Bovine tuberculosis, caused by M. bovis, has been recorded in animals in most developing nations, albeit the scale is difficult to determine because surveillance and control measures are frequently insufficient or nonexistent[16]. Detection of tuberculous lesions in cattle in Ethiopia during abattoir examination is a common procedure. In the present study, detailed postmortem examination resulted in a prevalence of lesions indicative of bTB as 4.58%, which is consistent with previous studies in Ethiopia [33] [34],[16], [35] and lower than other abattoir-based studies[36],[37] ,[38]. However, lower prevalence has also been reported elsewhere [39],[40] and the reported differences might be due to sample size, the origin of the cattle and bTB burden in the study communities. It could also be due to differences in the breed of study animals, as local breeds are relatively more resistant to bTB [26].
The burden of bTB in this study was highest among older age group cattle compared to the adult and young age groups, which is in agreement with the reports of municipal abattoir in Addis Ababa [16], Dilla municipal abattoir [41], [42] and Gondar abattoir [43]. This finding is logical; as the age of cattle increases, the probability of acquiring TB infection also increases which can be explained by the declining protective capability in aging animals[44]. The chronic nature of bTB, stress, malnutrition and immunosuppression all increase with age, which contribute to higher susceptibility to disease in older animals [45]. Poor body condition of the animals was one of the factors significantly associated with increased bTB prevalence in the current study. This has been observed in previous reports [42, 46], and could be related to the extensive lesions and wasting of the body caused by the chronic nature of bTB disease. The gross tuberculosis lesions were most frequent in lung and associated lymph nodes, followed by lymph nodes of head and mesenteric lymph node. Similar distributions of lesions have been reported elsewhere [43, 47] alongside lower prevalence of gross tuberculosis lesions compared to the current study [34],[33]. Higher prevalence of lesions have also been described [40],[38], highlighting the variation of disease across regions. Detection of the majority of tuberculous lesions in lung and associated lymph nodes suggests that the animals acquired the infection through the respiratory route [33, 40, 48].
RD4 deletion typing confirmed four isolates to be M. bovis (with PCR product sizes of 446 bp). The spoligotype analysis of these mycobacterial isolates identified three different spoligotypes (SB1176, SB1477 and SB0133), which have been previously reported in Ethiopia [16, 20, 21, 33] as a cause of bTB circulating in Ethiopian cattle. All three spoligotypes have a unique spoligotype feature (lacking spacers 3–7) which is indicative of members of the M. bovis African 2 (Af2) clonal complex that has only been reported in East Africa so far [49]. The high isolation frequency of spoligotype SB1477 conforms to the findings of a previous study [16] from the same Addis Ababa municipal abattoir. The SB0133 and SB1176 strains were also reported as the most common spoligotype pattern among Ethiopian cattle isolates [12, 33, 49]. Even though M. tuberculosis was not isolated in the current study, previous studies in Ethiopia have reported both M. bovis and M. tuberculosis strains in cattle and humans [12, 15, 18], suggesting that M. bovis infection could lead to zoonotic TB. M. bovis should be considered a possible hazard for human tuberculosis in countries, like Ethiopia, where eating of raw meat and unpasteurized milk is common in the context of high prevalence of bTB without enhanced management efforts to prevent zoonotic events.
In the present study, special attention was focused to determine if the detection of tuberculous lesions was associated with slaughterhouse work shifts. We found that the detection of tuberculous lesions was higher (1.54 adjusted OR, although not significantly associated with prevalence) in animals slaughtered during the night shift compared to the day shift. This needs further investigation to confirm and to understand the reasons behind such an observation. However, this finding suggests that improved meat inspections during the night shift might reduce the risk of bTB zoonotic events at slaughter. In the workforce survey, 79.31% of abattoir worker and 6.90% of butchers reported having awareness about bTB, which is similar to a previous study [50], and likely reflects the higher exposure of abattoir workers to diseased cattle pathologies. Most of the abattoir workers knew of methods used to reduce transmission of bTB from cattle to humans and vice versa. This good level of awareness about transmission of bTB is extremely important for the control and prevention of zoonotic bTB. In the current study, 53.46% of respondents said that ante-mortem examination is performed regularly in the abattoir and 44.82% of the respondents knew that raw or undercooked meat is a potential source of infection. However 67% of the respondents still consume raw beef meat and 60.3% of butchers perceived that the habit of consuming raw meat is increasing. The consumption of raw animal products and a low level of awareness about the zoonotic implications of bTB amongst butchers (and by extension the general public) are factors likely to promote the spread of tuberculosis as zoonosis in Ethiopia.