Contact investigations of two pulmonary tuberculosis cases resulting from Mycobacterium bovis infection suggest that multiple instances of potential airborne transmission may have contributed to the bacterium’s spread among Nebraskan residents.
M. bovis is a zoonotic pathogen responsible for pulmonary infection in cattle, but also is capable of spreading to deer, humans and other mammals, according to an MMWR study. Human M. bovis and M. tuberculosis are clinically indistinguishable, but require unique regimens due to inherent treatment resistances.
“Human M. bovis disease is typically attributed to consumption of unpasteurized milk (or dairy products made from unpasteurized milk) in or imported from countries with affected cattle herds,” researchers wrote. “Person-to-person airborne transmission of M. bovis has been reported infrequently, with uncertainty remaining about dietary exposures.”
Airborne transmission likely in some cases
In April 2014, a Nebraskan man aged 42 years who emigrated from Mexico in 2010 sought care for persistent cough, fever, weight loss and progressive debilitation. Nucleic acid amplification testing of sputum suggested pulmonary TB resulting from M. bovis. The man reported being currently employed on a dairy farm, as well as frequent consumption of raw milk.
Two months later, a girl aged 16 years who was born in Nebraska also sought medical care for persistent cough, which was eventually diagnosed as M. bovis TB. The patient reported no history of international travel and no memory of consuming Mexican dairy products.
Cultures grown from the patients’ sputum samples revealed the two isolates to be indistinguishable, with later whole-genome sequencing results suggesting that the two were closely related and shared a common ancestor with isolates collected from Mexican cattle. The only social connection identified between the patients was attendance at the same church, where their interactions were reported as minimal.
Contact investigations of household, community and work acquaintances found 24 individuals with extended close exposure, 10 of whom were determined to have latent infection. Another latent infection was detected among 57 school contacts of the second patient, while 28 of 100 individuals exposed to either patient at the church also showed infection. Two months after initial testing, follow-up examination of these contacts identified six more infections, resulting in an overall latent infection rate of 25%.
An investigation of possible transmission events suggested that airborne transmission was likely in approximately one-third of the identified infections, the researchers wrote, but unrecognized consumption of imported dairy products carrying the pathogen should not be ruled out.
“This report adds to the evidence for airborne person-to-person spread of M. bovis,” the researchers wrote. “Public health responses to M. bovis pulmonary TB should be the same as those for M. tuberculosis TB, with the 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.”
M. bovis TB rises in California
M. bovis has gradually become a greater concern in other states such as California, where resulting TB cases appear to have increased, according to a retrospective review published last year in Emerging Infectious Diseases.
Using data from the state’s TB registry collected from 2003 to 2011, researchers identified culture-confirmed patients whose initial drug susceptibility results demonstrated resistances akin to M. bovis. They reviewed the registry for sociodemographic, clinical and treatment outcome information for all patients, and conducted bivariate analysis of these patient characteristics.
They found that the proportion of TB cases in California for all ages caused by M. bovis increased from 3.4% of 2,384 cases in 2003 to 5.4% of 1,808 cases in 2011 (P = .002), an increase over the 1% to 2% of TB cases in the U.S. reportedly attributable to M. bovis infection. In multivariate analysis, the researchers determined independent associations between M. bovis disease and Hispanic ethnicity, extrapulmonary disease, diabetes and immunosuppressive conditions. Also of note, all six cases of M. bovis disease in children during 2010 and 2011 had at least one parent or guardian who was born in Mexico, as opposed to 38% of child case-patients with disease attributable to M. tuberculosis (P = .005).
“In California, there are ongoing interventions designed to limit the demand for and distribution of unpasteurized and contaminated dairy products, which are associated with M. bovis disease and other foodborne diseases,” the researchers wrote. “Elimination of human M. bovis disease in California likely requires further implementation of programs to reduce M. bovis contamination.” – by Dave Muoio
Disclosure: The researchers report no relevant financial disclosures.