Relationship of human and animal tuberculosis in the interface areas of the National parks of the Kafue Basin in Zambia
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Tuberculosis is a disease of worldwide public health and economic importance. In Zambia the disease has been consistently reported in the Kafue Basin without a clear understanding of transmission dynamics in the livestock-wildlife interface. The main objective of this study was to determine the relationship between human and animal tuberculosis in the livestock-wildlife interface. In order to determine TB prevalence in cattle, a cross-sectional study was conducted using the mid-cervical comparative tuberculin test in the Kafue Basin (Lochinvap and Blue Lagoon) and a control area outside the interface areas (Kazungula District). Farmers were interviewed using a pre-tested questionnaire in order to determine TB risk factors. Tissue samples were collected from abattoirs and hunter harvested wildlife animals for isolation of TB. Sputum samples were also collected from health centers for TB isolation from humans. All cultures from these species were subjected to DNA analysis using spoligotyping to determine the Mycobacterium species causing disease in these hosts. Questionnaire results were used to build a model on risk factors for cattle TB in the study areas. A total of 944 cattle from 111 herds were subjected to the mid-cervical comparative tuberculin test. Overall at individual animal level, the prevalence of TB in all the study areas was found to be 5.50%. In Lochinvar and Blue lagoon, the individual cattle prevalence was 5.35% and 7.34% respectively. However, in the control area (Kazungula) the individual animal prevalence was lower at 0.57%. At herd level, the overall TB prevalence in cattle was herds 24.32% and a trend similar to individual cattle results was observed. These results, suggest an existence of foci of TB infection in the Kafue Basin of Zambia. Tuberculosis prevalence was observed to vary according to the type of grazing system (P = 0.000) with interface herds recording higher prevalence (11.6%) than transhumance and village herds. Herds of cattle that practiced interface grazing system were eighteen times more likely to be TB positive than those that practiced village grazing system (Odds ratio = 18.07; CI 95% = 4.3-81.0); p=0.000). These results based on the grazing system suggest that TB prevalence increased with increasing level of contact with wildlife. From the questionnaire results, logistic regression analysis revealed that herd size and grazing system were the two most significant risk factors influencing TB herd status. From the abattoir survey, gross postmortem examinations revealed a 13% organ condemnation. However, only 6.62% of the overall condemnations had characteristic TB colonies on culture. The abattoir study provided evidence of the respiratory tract as a primary route for the establishment of tuberculosis infection in cattle as 87.64% of condemned organs were lungs. In the wild animal survey, gross postmortem examination indicated a prevalence of 43% in the lechwe. Based on these findings, TB may appear to be probably maintained principally by the lech we may be due to its abundance or owing to the small sample sizes obtained from the other wildlife species. Based on the questionnaire results, no statistical association was found between keeping cattle and having a TB patient in the household (Relative risk = 1.14; CI = 0.64-1.94); p = 0.83). Further, DNA analysis of animal and human isolates indicated that there was no sharing of infection. From the DNA results, all animal isolates were M. bovis while all human isolates were M. tuberculosis. These results may indicate species adaptability in certain hosts. However, animals may still be able to transmit TB to humans, albeit at a low prevalence. Based on the generated TB model, in terms of TB control in the interface areas, the most critical factors to consider will be herd size and the grazing system.
- Veterinary Medicine