Burden of Mycobacterium tuberculosis in Animals Versus M. bovis in Human

It appears interesting to write on Mycobacterium tuberculosis, primarily pathogen of human tuberculosis (TB) and M. bovis, primarily pathogen of cattle (BTB), other domestic animals and wild life in context of burden of these two closely related species in human and animals. The common notion in the mind of researchers has been their strong believe that M. tuberculosis does not infect cattle vice-a-versa M. bovis seldom infects human. As a result of this notion, laboratories handling human sample only cultivates them on Lowenstein-Jensen (LJ) medium containing glycerol but not onto LJ medium containing 0.5% sodium pyruvate for cultivation of M. bovis. This appears one of the strongest reason that neither burden of M. tuberculosis in animals nor burden of M. bovis is precisely known or reported anywhere in the world except of the fact both of these closely related species have been reported both in human and animals. They are members of M. tuberculosis complex (MTBC) which includes human and animal-associated pathogens. Other members of MTBC include M. caprae (goats), M. microti (voles), M. pinnipedii (seals), M. mungi (infecting mongoose), M. orygis (antelopes) and the chimp bacillus. The phenotypic similarity is less than 65% among the members of MTBC, there is about 99.9% genetic identity observed at deoxyribonucleic acid (DNA) level [1] and 99.95% at the nucleotide level [2]. M. tuberculosis may have developed through the domestication of cattle infected with M. bovis, some 15 to 20 thousand years ago. M. tuberculosis can also infect cattle; however, it is not as virulent as M. bovis in cattle [3,4]. M. tuberculosis causes less severe disease in cattle than that caused by M. bovis [5] and usually causes minor nonprogressive infections in domestic cattle [6]. Not much is known about the prevalence of M. tuberculosis in its spill over hosts. M. tuberculosis infected animals including cattle react positively when subjected to tuberculin skin testing (TST); however, the infection seems to vanish rather quickly and does not normally lead to a progressive disease [7-9]. Ayele et al. [10] reported human to cattle transmission of M. tuberculosis and according to them, the isolation of M. tuberculosis from any species other than human, especially from cattle, was interesting and important. The infective dose of M. bovis is low with studies demonstrating less than 10 viable bacilli are sufficient to cause infection [11]. Cattle infected with M. tuberculosis by tuberculous owners can be sensitized to tuberculin and develop a false-positive reaction for M. bovis infection [12].