.2 0
 Trumbull 0.2 0
 Tuscarawas 0.5 0
 Union 0.6 0
 Van Wert 0.0 0
 Vinton 0.7 0
 Warren 0.1 0
 Washington 0.2 0
 Wayne 0.5 0
 Williams 0.3 0
 Wood 0.1 0
 Wyandot 0.1 0
TOTAL 0.2 8

Source: Ohio Department of Health.
* Data as of September 15, 2022.
County-level data are based on the county of residence of the case.

What are the signs and symptoms of La Crosse virus disease?

Many people infected with La Crosse virus have no apparent symptoms.  For those who do, symptoms typically begin five to 15 days after a mosquito bite and include non-specific symptoms such as:

  • Fever.
  • Headache.
  • Nausea.
  • Vomiting.
  • Lethargy.

Severe disease most often occurs among children less than 16 years old and is characterized by:

  • Seizures.
  • Coma.
  • Paralysis.
  • A variety of neurologic complications after recovery.

Death from infection with La Crosse virus is rare and occurs in less than 1% of cases.

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

How is La Crosse virus disease diagnosed?

La Crosse virus disease 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 La Crosse virus disease?

There is no specific treatment for La Crosse virus disease, and care is based on symptoms.

Who is at risk in Ohio?

Anyone who spends outdoors can be at risk for La Crosse virus infection.  The mosquito that transmits La Crosse virus disease, the eastern treehole mosquito, is found in woodlots and wooded areas, so people who live or recreate near these areas are at increased risk.

Ohioans of all ages can get sick with La Crosse virus, but most cases reported each year are among children.  More than 90% of cases reported in Ohio are among children younger than 15 years of age.

Graph: La Crosse virus disease by age and sex

La Crosse Virus Disease by Age and Sex, Ohio, 2012-2021

Age Female Male
 0-4 years 17 23
 5-9 years 46 57
 10-14 years 27 40
 15-18 years 2 5
 19-25 years 0 0
 26-60 years 2 0
 61+ years 3 1

Source: Ohio Department of Health

What time of year are Ohioans at risk for contracting La Crosse virus disease?

In Ohio, La Crosse virus infection can occur anytime during mosquito season, which typically runs from May through October.  Most of the cases are reported July through September.

It can take anywhere from five to 15 days from when the mosquito bite occurs to when symptoms of La Crosse virus disease appear.  Since most cases become ill in July through September, that means most are bitten by an infected treehole mosquito between mid-June and mid-September.  Therefore, summer through early fall is the time of year when Ohioans are most at risk for contracting La Crosse virus disease.

Graph: La Crosse virus disease by week of illness onset in Ohio

La Crosse 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 1
 June 22 1
  23 0
  24 2
  25 4
  26 6
 July 27 5
  28 13
  29 11
  30 13
 August 31 21
  32 22
  33 15
  34 20
 September 35 15
  36 14
  37 13
  38 16
  39 8
 October 40 7
  41 7
  42 8
  43 1
 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 cases of La Crosse virus disease since 1963, and more cases have been reported from Ohio than any other state in the United States.  An average of 20 cases are reported each year in Ohio.  However, epidemics can flare up under certain environmental conditions as was seen in 2011 in Ohio where 50 cases were reported from 34 counties.

Ohio La Crosse Virus Disease Annual Case Statistics
Year Human Cases Deaths Median Age (Years) Age Range of Cases (Years) Earliest Date of Symptom Onset Counties with Reported La Crosse Cases
2012 14 0 8.5 3 – 42 May 21 12
2013 16 0 9 0 – 78 Jul 12 15
2014 31 0 7 2 – 13 Jul 4 22
2015 24 0 7 0 – 14 Jun 26 18
2016 9 0 12 4 – 51 Jul 9 8
2017 13 0 8 4 – 65 Jun 16 12
2018 39 0 7 3 – 17 Jun 20 23
2019 26 0 7.5 0 – 75 Jun 27 16
2020 33 0 6 3 – 15 Jul 2 21
2021 18 0 7 0 – 78 Jun 21 12
 AVERAGE  22 0 9 n/a n/a 16
TOTAL 223 0 n/a n/a n/a n/a

How can I reduce my risk of La Crosse virus infection?

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

What are the roles of other animals in La Crosse virus transmission?

Mosquitoes become infected with La Crosse virus primarily through taking blood meals from infected mammals, especially squirrels and chipmunks.  However, the virus can be transmitted from infected female mosquitoes to their eggs, which results in infected offspring.  Because of this, La Crosse virus can persist in an area for years if mosquito breeding is not controlled.
Squirrels, chipmunks, and other small mammals are amplifying hosts for La Crosse virus, meaning they serve as a source of infection to mosquitoes that bite them and humans.