Long-term trend analysis of Wisconsin’s climate indicates that Wisconsin is becoming warmer and wetter. After analyzing historical climate data from 1950 to 2006 and developing climate models, University of Wisconsin climate scientists created climate projections based on the historical trends and scientifically validated models. Some of these projections show that increased flooding and heat events are likely to occur over the coming decades. These trends could influence the physical and biological conditions that allow for an increased occurrence of vector-borne diseases.
With the warmer and wetter climate, changes may occur within vector populations. A vector is a species such as a mosquito or tick that can transmit a disease agent.
Of concern are mosquito species that transmit arboviral diseases, including West Nile virus (WNV), La Crosse/California encephalitis (LAC/CA), Jamestown Canyon virus (JCV), St. Louis encephalitis (SLE), and Eastern equine encephalitis (EEE) infections. Mosquito populations may increase throughout Wisconsin as annual average temperatures are projected to increase. Wet or dry weather conditions may contribute to the selection of certain mosquito species, allowing them to thrive in their natural habitats.
In areas with existing mosquitoes that are known carriers of WNV, warmer winters with fewer hard freezes can be associated with a higher incidence of WNV disease. Winters with fewer hard freezes result in fewer mosquitoes dying off during the winter and could lengthen the season of WNV transmission. As a result, human cases of WNV in Wisconsin may increase.
Underground storm water catch basins found in older urban areas can produce standing water. Because stagnant water is a perfect home for mosquito breeding, this environment means a possible increase in the number of WNV cases.
In addition, warmer and wetter environmental conditions could lead to newly emerging mosquito species in Wisconsin, which can transmit viruses including the Jamestown Canyon virus, Chikungunya, and Dengue Fever. The Jamestown Canyon virus has been isolated from 22 different mosquito species, including the Anopheles sp. recently detected in water samples collected from Milwaukee urban ponds.
More information about mosquito-transmitted diseases is available elsewhere on the DHS website.
Tick populations and distribution may also be changing in response to changing weather and climate. The blacklegged tick (commonly known as deer tick) is the main vector for the two most common tickborne diseases in Wisconsin: Lyme disease and anaplasmosis. Other tickborne diseases in Wisconsin include ehrlichiosis, Powassan virus, and Babesiosis.
Deer ticks are abundant in northwestern Wisconsin, and recent tick surveillance has shown that deer tick populations are expanding across Wisconsin into the state's northeastern and southern regions. In Wisconsin, there have been documented cases of several varieties of tickborne diseases, including Lyme disease. These cases are most likely to occur during the summer months when ticks are active. There is ongoing surveillance throughout the state to track the occurrences of Lyme disease and other vector-borne diseases.
Since 2008, there has been an increase in reported cases of Ehrlichiosis in the Eau Claire region of Wisconsin. A new Ehrlichia species (Ehrlichia muris-like) was discovered in Wisconsin and Minnesota in 2009, and has been linked to these cases.
Another emerging tick is the Amblyomma americanum, commonly known as the lone star tick. Recent tick surveillance has reported a few lone star ticks found in southeastern Wisconsin. However, warmer annual temperatures, especially during the winter months, may allow the lone star tick to move into central and northern Wisconsin, bringing with it an increasing risk for Ehrlichiosis (exit DHS), a bacterial illness that can cause fatigue, fever, and headache.
Tick surveillance and tracking projects are ongoing in the northwestern corner and south central parts of Wisconsin in collaboration with the U.W. Department of Entomology, DHS Bureau of Communicable Diseases (BCD), and Eau Claire City-County Health Department, to assess disease prevalence, varying tick phenology, and habitat changes.
Several emerging tick-related diseases have the potential to become more common in Wisconsin. Atypical Lyme disease caused by Borellia spp. has been found in the La Crosse region and has the potential to become more widespread. The lone star tick has the potential to establish a population in Wisconsin.