Possible Person-to-Person Transmission of Mycobacterium Bovis — Nebraska 2014–2015

BACKGROUND: Mycobacterium bovis, part of the M. tuberculosis complex, is a global zoonotic pathogen that primarily infects cattle; humans most commonly become infected by consuming contaminated, unpasteurized dairy products. Possible person-to-person airborne transmission has been reported. In April 2014, a Mexico-born man in Nebraska was found to have extensive pulmonary tuberculosis (TB) disease caused by M. bovis. In August 2014, a U.S.-born Hispanic girl from a nearby town was also found to have pulmonary M. bovis TB. Both patients had close contact while attending the same church.

METHODS: Case investigations were conducted. After initial genotyping, both patients’ isolates were sent to USDA National Veterinary Services Laboratory for whole-genome sequencing (WGS). Contact investigations were conducted following published guidelines. Tuberculin skin tests and interferon gamma release assays (IGRA) were used for testing U.S.- and foreign-born contacts, respectively; IGRA was used for church members, most of whom were foreign-born. Contacts with initial negative results were retested if their last exposure occurred <2 months earlier. All contacts reported country-of-birth except those at the second patient’s school (all were assumed U.S.-born). Contacts with positive results were asked about travel abroad and consumption of Mexico-origin unpasteurized dairy products. 

RESULTS: Both patients had pulmonary cavities on radiography and acid-fast bacilli (AFB) on sputum-smear microscopy, both indicators of contagiousness. No common dietary exposure was found. Both patients' isolates were closely related; WGS differed by only three single nucleotide polymorphisms. Initial examination of 181 contacts found 39 (22% [11 US-born]) with latent TB infection: 10 (42%) of 24 who had close exposure to either patient, 28 (28%) of 100 exposed to one or both in the church, and one (2%) of 57 exposed to the second patient at the school. Latent infection was diagnosed in 6 contacts on follow-up examination, 2 months after a negative test result, for an overall latent infection rate of 25%. No infected contacts recalled consuming unpasteurized dairy products or had active TB disease at initial or secondary examination.  

CONCLUSIONS: Potential spread of M. bovis from the first to the second patient and to both patients’ contacts adds evidence for possible person-to-person airborne transmission. WGS is an emerging tool for investigating transmission. Public health responses to M. bovis pulmonary TB should be the same as 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 TB globally and to pasteurize dairy products.