Airborne transmission suspected in human M. bovis cases

Investigations into two human cases of Mycobacterium bovis infection resulting in tuberculosis (TB) in Nebraska indicate airborne transmission. The zoonotic M. bovis pathogen, usually associated with cattle, can spread to other mammals including humans. Human cases often are associated with consumption of raw milk.

A team of researchers led by Bryan F. Buss, DVM, with the Centers for Disease Control and Prevention investigated the cases in Nebraska. The researchers reported their investigation in the CDC's Weekly Morbidity and Mortality Report.

The researchers report that a Nebraska man from Mexico experienced cough and fever for several months and was subsequently diagnosed with extensive pulmonary TB caused by M. bovis. The patient had previously worked on a dairy farm and indicated he had frequently consumed unpasteurized milk. Four months after he was diagnosed, a girl from a nearby town also was determined to have pulmonary TB caused by M. bovis. She had no apparent contact with cattle or known consumption of raw milk. The researchers used whole-genome sequencing of M. bovis isolates from both patients to determine the bacteria were essentially identical.

The only connection between the two patients was they attended the same church, so the researchers examined 181 possible contacts from that church, the second patient's school, the first patient's workplace and both patients' families. They found that 39 (22%) had latent infections with M. bovis. Of 24 people who had close contact with either patient, 10 (42%) had latent infections. In a follow-up two months after the initial examinations, six more contacts tested positive for latent infections, for a total infection rate of 25 percent. None of the infected contacts reported consuming unpasteurized dairy products.

The researchers note that investigating human M. bovis infections is challenging, in part because the incubation period, which has not been well studied, could range from months to years and might obscure ascertainment of time and nature of exposure.

They conclude, however that the investigation adds to the evidence for airborne person-to-person spread of M. bovis. They also note that whole-genome sequencing is an emerging tool for investigating transmission and that public health responses to M. bovis pulmonary TB should be the same as those for M. tuberculosis TB, with additional inquiries about consumption of unpasteurized dairy products. "The ongoing incidence of M. bovis TB in humans substantiates the need to control bovine tuberculosis globally and to pasteurize all milk and dairy products," they write.

Read more in the Weekly Morbidity and Mortality Report from the CDC.