Doctoral Degrees ((SACEMA) South African Centre for Epidemiological Modelling and Analysis)

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    Extinction probabilities for tsetse (Glossina spp.) in a world of changing climate
    (Stellenbosch : Stellenbosch University, 2020-12) Are, Elisha Bayode; Hargrove, John; Faulty of Medicine and Health Sciences. SACEMA: South African Centre for Epidemiological Modelling and Analysis
    ENGLISH ABSTRACT: Tsetse (Glossina spp) transmit trypanosomiases, as sleeping sickness to humans and nagana to livestock. These continue to have negative impacts on health and wealth in the African continent. In recent years, treatment, and vector control, have helped to reduced disease burden and the World Health Organization set 2020 as a target year for eliminating the disease in humans. Tsetse populations have also declined in parts of Africa due to climate change and human encroachment. In the fight against trypanosomiases, there is a continuing need to improve understanding of tsetse population dynamics – and particularly the conditions under which tsetse populations persist, and the implications for tsetse control/eradication in a changing world. We explore here five primary objectives. Firstly, we revisit a branching process model developed for tsetse population growth and estimates for extinction probabilities. We improve the model by modifying it to work for more realistic situations where, for example, male to female sex ratios in the population are not necessarily one-to-one. We estimate extinction probabilities as a function of the probability that a deposited larva is female, and show that tsetse populations will thrive better when there are slightly more females than males in the population. We confirm that daily mortality rates 3.5% ensure eradication of closed populations of tsetse. Secondly, we simplify the mathematical derivation of earlier estimate for extinction probabilities and carry out global uncertainty and sensitivity analyses on extinction probabilities, using Latin Hypercube Sampling and Partial Rank Correlation Coefficient methods. We show that adult female mortality has the highest correlation with extinction probability. We caution that a new tsetse control method, which proposes a strategy combining Sterile Insect Techniques (SIT) with increased pupal mortality, may not offer any added benefit for tsetse eradication. Thirdly, we estimate extinction probabilities, times to extinction and growth rates as a function of temperature for tsetse populations. We provide temperature bounds for tsetse persistence, and suggest that future control efforts should consider the impact of changing climate on the distribution and abundance of tsetse populations. Fourthly, we develop a general model for tsetse population persistence, and show that previous models are special cases of our current model. While extinction probabilities are sensitive to changes in the point of the life cycle at which we count the population, the reproduction number is independent of the counting point chosen. Finally, we derive the intrinsic rate of increase for tsetse populations using the Euler-Lotka equation. We use temperature data, and tsetse population estimates from a mark–recapture exercise, to test our model’s validity, and show that our results are comparable to estimates derived from the data. We estimate the intrinsic rate of increase for tsetse populations in the neighbourhood of Rekomitjie Research Station in Zimbabwe, using as input average daily temperatures from 1960–2018. We created multiple climate change scenarios, using 2018 daily temperatures as a baseline. We predict that a warming rate of 0.08°C per-year could drive tsetse populations to extinction in the neighbourhood of Rekomitjie within the next 50 years.