The prevalence of bovine tuberculosis (TB) in cattle, bison, and captive cervids in the U.S. is low – about seven per one million cattle screened, according to the Centers for Disease Control and Prevention. Still, low prevalence does not mean no prevalence, and researchers are increasingly concerned about the spread of the disease.

Of particular concern is the spread of the disease to and from wild animals. White-tailed deer are a reservoir host for bovine TB in the U.S.

“Once (the bacteria) gets into the wildlife population, it spreads unchecked through oral secretions and aerosol,” Srinand Sreevatsan, dean of the University of Missouri’s College of Veterinary Medicine, said. “Sometimes when wild animals, like deer, come into dairy or beef holdings and consume feed that is placed for cattle, cattle can potentially be exposed and develop the disease.”

This is the case in the northeastern region of Michigan, the only state with a region still not classified as bovine TB-free by the USDA. Since 2014, the state has been split into two TB zones: 79 counties in an Accredited Free Zone and a four-county area, the Modified Accredited Zone, in the northeastern lower peninsula, where bovine TB is endemic in the free-ranging deer population. 

As of 2022, Michigan has detected bovine TB in 82 cattle herds, six captive cervid herds, and six feedlots since 1997. Three cattle herds and two cervid herds were detected in the past three years. During the 2023 hunting season, bovine TB was detected in wild deer in three counties outside the Modified Accredited Zone. This prompted the state to set up designated testing areas for all cattle and bison herds near where the infected deer were found this past May.

Bovine TB is also a public health concern. In 2020, five workers at Michigan’s Wildlife Disease Laboratory were diagnosed with bovine TB, likely contracted from white-tailed deer. In 2017, the state reported that a hunter contracted bovine TB. People who consume raw dairy products are also at risk of contracting the illness. Bovine TB is curable in humans, although it requires six to 12 months of antibiotic treatment.

The disease is devastating and difficult to treat and prevent in cattle.

“Treatment against bovine TB (in animals) would be next to impossible because it would take about five to six months of treatment,” Sreevatsan said. “We would have to deliver three different types of drugs to the animals, during which time they can’t be productive. We can’t use the milk they make or the meat because of drug residues and the associated withholding times to assure that these drugs do not enter the human food chain.”

Instead of treatment, the official recommendation is to cull the infected animal or depopulate the herd depending on how many animals are infected.

“The standard test and slaughter program can be very devastating to both the farmer, the veterinarian, and the country itself,” Sreevatsan said. “Economically, indemnities need to be paid to the farmer, and even if we pay the farmer indemnities for depopulation of their herds, it still has a psychological impact on a farmer and their families.”

Sreevatsan is pioneering a novel vaccine to help prevent the spread of bovine TB. While a vaccine called BCG exists for bovine TB, it isn’t consistent in reducing the severity of the disease, and it doesn’t prevent infection. Because it uses a live, attenuated version of the bacteria strain, cattle in the U.S. are not vaccinated using BCG. It would be difficult to differentiate a vaccinated animal from one actively infected with bovine TB.

Instead of using a live version of the bacteria, Sreevatsan and his team developed a vaccine using a probiotic delivery system. Sreevatsan has identified components essential for the bacteria to survive and that elicit an immune response from an animal. These components are transferred onto a probiotic organism found in soil and in foods like yogurt.

“It’s like a nanoparticle that doesn’t need refrigeration or a cold chain for transportation, and we could deliver it orally,” he said. “It would elicit a mucosal immune response, the place which is most important in getting robust protection against bovine TB because the mucosa are the first portals of entry for this organism. The idea is that it can be used in cattle and potentially has opportunities to be applicable in the wildlife population.”

Currently, Sreevatsan is testing the vaccine in an experimental study on cattle at a biosafety level three facility in Missouri. He aims to learn how efficient the vaccine is at establishing protective immunity within a cattle population and how the bacteria are triggering the immune system.

“(The vaccine) would impact farmers quite significantly in reducing the disease burden and help (Michigan) go toward eradication of the disease from at least the domesticated animal population and creating immune buffer zones around the farms by vaccinating wildlife,” he said. “It’s a work in progress. We first must establish efficacy before we can make recommendations.”

Sreevatsan is also validating a diagnostic test that wouldn’t require the culling of animals to confirm bovine TB. The test screens blood from an animal for a biomarker specific to bovine TB.

Michigan recently conducted a vaccination study in white-tailed deer, deploying vaccine delivery units across Alpena County. Alfalfa and molasses cubes containing an edible, liquid version of the BCG vaccine were made available for deer. The project’s results are estimated to be available in September.