Zoonotic Mycobacterium bovis–induced tuberculosis in humans

We aimed to estimate the global occurrence of zoonotic tuberculosis (TB) caused by Mycobacterium bovis or M. caprae infections in humans by performing a multilingual, systematic review and analysis of relevant scientific literature of the last 2 decades. Although information from many parts of the world was not available, data from 61 countries suggested a low global disease incidence. In regions outside Africa included in this study, overall median proportions of zoonotic TB of ≤1.4% in connection with overall TB incidence rates ≤71/100,000 population/year suggested low incidence rates. For countries of Africa included in the study, we multiplied the observed median proportion of zoonotic TB cases of 2.8% with the continental average overall TB incidence rate of 264/100,000 population/year, which resulted in a crude estimate of 7 zoonotic TB cases/100,000 population/year. These generally low incidence rates notwithstanding, available data indicated substantial consequences of this disease for some population groups and settings.

Tuberculosis (TB) is among the most devastating human infectious diseases worldwide. An estimated 8.8 million new cases, a global average incidence rate of 128/100,000 population/year, and 1.5 million deaths were attributed to TB in 2010. Human TB is caused principally by M. tuberculosis. The main causative agents of bovine TB are M. bovis and, to a lesser extent, M. caprae; however, zoonotic transmission of these pathogens is well described and occurs primarily through close contact with infected cattle or consumption of contaminated animal products such as unpasteurized milk. TB cases caused by transmission of other mycobacteria from other animal reservoirs (e.g., wildlife) have been anecdotally reported. Globally, most cases of zoonotic TB are caused by M. bovis, and cattle are the major reservoir. Therefore, for the purpose of this study and the remainder of this report, we refer to zoonotic TB as TB in humans caused by M. bovis or M. caprae. There is evidence to suggest that zoonotic TB accounted for a significant proportion of the TB cases in the Western world before the introduction of regular milk pasteurization programs. Currently, in high-income countries, bovine TB is well controlled or eliminated in most areas, and cases of zoonotic TB are rarely seen. However, reservoirs of TB in wildlife populations have been linked to the persistence or increase of the incidence of bovine TB in some countries, most notably the United Kingdom (UK). The absence of zoonotic TB despite an upsurge in the incidence of bovine TB in the United Kingdom sparked a controversy over the large financial expenditures for disease control in cattle.

The situation may be fundamentally different in other regions. For example, in most countries in Africa, bovine TB is prevalent, but effective disease control, including regular milk pasteurization and slaughterhouse meat inspection, is largely absent. This situation is exacerbated by the presence of multiple additional risk factors such as human behavior and the high prevalence of HIV infections. Although HIV/AIDS is thought to facilitate transmission and progression to active disease of any form of TB, some studies showed a significantly increased proportion of M. bovis infections among HIV–co-infected TB patients compared with HIV-negative TB patients.

Emerging Infectious Diseases
June 11, 2013

Original web page at Emerging Infectious Diseases