West Nile virus: Facts and Figures


West Nile virus cases in Minnesota

Data source - MDH Vectorborne Disease Program surveillance data.

West Nile virus (WNV) cases were first found in Minnesota in 2002. The number of WNV cases is variable from year to year and influenced by environmental factors, such as temperature and precipitation. Warm, dry conditions are optimal for the mosquito to breed and the virus to reproduce. The weather can also play a role in influencing human behavior and decisions about time spent outdoors, type of clothing worn, and repellent use.


Average number of West Nile virus cases by county

Source: MDH Vectorborne Disease Program surveillance data, 2002-2016. Unadjusted rates per 100,000. Click to download data.

The risk for West Nile virus (WNV) is highest in open, agricultural areas of the state, primarily in western and central Minnesota. People are typically exposed to WNV around their home, not from traveling to other areas of the state.


West Nile virus cases by month of illness onset

Data source - MDH Vectorborne Disease Program surveillance data. Chart represents the month of symptom onset for confirmed cases of WNV from 2002-2016. Onset is not known for all cases.

People may become ill with West Nile virus (WNV) from May through November, with most people developing WNV symptoms in August. The time from when someone is bitten by a WNV infected mosquito to when they start developing symptoms is usually about 2-6 days, although it can be as long as 15 days. The late summer and early fall are high-risk times for WNV infection.


West Nile cases by age and sex

Data source - MDH Vectorborne Disease Program surveillance data. 2002-2016

There are more reported West Nile virus (WNV) cases in males than females for all age groups, except for those under age 5. Most reports of WNV cases are in males 60 to 65 years. There may be more cases in older males due to their outdoor activity, which puts them at greater risk.

For both sexes, there are more WNV cases in middle-aged to older adults. Older adults are more likely to develop severe symptoms from WNV, such as encephalitis (brain swelling).


West Nile virus cases by symptom severity

Data source - MDH Vectorborne Disease Program surveillance data.

People with mild West Nile virus (WNV) disease develop symptoms like fever or headache. Severe WNV disease symptoms include encephalitis or meningitis (brain and spinal cord inflammation).This chart shows the number of cases that have milder WNV illness compared to those who develop severe WNV symptoms.

Most people infected with West Nile virus will either have no symptoms or experience a flu-like illness. Approximately 20% of people bitten by an infected mosquito will develop the milder symptoms of West Nile fever. Less than 1% of people bitten by an infected mosquito will develop the more severe form of WNV, West Nile encephalitis.