Eco-epidemiology of Bovine Tuberculosis (bTB) in an African savanna : The conflict between traditional pastoralist adaptations and disease transmission in the modern era

Abstract

Bovine tuberculosis (bTB) is a zoonotic disease, and remains a cause of concern for livestock, wildlife and human health, especially in Ethiopia. It is a contagious disease, so close contact between animals or sharing of feed between infected and non-infected animals are major risk factors for transmission. Thus, improving the understanding of the factors that promote contact between hosts (i.e., livestock animals but also wild ruminants) is critical for limiting bTB transmission in pastoral, multi-host communities. I found that the older the age of the cattle and the lower the body condition, the higher the chance of a positive bTB test result at the individual animal level. Moreover, at herd level, herd size, contact with wildlife, and the interaction of herd size and contact with wildlife were identified as significant risk factors for bTB prevalence in cattle in Ethiopia. Further to what is already known from the past studies, I found that the probability of contact with wildlife was positively influenced by herd size, through herd movement. As larger herds moved more and grazed in larger areas, the probability of grazing in an area with wildlife and contact with either infected cattle or infected wildlife hosts increased; this also increased the chances for bTB infection. I detected a possible ‘dilution effect’ in bTB, where a higher evenness of mammal species reduced the probability of bTB occurrence. This dilution effect might be caused by encounter reduction. Because the encounter rate is proportional to the distribution of the host species; evenness would then capture the probability of encounter between pathogens and each host species. Thus, species evenness can be an appropriate measure of biodiversity to explain disease risk. I also showed that bTB prevalence was positively associated with the invasion of the plant Prosopis (Prosopis juliflora), maybe due to the loss in host species evenness and the increase in cattle movement as a consequence of the loss of palatable grasses in Prosopis-infested areas. Moreover, social contacts between herd owners are also important, as I found that herds with a greater number of edges in a (social) network had more connections in the livestock transfer network, increasing the probability of becoming infected with bTB. Thus, cultural components like large herd size and social contacts are at odds with the global One Health rationale to reduce bTB.