Wisconsinites know they’re likely to encounter deer ticks, and along with them the bacteria that cause Lyme disease. First diagnosed in 1975, the disease is found across the Northern Hemisphere. In the U.S., it’s most heavily concentrated in New England and the mid-Atlantic states.

However, Minnesota and Wisconsin form a second zone of high incidence in the U.S.. In 2015, state and federal estimates indicated that Wisconsin had 22.7 cases of Lyme disease per 100,000 people, compared to 8.9 in the United States as a whole. This incidence rate in Wisconsin has been well above the national average since at least 2005, and is rivaled by only a handful of other states.

But these numbers don’t represent a definitive count of how many people actually come down with Lyme disease in Wisconsin each year.

Public health officials at local, state, and federal levels acknowledge that Lyme disease is underreported, especially in states like Wisconsin and New York where it is endemic. This is largely because of the choices physicians make when diagnosing and reporting Lyme cases, and the limited staff health agencies can devote to following up on those diagnoses.

“I’m going to guess probably maybe 30 percent more than are actually confirmed,” said Cindy Zach, health officer for Ashland County, when asked how underreported Lyme disease might be there. The northern Wisconsin county doesn’t have the highest incidence of the disease in the state, but it still has more cases per 100,000 people than most, and borders Price County, which ranks among the state’s Lyme hot spots.

This doesn’t mean that people aren’t going to their doctors and getting treatment. In fact, health officials consider the common methods for diagnosing Lyme disease — especially checking the patient for symptoms like a telltale rash or testing blood samples for bacterial antibodies — to be quite reliable.

“I think it’s also important to really understand that under-reporting does not mean under-diagnosis,” said Kiersten Kugeler, a Colorado-based epidemiologist with the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases.

In many cases, doctors check for the rash, diagnose the patient and prescribe antibiotics without running a blood test. Without that test, though, public health officials cannot count the case as a “confirmed” case of Lyme disease. In public health, a case of a given disease is either “suspected,” “probable” or “confirmed.” Epidemiologists define those gradations a bit differently for each disease.

For Lyme disease, the medical profession has agreed that “confirmed” means a blood sample testing positive for the bacterial antibodies, said Jeff Engel, executive director of the Council of State and Territorial Epidemiologists, which coordinates among state health agencies and the CDC to gather national data on diseases. A “probable” case occurs when a patient has a telltale rash and a history of exposure to deer ticks. “Suspected” cases can involve a whole variety of circumstances that don’t include the evidence of a rash, exposure history or blood test.

These differences matter because the “confirmed” number is what the CDC uses to aggregates disease statistics across the country to create a national picture of a given disease’s prominence between 2005-15. Public health officials began formally tracking the disease in the early 1990s.

Engel, who is a medical doctor, does not begrudge physicians who skip a blood antibody test. If a health care provider identifies the rash and knows the patient has been in a deer tick habitat or had a one on their body, they can generally feel confident in a Lyme disease diagnosis. The blood test doesn’t directly looks for the bacteria that cause Lyme disease, but for antibodies produced by the immune system in response.

But some people infected by Lyme-causing bacteria might not begin generating antibodies until a month or so after infection. “By then the patient is treated and feeling well, and is not going to come back to the doctor for a lab test,” Engel said.

Many doctors often reason that if they already have made a diagnosis, they needn’t order lab work that will end up costing their patient more money. Engel is fine with that, and Zach said she understands it too. Even if a patient has health insurance, the test could cost them out of pocket, especially if they’re on a high-deductible plan.

“It’s a cost savings and Ashland County is a very poor county,” Zach said. “We have a lot of residents who are on social assistance, Medicaid and Medicare, and when they’re having to pay copays and out-of-pocket expenses for healthcare, then anything a physician can do to help them improves our population’s health. It just doesn’t improve the science behind the disease in terms of finding out how many people are infected.”

Accepting incomplete numbers

Local health departments are the front lines of data gathering on diseases, passing information they gather onto the Wisconsin Department of Health Services’ Division of Public Health, which in turn sends it along to CSTE and the CDC. Local agencies’ staff are tasked with making follow-up calls to medical practitioners and patients when they need more information on a given case, and often there just aren’t enough people or hours to go around, especially in rural areas.

Bayfield County Health Department director Sarah Wartman is one of just five public-health nurses in the agency, two of whom work only part-time.

“We do receive quite a significant number of lab reports, but without any other information,” Wartman said. “We try to follow up each lab report we receive, but we simply don’t have the staffing.”

Like other public health officials interviewed for this article, Wartman said she doesn’t really blame doctors or others in the healthcare system for obstacles to more thorough info about how widespread Lyme disease is.

“But I feel like there could be more dedicated funding if we had a more accurate, realistic look at the numbers in our county,” she said.

While people get Lyme disease all over Wisconsin, Wartman said people in urbanized areas like Dane and Milwaukee counties (which have more public health resources) might not quite understand the level of exposure people elsewhere in the state have to tick habitat.

“Almost anyone in our county is exposed to ticks,” Wartman said. “Even if you live in the city of Washburn, you’re exposed to wood ticks or deer ticks. You could be mowing your lawn in the middle of the town of Mason and be exposed to deer ticks.”

Epidemiologists at the DHS are responsible for compiling data that comes in from the counties, but don’t do a lot of the shoe-leather work of gathering information about individual cases.

“We work with providers and local health departments about questions that may come up, but when it comes to patient follow-up in large part that’s done by the local health department,” said Rebecca Osborn, a vector-borne disease epidemiologist with the DHS Division of Public Health.

Osborn and her colleagues know that underreporting of Lyme disease make it difficult to treat the raw numbers as an accurate measurement of incidence in the state, so the agency uses a statistical estimation algorithm to try and estimate the true burden of infection. This algorithm draws on a 20-year span of data about Lyme disease in Wisconsin and information including patients’ age, sex and symptoms to try and account for potential blind spots, explained Christine Muganda, a population data analyst with DHS.

The data from such an algorithm can help state and local officials get a better handle on the scale of underreporting. Still, it can’t quite make up for the hard numbers the CDC needs when gathering national statistics.

“That’s a dilemma and it’s not going to be resolved, because the national notifiable disease system is event-based and case-based, and to my knowledge it’s going to remain that way,” said Engel. “The thing is you just need to be transparent with the public.”

The discovery of another bacteria that causes Lyme disease, Borrelia mayonii, could further complicate the search for solid numbers. Previously, scientists knew of three species of bacteria that cause Lyme disease: Borrelia burgdorferi in North America, and Borrelia afzelii and Borrelia garinii in Europe and Asia.

So far, the Mayo Clinic researchers who discovered (and, of course, named) Borrelia mayonii have found it only in Minnesota and Wisconsin, and only a few cases at that. Like Borrelia burgdorferi, mayonii is carried by deer ticks, so cases of Lyme disease caused by the two organisms may follow a similar distribution.

“I think it does raise the question of whether there are other species that can cause Lyme disease,” said Dr. Bobbi Pritt, who leads the Mayo Clinic lab group that discovered the bacterium.

States and local governments could use more funding to improve their reporting, but paradoxically need solid data to make a strong case for such support. Then again, gaps in reporting have not obscured the fact that Wisconsin has a high incidence of Lyme disease.

Despite leading a national organization of epidemiologists, Engel said this data isn’t always the most important factor.

“Sometimes the funding is directly proportional to the morbidity the communities experience,” he said. “But in Lyme disease I think we overemphasize surveillance sometimes, especially in endemic areas. What public health really needs to focus on is the prevention message.”