Deer-to-human M. bovis transmission identified in Michigan
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Key takeaways:
- Between 2019 and 2022, three confirmed and one probable case of Mycobacterium bovis were identified in humans.
- The investigation revealed connections to M. bovis detected in free-ranging deer in Michigan.
Several cases of Mycobacterium bovis — an enzootic disease typically affecting cattle — transmitted from deer to humans were reported in Michigan between 2019 and 2022, according to a recent study.
“M. bovis is usually a disease of cattle. Veterinarians thought they had eliminated bovine tuberculosis (TB) in cattle by the 1980s across the U.S., but in northeastern Michigan, which is heavily forested, there was unrecognized spillover into wild deer,” James Sunstrum, MD, an infectious disease specialist with Corewell Health East, told Healio. “This study was prompted by the discovery of M. bovis in a human sample, matching the DNA pattern of known samples from deer and cattle [which] prompted epidemiological investigations.”
For the study, researchers collected data on human TB cases with cultures of M. bovis from the Michigan Department of Health and Human Services (MDHHS) TB database. According to the study, researchers performed clinical review and interviews, during which they focused on risk factors for contact with wildlife and cattle. Whole-genome sequences of human isolates were then compared with a veterinary library of M. bovis strains to identify cases linked to the enzootic focus.
In total, four cases of M. bovis in humans were identified — three confirmed and one probable — between 2019 and 2022. These cases included cutaneous disease, two severe pulmonary disease cases and human-to-human transmission.
The investigation revealed that the three human isolates had 0-3 single-nucleotide polymorphisms with M. bovis strains circulating in wild deer and domestic cattle in Michigan, signaling a connection between the human cases and the wild deer.
According to the study, enzootic focus of M. bovis in free-ranging deer began in Michigan in 1994 after evidence of transmission to local cattle herds was reported. Because of this focus, three Michigan deer hunters with M. bovis were identified between 2002 and 2017, followed by the four additional cases identified during the 2019-2022 investigation.
Based on these connections, the researchers hypothesized that field dressing a deer carcass can trigger a brief but intense aerosol exposure to pulmonary secretions during removal of thoracic and abdominal viscera, which could lead to infection. Consuming undercooked venison could have also been a mode of transmission. However, the researchers noted that no patients in the investigation reported consumption of undercooked venison or “uninspected freezer beef.”
“Infectious disease physicians need to recognize the potential for zoonotic TB in selected circumstances,” Sunstrum said. “The infectious disease community needs to establish working relationships with wildlife and cattle veterinarians to collaborate on eradication.”
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