South African Centre for Epidemiological Modelling and Analysis (SACEMA)

Permanent URI for this community

https://scholar.sun.ac.za/handle/10019.1/96225
The South African Centre for Epidemiological Modelling and Analysis (SACEMA) is a national research centre established under the Centre of Excellence programme of the Department of Science and Technology and the National Research Foundation. The Centre focuses on research in quantitative modelling of the spatial and temporal patterns of disease. The immediate aim of the research is to understand and predict the development of various diseases, and thereby to provide advice on how best to combat them. Our research focuses on issues pertaining to HIV, TB and malaria, although not to the exclusion of other epidemiological problems.

Browse

Browsing South African Centre for Epidemiological Modelling and Analysis (SACEMA) by browse.metadata.advisor "Hargrove, John"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Investigating the simultaneous effect of age and temperature on the population dynamics of female tsetse flies
    (Stellenbosch : Stellenbosch University, 2011-12) Elama Ameh, Josephine; Ochigbo, Josephine Elanma; Ouifki, Rachid; Hargrove, John; Stellenbosch University. Faculty of Science. Dept. of Mathematical Sciences.
    ENGLISH ABSTRACT: Age and temperature are two factors that affect mortality in adult tsetse flies. Both are found to be very important, but the simultaneous effect of these factors on the mortality rate have not been studied. This study seeks to address this, with an application to a population of female tsetse, using a model based on partial differential equations. Adult mortality is agedependent and is modelled as the sum of two exponentials, with four parameters (coefficients of each exponential): numerical analysis of a population model with this mortality structure predicts exponential growth. Analysis of each of the parameters through parameter variation shows that two of these parameters control the mortality of the nulliparous (ages 0 − 10 days) flies only while the other two only take care of flies of mature ages. Measurement of the impact of these parameters on the mortality of tsetse of different ages by the normalized forward sensitivity index method is also carried out. This is followed by fitting the model based on the age-dependent mortality along with a constant tsetse birth rate to data representing the catches of female Glossina pallidipes at Rekomitjie Research station, Zimbabwe. Considering a three parameter adult tsetse mortality, parameter analysis shows the effect of one of the parameters to affect the mortality of flies of all ages while a second controls only the mature tsetse flies of reproductive ages. A further analysis resulted in the estimate of these parameters as functions of temperature, thereby leading to the establishment of an age and temperature-dependent adult tsetse mortality. Using data for the daily average temperature records obtained in 1981 on Antelope Island, Lake Kariba, Zimbabwe, daily changes in the pupal duration (adult tsetse birth rate) changes negatively with temperature change. Incorporating this (temperature-dependent ) birth rate into the model, together with the established age and temperature-dependent adult mortality, the adult tsetse population dynamics is explored numerically. The latter model is then fitted to population data of female Glossina morsitans morsitans obtained from the same Island and for the same period as used for the temperature data. The data suggests peak tsetse population to be in the month of July and lowest in the month of December. The first quarter of the year is predicted to be most favorable for breeding tsetse while the second, showed a period of stable growth rate and a time of tsetse abundance. In addition, the dynamics with both age and temperature showed a non-uniform daily population growth contrary to that with age effect only. This study has enhanced our understanding of tsetse population dynamics for age and temperature-dependent adult mortality with temperature-dependent pupal duration and suggests the period of tsetse abundance on Antelope Island.