Interactions Amongst the Community of Endemic Pathogens of African Cattle: A Longitudinal Study in South East Uganda

Abstract

The work presented in this thesis is focused upon the community of endemic pathogens of African cattle in Sub-Saharan Africa, which has long constrained livestock production in these areas. The first aim of this work is to investigate whether the pathogen community as a whole shapes the ensuant epidemiology and morbidity which are currently attributed to any of its individual pathogens. The second aim is to determine if a greater understanding of the interactions present amongst genetically distinct parasites of the same species can be used to better explain epidemiological features that are at present poorly understood. Emphasis is placed on examining spatial variation in the epidemiology of Theileria parva, a tick-transmitted protozoan that causes East Coast Fever. To achieve these aims, this work examines field data collected from a large and comprehensive study conducted in south east Uganda. Through application of apposite statistical techniques and mathematical modelling, aspects of the complex relations amongst the pathogen community and their environment are explored. Evidence is presented that demonstrates the paramount role of the pathogen community as a whole in shaping the infection dynamics and pathogenicity of any of its individual components. By focusing on a single member of this pathogen community (Theileria parva), some of the influences of host, vector, geographical location, temporal dynamics and intra-species pathogen interactions are elucidated. Application of a polymorphic molecular marker to Theileria parva infected blood samples and the use of Cox proportional hazard analysis, show variability in the survival of infections in cattle in high and low tick challenge areas. Moreover infection survival, which plays a pivotal role in parasite transmission, is shown to be a function of the interactions established amongst genetically distinct co-infective parasites. In consequence, vector intensity alone is insufficient to develop reliable transmission models which can accurately predict the epidemiology of the parasite inside and outside enzootic belts. Finally, a theoretical model is developed which, based upon the field evidence obtained throughout this work, provides a possible explanation for the mechanics of T. parva survival in cattle. In summary, this thesis makes a case that consideration of both inter- and intra-species pathogen interactions, can greatly augment understanding of the epidemiology of these pathogen communities. An integrated approach to pathogen dynamics can better equip an integrated approach to control of important diseases of African cattle.