Epidemiology of bovine tuberculosis and influence of liver fluke co-infection in Cameroon, Central Africa
Kelly, Robert Francis
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Despite Africa accounting for ~20% of the global cattle population, prevalence estimates and related risk factors of bovine tuberculosis (bTB), caused by Mycobacterium bovis, are still poorly quantified in many countries across the continent. Control of bTB in Africa is difficult due to poor monitoring of cattle movements and limited abattoir surveillance. Also M. bovis is zoonotic and risk factors for transmission include living in close contact with cattle and consumption of unpasteurised milk. Cattle keeping is integral to some rural populations in Cameroon and understanding the epidemiology of bTB in cattle populations is important both to bovine and public health. Detection of bTB in cattle is difficult due to variability of immune responses to M. bovis infection. The interferon-γ (IFN-γ) assay maybe useful to estimate bTB prevalence and identify bTB risk factors in Cameroon. However its performance can vary at different stages of bTB pathogenesis and in different cattle populations. Recently Fasciola hepatica co-infections have been reported to suppress IFN-γ responses in M. bovis infected cattle but the potential effect with F. gigantica co-infections on bTB prevalence estimates in Cameroon is unknown. An abattoir study was conducted in Cameroon to assess the performance of the IFN-γ assay. In 2012-13; 2064 slaughtered cattle were sampled from Bamenda abattoir (North West Region; NWR) and Ngaoundere abattoir (Vina Division; VD). Individual animal data was collected from routine meat inspection including identification of bTB and Fasciola pathology. Cattle were also tested for bTB using the IFN-γ assay and an M. bovis antibody ELISA. In the absence of a gold-standard diagnostic, the IFN-γ assay was compared to other diagnostic tests to assess agreement and identify factors that affected performance of the assay. Agreement between IFN-γ assay, TB lesion identification and an M. bovis antibody ELISA was poor-moderate, probably partly related to differences in immune response detected. A presence of Fasciola gigantica also increased the odds of false negative IFN-γ assay results. On further investigation co-infected cattle had increased odds of TB lesions and reduced IFN-γ responses that potentially could lead to ~20% reduction in test sensitivity. In an attempt to take into account the potential impact of F. gigantica, when estimating bTB prevalence, an antibody ELISA was developed to detect the exposure in live cattle. To highlight the awareness of disease in cattle-rearing communities, estimate prevalence and identify risk factors of bTB in cattle populations; two cross-sectional studies were conducted in 2013. A stratified clustered cross-sectional study of pastoral cattle herds, in the NWR and the VD, sampled 1448 pastoral cattle reared by 100 pastoralists. A smaller cross-sectional study sampled 60 dairy cattle from 46 small-holder co-operative dairy farmers. Individual animal data and herd-level data were collected and animals were screened by both the single comparative intradermal skin test (SCITT) and IFN-γ assay. Awareness of zoonotic TB was low yet consumption of raw milk was high in cattle-keeping communities highlighting the need for accurate bTB prevalence estimates. Despite the high awareness of the clinical presentation of bTB, clinical signs identified by pastoral herdsmen were not associated with cattle being bTB positive. The SCITT was used to compare two manufacturers cut offs for the IFN-γ assay, ≥0.05 and ≥0.1, and highlighted that these two diagnostics may detect different populations of bTB positive cattle. Using the IFN-γ assay at ≥0.1, bTB prevalence was highest in dairy cattle (21.67%) and was also present in pastoral cattle in the NWR and VD (11.33% and 6.55% respectively). Importantly, as F. gigantica is endemic in Cameroon and its influence could mean the true prevalence of bTB could be higher. Female pastoral cattle were at lower odds of being IFN-γ assay positive potentially due to immunosuppressive factors had lower odds of disease. Husbandry practices also decreased the odds of being IFN-γ assay positive such as drinking from streams, antelope and contact with herds at grazing. Age increased the odds of pastoral cattle being IFN- assay positive potentially being a confounder to chronicity of bTB and other co-infections may influence IFN-γ responses. Dairy cattle herds had different risk factors for being IFN- positive likely due to differences in husbandry practices. Considering the potential risk to public health of M. bovis this thesis highlights the extent of bTB across two major cattle keeping regions in Cameroon and the public health risk in cattle-rearing communities. Furthermore the relationship between Fasciola co-infection and IFN- responses to M. bovis described has potential implications for bTB diagnosis in cattle populations where the parasite is present across the globe.