A recent article from the U.S. Centers for Disease Control and Prevention (CDC), published in the Morbidity and Mortality Weekly Report (MMWR), describes the results of an intensive contact tracing program initiated after it was suspected that a Nebraska man, who had pulmonary tuberculosis attributable to Mycobacterium bovis, had acted as the primary focus for person-to-person transmission to other susceptible people, through airborne routes.
The CDC MMWR reports a case of possible person-to-person transmission of Mycobacterium bovis pulmonary disease. The article states:
Mycobacterium bovis, one of several mycobacteria of the M. tuberculosis complex, is a global zoonotic pathogen that primarily infects cattle. Humans become infected by consuming unpasteurized dairy products from infected cows (1,2); possible person-to-person airborne transmission has also been reported (3). In April 2014, a man in Nebraska who was born in Mexico was determined to have extensive pulmonary tuberculosis (TB) caused by M. bovis after experiencing approximately 3 months of cough and fever. Four months later, a U.S.-born Hispanic girl from a nearby town who had been ill for 4–5 months was also determined to have pulmonary TB caused by M. bovis. The only social connection between the two patients was attendance at the same church, and no common dietary exposure was identified. Both patients had pulmonary cavities on radiography and acid-fast bacilli (AFB) on sputum-smear microscopy, indicators of being contagious (4). Whole-genome sequencing results of the isolates were nearly indistinguishable. Initial examination of 181 contacts determined that 39 (22%) had latent infection: 10 (42%) of 24 who had close exposure to either patient, 28 (28%) of 100 who were exposed to one or both patients in church, and one (2%) of 57 exposed to the second patient at a school. Latent infection was diagnosed in six contacts on follow-up examination, 2 months after an initial negative test result (4), for an overall latent infection rate of 25%. No infected contacts recalled consuming unpasteurized dairy products, and none had active TB disease at the initial or secondary examination. Persons who have M. bovis TB should be asked about consumption of unpasteurized dairy products (2), and contact investigations should follow the same guidance as for M. tuberculosis TB (4).
This paper is important mainly because it establishes the much-suspected person-to-person airborne transmission of M bovis, which is conventionally associated with extrapulmonary tuberculosis, especially disease of the gut acquired through ingestion of infected milk.
This study is basically the results of a large contact tracing program, which established the single point source of the infection in the exposed people, through molecular characterization of the bacteria.
The public health implications are several, including the need to establish contact tracing protocols for patients suffering from bovine tuberculosis.
Buss BF, Keyser-Metobo A, Rother J, et al. Possible Airborne Person-to-Person Transmission of Mycobacterium bovis — Nebraska 2014–2015. MMWR Morb Mortal Wkly Rep 2016;65:197–201. DOI: http://dx.doi.org/10.15585/mmwr.mm6508a1