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 Last update 2019/11/19

Etiology of bovine tuberculosis

M. tuberculosis complex


Mycobacterium Bovine Tuberculosis

Mycobacterium scanning electron micrograph (SEM). National Institute of Allergy and Infectious Diseases (NIAID).

Mycobacterium bovis is the main etiological agent of bovine TB although other species of the M. tuberculosis complex (MTBC) such as M. caprae or M. tuberculosis has been isolated from infected cattle [1][2]. The MTBC is characterized by a 99.9% of similarity at the nucleotide level and virtually identical 16S rRNA sequences [3]. Despite this high similarity at nucleotide level, the members of the MTBC can be distinguished by several molecular markers [4]. Nowadays, the species included in the MTBC are M. tuberculosis (sensu stricto), M. africanum, Dassie bacillus, M. mungi, M. orygis, M. pinnipedii, M. microti, M. caprae, M. bovis and M. bovis BCG [4].

Cattle and other bovids are mainly infected by M. bovis and in a lesser extent by M. caprae. Theobald Smith reported in 1898 that human and bovine TB were infections caused by different bacteria. Some contemporary researchers misunderstood the hypothesis of Smith. Moreover, in a conference in London in 1901, Koch reported that the aetiological agent of TB in animals was not a threat for humans. Fortunately, this hypothesis was not accepted without questions and a commission of experts was requested to elucidate this enquiry. In 1911, this commission concluded that TB could be transmitted between animals and humans and, therefore, it was a zoonosis [5]. In fact, in spite of the general acceptance of M. bovis as different from M. tuberculosis, it was not until 1970 that M. bovis was officially recognised as a species [6].


M. bovis


The first whole genome sequence of a M. bovis strain was completed in 2003 [7] and the results showed that it was much smaller than the M. tuberculosis genome suggesting an evolution from a M. tuberculosis-like organism. TB in cattle has been reported worldwide and movements is the most likely cause of introduction and distribution of the disease [8]. M. bovis has been isolated also from several other domestic and wildlife species including goat, sheep, dog, cat, horse, camelids, wild boar, red deer, fallow deer, badger, bison, brushtail possum and many others [9][10][11][12][13][14][15][16]. Several both domestic and wildlife species can act as reservoirs of infection being of great importance in the epidemiology of TB.

Some authors have speculated regarding differences in susceptibility to M. bovis infection between different species and even cattle subspecies and productive types (B. taurus vs B. indicus) [17][18]. Furthermore, the genotype of the field isolates of M. bovis can vary in virulence and pathology in cattle [19][20].


M. caprae


M. caprae [21] was previously classified as M. bovis [22] and it has been isolated from a great variety of domestic and wildlife species. It has been described as the main etiological agent of TB in goats in Spain [1]. In the last years, this pathogen has been recognized in many Central and Western European countries where it has been isolated from cattle causing an indistinguishable disease from that outbreaks caused by M. bovis attending to the ante-mortem diagnosis performance, clinical signs and macroscopic lesions.


M. tuberculosis


M. tuberculosis is the main etiological agent of TB in humans although several outbreaks has been reported in cattle and other domestic and wildlife animals by this species. When epidemiological studies were available, a human origin of the infection [2] was demonstrated. Previous studies suggested that M. tuberculosis infection does not sustain in animal populations. In fact, some findings supported the attenuation of M. tuberculosis H37Rv in bovine hosts [23] [24] reducing the probability of dissemination.


References


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