Marshfield study finds 51% of deer ticks carried Lyme bacteria: full analysis

A Wisconsin tick surveillance project is adding new weight to concerns about Lyme risk in the Upper Midwest. Marshfield Clinic Research Institute reported that 51% of 707 adult female deer ticks, also called blacklegged ticks, tested through its Tick Inventory via Citizen Science study carried Borrelia burgdorferi, the bacterium behind Lyme disease. The result was higher than researchers expected and drew broad local coverage after a May 15 preprint outlined the study’s early findings. (wispolitics.com)

The finding builds on a broader surveillance effort Marshfield launched in 2024 to map tick species, distribution, and pathogens using ticks submitted by the public. Over the last two years, the program has received nearly 12,500 ticks from Wisconsin and surrounding areas, giving researchers a larger passive-surveillance dataset than most clinics or local agencies could assemble on their own. Marshfield has already used the project to highlight the growing importance of deer ticks, which are smaller than wood ticks and more likely to carry pathogens of concern. (wpr.org)

The headline number comes with some important context. The 51% figure applies specifically to adult female deer ticks tested in the study, not to all ticks in Wisconsin and not to the odds that a person or animal will develop Lyme disease after a bite. Linz told Wisconsin Public Radio she was surprised by the prevalence, but also cautioned against interpreting the result as a simple 50-50 infection risk from any single bite. The researchers also found Lyme-causing pathogens were more common in western Wisconsin and during the fall, reinforcing the point that tick season does not end with summer. (wpr.org)

That pattern fits with what Wisconsin public health agencies have been saying. Wisconsin DHS describes Lyme disease as the most commonly reported illness spread by ticks in the state and warns that tick exposure risk persists whenever temperatures are warm enough for ticks to be active. CDC likewise emphasizes prevention on pets, including veterinary-guided tick control and discussion of Lyme vaccination for dogs in endemic areas. (dhs.wisconsin.gov)

Industry and expert commentary also points in the same direction. CAPC’s 2026 Pet Parasite Forecast says Lyme disease, anaplasmosis, ehrlichiosis, and other vector-borne threats are continuing to expand geographically, with Wisconsin named among the high-risk states influencing spread in the Midwest. That forecast is based on what veterinarians and laboratories are expected to see in dogs, making it especially relevant for companion animal practice. (capcvet.org)

Why it matters: For veterinary teams, the Marshfield data is less about a surprise pathogen and more about sharpening the risk conversation. In endemic regions, pet parents may still think of tick prevention as a spring or summer issue, but these results support counseling that risk can remain meaningful into fall, and in some settings effectively year-round. The study also offers a useful local hook for discussing canine Lyme vaccination, adherence to preventives, prompt tick removal, and the reality that blacklegged ticks can carry more than one pathogen. In practices serving Wisconsin, Minnesota, northern Illinois, or traveling pets, this is the kind of regional surveillance signal that can help improve compliance because it feels immediate and local. (wsaw.com)

There are still limits to the dataset. This is a citizen-science, passively collected sample, and the headline figure comes from a subset of adult female ticks rather than a randomized statewide surveillance design. Even so, the scale of the program and the consistency of the signal across Marshfield, Wisconsin media, and public health context make it a meaningful indicator of exposure pressure. (wispolitics.com)

What to watch: Marshfield’s project is now in its third year, so the next things to watch are whether the preprint advances to peer-reviewed publication, whether larger seasonal analyses confirm the western Wisconsin and fall trends, and whether veterinary and public health messaging in the region becomes even more explicit about sustained tick prevention beyond summer. (wispolitics.com)