Zoonotic (M. bovis) TB in Humans in Cameroon

Tuberculosis (TB) is globally a leading cause of human death by a single causal infectious agent. The extent of human TB caused by M. bovis is unknown, but it seems to account for only a small percentage of the cases of TB reported in humans.

Thus, in the developed countries, M. bovis accounts for less than 0.5-7.2% of the cases of human TB, while in developing countries, 10-15% or more are estimated to be caused by the infection (de la Rua-Domenech 2006). Reliable data are unavailable about the number of human cases of TB caused by M. bovis in Cameroon, due to the lack of attention to the problem by policy makers, and the limited number of available diagnostic facilities able to identify M. bovis. In most of the rural areas, for TB, the main diagnostic approach is the detection of acid-fast bacilli (AFB) in the sputum of suspected cases by direct microscopy, and only urban reference laboratories attempt to isolate the organism. These referral laboratories, however, make little or no attempt to identify the isolates to the strain/species level, thus probably missing many M. bovis cases. Their argument is that the same drug regimen would be used to treat all TB cases, although resistance of M. bovis to some of the first-line anti-TB drugs has been reported worldwide (Diguimbaye et al. 2006b; Gibson et al. 2004), including in Cameroon (Kuaban et al. 2000a, b). Currently, the level of public awareness of the zoonotic risk of BTB, and the lack of animal and human health surveillance programs to control TB, is very limited; this is in spite of an established epidemiologic association between tuberculin-positive cattle and human TB in several countries, such as in Ethiopia and Zambia (Regassa et al. 2008).

Because of a number of practices that may enhance the transmission of the disease, there is concern that zoonotic M. bovis infections may be more prevalent in Cameroon than anticipated.

Given the close association between livestock and humans because of prevailing farming practices in Cameroon, the high prevalence of BTB in the indigenous cattle could contribute to an increase in the prevalence of human TB. Opportunities exist for the transmission of M. bovis to humans because of the very close human-livestock interaction (Table 12.4) favoring aerosol transmission, and the habit of consuming unpasteurized milk (Thoen et al. 2006) and raw meat (Table 12.3). Other people at risk, such as butchers, abattoir workers, and those with low levels of education, appear not to be well informed about the risk posed by and the modes of transmission of zoonotic TB. OfTB patients surveyed in Bamenda (Awah-Ndukum et al. 2014), only 17.3% were aware of zoonotic TB, and Kelly et al. (2016) too reported a low level of awareness by both dairy farmers and cattle owners/pastoralists that consuming milk could cause zoonotic TB. A large segment of the population (32.1%) drink fresh milk (pasteurized or unpasteurized), 19.8% drink unpasteurized milk, 2.5% eat raw meat, and 61.3% eat Suya that is meat briefly roasted over hot charcoal or fire. The practice of pooling milk from cows of several owners could further increase the risk to a larger number of people consuming the raw product (Awah-Ndukum et al. 2014). Under these conditions, particularly those suffering from HIV/AIDS (Noeske et al. 2004) are at risk (Berg et al. 2009).

PCR-based genomic deletion analyses showed evidence of M. bovis in human and M. tuberculosis in cattle samples (Awah-Ndukum et al. 2011) suggesting possible animal-to-human and human-to-animal transmission cycles. Molecular analyses confirmed the presence of M. bovis in humans with pulmonary TB in the Western and Northwestern regions of Cameroon (Niobe-Eyangoh et al. 2003; Egbe et al. 2017). This indicates the possibility of human-to-human transmission of M. bovis by the respiratory route. Similarly, a range of mycobacteria, including M.

tuberculosis and several mycobacteria other than tubercle (MOTT) bacilli, has been isolated from cattle (Egbe et al. 2016).

The epidemiologic link between TB in humans and BTB in cattle, with the possibility of a cattle-human-cattle cyclical transmission, is of serious concern in Cameroon due to issues related to drug resistance. Mycobacterium bovis strains isolated from human TB cases also have different drug susceptibility profiles compared to M. tuberculosis strains (Diguimbaye et al. 2006b; Gibson et al. 2004).

The contribution of BTB to the overall TB morbidity and mortality in Cameroon thus needs a broad investigative approach. This would involve establishing the epidemiology of the disease in cattle and human populations, identification of TB-causing agents and their respective sources, maintenance hosts, possible routes of transmission, and associated risk factors.

The public health threat of M. bovis in Cameroon therefore requires the urgent attention of veterinary and medical professionals, biomedical and ecological experts, social workers, and policy makers within the context of the “One Health” approach.

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Source: Dibaba A.B., Kriek N.P.J., Thoen C.O. (eds.). Tuberculosis in Animals: An African Perspective. Springer,2019. — 453 p.. 2019

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