Population dynamics of African Trypanosomiasis

Milligan, P 1991, Population dynamics of African Trypanosomiasis , PhD thesis, University of Salford.

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Abstract

At the end of his book The Role of the Trypanosomiases in African Ecology, Ford (1971) argued that quantitative methods in the tradition of the work of Ross and MacDonald, should be adopted in the study of the epidemiology of the trypanosomiases. This is now feasible in light of recent advances in our understanding of these diseases, and this thesis aims to develop this approach. Current knowledge of the factors involved in the transmission of the diseases is reviewed in the context of the construction of a model, and parameter values are estimated from published data. Time lags between seasonal vector density and prevalence in hosts are calculated; a natural experiment in The Gambia where host density also varies due to annual migrations provides a test of the seasonal model. A simple SIRS model fails to account for the lengthening interval between infections as animals age. A model of acquired immunity is proposed, based on consideration of the life experience of infection of individuals, which gradually acquire immunity to the various serodemes to which they are exposed. The model combines published data from Kilifi on serodeme abundance, tsetse challenge and cattle infection and predicts the lengthening intervals between drug treatments that were observed. This model leads to a relationship between tsetse challenge and equilibrium prevalence in hosts which depends on the degree of trypanotolerance in the animals, and which agrees well with the observed relationship. Implications of vector-parasite relationships are discussed in light of analysis of published data on parasite-induced tsetse mortality and the genetics of tsetse susceptibility to midgut and salivary-gland infections. Calculated selective forces are low but sufficient to lead to large changes in gene frequency over the time scale observed between epidemics. A model of animal trypanosomiasis is compared against field data from the International Trypanotolerance Centre, The Gambia. Tsetse population parameters were investigated in a recapture study, and a method of analysis of these data which allows for cyclic availability to the traps was developed. The model was modified to calculate the expected fraction of positive blood films observed. The model could generate the observed age-prevalence patterns and the observed relationships between tsetse challenge and prevalence, once age-dependent exposure and mechanical transmission have been incorporated. An age-structured model, which takes account of gradual acquisition of immunity with age, is used to explore expected effects of alternative control measures by vaccination and chemotherapy.

Item Type: Thesis (PhD)
Contributors: Molyneux, D (Supervisor)
Schools: Schools > School of Environment and Life Sciences
Funders: The Wellcome Trust, Deutsche Gesellschaft fiir Technische Zusammenarbeit (GTZ)
Depositing User: Institutional Repository
Date Deposited: 23 Jul 2021 12:48
Last Modified: 27 Aug 2021 21:55
URI: http://usir.salford.ac.uk/id/eprint/61276

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