Population dynamics of African Trypanosomiasis

At the end of his book The Role of the Trypanosomiases inAfrican Ecology, Ford (1971) argued that quantitative methodsin the tradition of the work of Ross and MacDonald, should beadopted in the study of the epidemiology of thetrypanosomiases. This is now feasible in light of recentadvances in our understanding of these diseases, and thisthesis aims to develop this approach.Current knowledge of the factors involved in the transmissionof the diseases is reviewed in the context of the constructionof a model, and parameter values are estimated from publisheddata. Time lags between seasonal vector density and prevalencein hosts are calculated; a natural experiment in The Gambiawhere host density also varies due to annual migrationsprovides a test of the seasonal model. A simple SIRS modelfails to account for the lengthening interval betweeninfections as animals age. A model of acquired immunity isproposed, based on consideration of the life experience ofinfection of individuals, which gradually acquire immunity tothe various serodemes to which they are exposed. The modelcombines published data from Kilifi on serodeme abundance,tsetse challenge and cattle infection and predicts thelengthening intervals between drug treatments that wereobserved. This model leads to a relationship between tsetsechallenge and equilibrium prevalence in hosts which depends onthe degree of trypanotolerance in the animals, and which agreeswell with the observed relationship.Implications of vector-parasite relationships are discussed inlight of analysis of published data on parasite-induced tsetsemortality and the genetics of tsetse susceptibility to midgutand salivary-gland infections. Calculated selective forces arelow but sufficient to lead to large changes in gene frequencyover the time scale observed between epidemics.A model of animal trypanosomiasis is compared against fielddata from the International Trypanotolerance Centre, TheGambia. Tsetse population parameters were investigated in arecapture study, and a method of analysis of these data whichallows for cyclic availability to the traps was developed. Themodel was modified to calculate the expected fraction ofpositive blood films observed. The model could generate theobserved age-prevalence patterns and the observed relationshipsbetween tsetse challenge and prevalence, once age-dependentexposure and mechanical transmission have been incorporated.An age-structured model, which takes account of gradualacquisition of immunity with age, is used to explore expectedeffects of alternative control measures by vaccination andchemotherapy.

Level dc:type.qualificationlevel

Doctoral (Level 8)

Year dc:date.issued

1991

Author dc:creator

• Milligan, P

Language dc:language

en

Identifier

oai:salford-repository.worktribe.com:1338517

OAI identifier oai:identifier

oai:salford-repository.worktribe.com:1338517