Environmental Public Health Tracking: West Nile Virus Data

West Nile virus is an illness spread by the bite of an infected Culex species mosquito. West Nile virus cases occur throughout the U.S. In Wisconsin, West Nile virus was first found in wild birds in 2001, and the first human infections were reported in 2002. 

Anyone can get West Nile virus, but people who spend more time outdoors are at a higher risk of being bitten by an infected mosquito. Mosquitoes are usually most active in Wisconsin from May to September. The mosquitoes that spread West Nile virus can be found in areas near standing water, which they need to breed.

It is important to remove standing water sources, such as containers, leaves, and yard debris, to reduce breeding habitats in your yard.

Tracking West Nile virus gives public health professionals a better understanding of how often West Nile virus happens in their county. 

We monitor how many cases of West Nile virus occur in a county over time and can use that information to educate our communities and plan prevention efforts.

The source of the data is the Wisconsin Vectorborne Disease Program. Healthcare providers are required to report cases of West Nile virus.

The Vectorborne Disease Program aggregates the cases and sends this information to Wisconsin Environmental Public Health Tracking Program.

• Cases of West Nile virus – The total number of West Nile virus cases in Wisconsin in a given year, and in rolling 5-year intervals.
• Incidence of West Nile virus – The crude rate of new confirmed cases of West Nile virus in a given year, and in rolling five-year intervals, per 100,000 people. See our definitions and explanations of terminology on incidence rate for more information.

• Cases are based on the county of residence; some infections may have been acquired during travel to other areas.
• Data users should keep in mind that many factors contribute to illness. These factors should be considered when interpreting the data. Factors include:
  • Socioeconomic status
  • Demographics (age)
  • Geography (rural, urban)
  • Changes in the medical field (diagnosis patterns, reporting requirements)
  • Individual behavior (outdoor hobbies)
• There are more cases of non-neuroinvasive West Nile virus than neuroinvasive West Nile virus; however, the data show decreasing case numbers of non-neuroinvasive West Nile virus over time. This is likely because neuroinvasive West Nile Virus cases have more severe symptoms, which are more likely to require a person to seek medical care and get tested. Therefore, they are more likely to be detected and reported than non-neuroinvasive West Nile virus cases. People with non-neuroinvasive West Nile virus have have less severe symptoms and may not even know that they have West Nile virus.

• Wisconsin Department of Health Services – Diseases Spread by Mosquitos
• Wisconsin Department of Health Services – West Nile Virus
• University of Wisconsin-Madison Department of Entomology – Mosquito transmitted viruses
• Wisconsin Climate and Health Program – Wisconsin Vectorborne Disease Toolkit, P-01109 (PDF)
• Centers for Disease Control and Prevention – West Nile Virus

Click the link below to download the data you're looking for:

County Data (CSV)

State Data (CSV)

Number of West Nile virus cases

These data are obtained from the Wisconsin Electronic Disease Surveillance System (WEDSS). WEDSS is a secure, web-based system used by public health staff, infection control practitioners, clinical laboratories, clinics, and other disease reporters to report communicable diseases. "Month" indicates the month of illness onset. County-level data are based on the county of residence of the case; some infections may have been acquired during travel to other areas. The entire state of Wisconsin is considered to be endemic for West Nile virus; thus, any Wisconsin resident is considered to be "exposed." This dataset combines both “Confirmed” and “Probable” cases of West Nile virus. West Nile virus can be neuroinvasive or non-neuroinvasive.

Laboratory Criteria for Diagnosis

West Nile virus is diagnosed using a blood test. The laboratory criteria for diagnosing West Nile virus includes:

• Isolation of virus from, or demonstration of West Nile virus antigen or nucleic acid in, tissue, blood, CSF, or other body fluid, OR
• Four-fold or greater change in West Nile virus quantitative antibody titers in paired sera, OR
• West Nile virus IgM antibodies in serum with confirmatory West Nile virus neutralizing antibodies in the same or a later specimen, OR
• West Nile virus IgM antibodies in CSF or serum.

Definition of Neuroinvasive disease

West Nile virus can cause neuroinvasive diseases such as aseptic meningitis, encephalitis, or acute flaccid paralysis (AFP). These illnesses are usually characterized by the acute onset of fever with headache, myalgia, stiff neck, altered mental status, seizures, limb weakness, or cerebrospinal fluid (CSF) pleocytosis. AFP may result from anterior ("polio") myelitis, peripheral neuritis, or post-infectious peripheral demyelinating neuropathy (i.e., Guillain-Barre’ syndrome). Less common neurological manifestations, such as cranial nerve palsies, also occur. A clinically compatible case of neuroinvasive disease is defined as follows:

• Meningitis, encephalitis, acute flaccid paralysis, or other acute signs of central or peripheral neurologic dysfunction, as documented by a physician, AND
• Absence of a more likely clinical explanation. Other clinically compatible symptoms of arbovirus disease include: headache, myalgia, rash, arthralgia, vertigo, vomiting, paresis and/ or nuchal rigidity.

Definition of Non-neuroinvasive disease

West Nile virus is capable of causing an acute systemic febrile illness that may include headache, myalgias, arthralgia, rash, or gastrointestinal symptoms. A clinically compatible case of non-neuroinvasive disease is defined as follows:

• Fever (chills) as reported by the patient or a health-care provider, AND
• Absence of neuroinvasive disease, AND
• Absence of a more likely clinical explanation. Other clinically compatible symptoms of arbovirus disease include: headache, myalgia, rash, arthralgia, vertigo, vomiting, paresis and/ or nuchal rigidity.

Reference: Arboviral Diseases, Neuroinvasive and Non-neuroinvasive 2015 Case Definition | CDC

Incidence

Crude rates of confirmed cases per 100,000 population

These data are obtained from the Wisconsin Electronic Disease Surveillance System (WEDSS). WEDSS is a secure, web-based system used by public health staff, infection control practitioners, clinical laboratories, clinics, and other disease reporters to report communicable diseases. County-level data are based on the county of residence of the case; some infections may have been acquired during travel to other areas. Incidence rates were calculated using neuroinvasive and/or non-neuroinvasive West Nile virus cases as the numerator. The incidence, or crude rate, is then calculated by dividing the numerator by the total number of people in the population of interest (for example, a county). Population estimates are derived from the U.S. Census. This is expressed as a number per unit population such as "per 100,000 population." Note: a crude rate does not take into account the differences in age distributions across counties and are therefore subject to bias. For example, as neuroinvasive West Nile virus is more common people older than 65 years old, areas of the state with higher populations of older adults could appear, artificially, to have more neuroinvasive West Nile virus cases.

For more information on age-adjustment and biases see our definitions and explanations of terminology.