Population dynamics of African Trypanosomiasis

Milligan, P

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.

Citation

Milligan, P. Population dynamics of African Trypanosomiasis. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Jul 23, 2021
• Additional Information: Funders : The Wellcome Trust;Deutsche Gesellschaft fiir Technische Zusammenarbeit (GTZ); Grant Number: 17001/1.5
• Award Date: Jul 1, 1991