Research Articles ((SACEMA) South African Centre for Epidemiological Modelling and Analysis )

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/96226

Browse

Browsing Research Articles ((SACEMA) South African Centre for Epidemiological Modelling and Analysis ) by Subject "African trypanosomiases"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Optimal strategies for controlling riverine Tsetse Flies using targets : a modelling study
    (Public Library of Science, 2015-03) Vale, Glyn A.; Hargrove, John W.; Lehane, Michael J.; Solano, Philippe; Torr, Stephen J.
    Background: Tsetse flies occur in much of sub-Saharan Africa where they transmit the trypanosomes that cause the diseases of sleeping sickness in humans and nagana in livestock. One of the most economical and effective methods of tsetse control is the use of insecticide-treated screens, called targets, that simulate hosts. Targets have been ~1m2, but recently it was shown that those tsetse that occupy riverine situations, and which are the main vectors of sleeping sickness, respond well to targets only ~0.06m2. The cheapness of these tiny targets suggests the need to reconsider what intensity and duration of target deployments comprise the most cost-effective strategy in various riverine habitats. Methodology/Principal Findings: A deterministic model, written in Excel spreadsheets and managed by Visual Basic for Applications, simulated the births, deaths and movement of tsetse confined to a strip of riverine vegetation composed of segments of habitat in which the tsetse population was either self-sustaining, or not sustainable unless supplemented by immigrants. Results suggested that in many situations the use of tiny targets at high density for just a few months per year would be the most cost-effective strategy for rapidly reducing tsetse densities by the ~90% expected to have a great impact on the incidence of sleeping sickness. Local elimination of tsetse becomes feasible when targets are deployed in isolated situations, or where the only invasion occurs from populations that are not self-sustaining. Conclusion/Significance: Seasonal use of tiny targets deserves field trials. The ability to recognise habitat that contains tsetse populations which are not self-sustaining could improve the planning of all methods of tsetse control, against any species, in riverine, savannah or forest situations. Criteria to assist such recognition are suggested. Author Summary: We employed a deterministic model to simulate the efficacy of various ways of using the tiny, ~0.06m2, insecticide-treated targets recently recommended as replacements for the larger, ~1m2, types previously used to control riverine species of tsetse fly, the main vectors of sleeping sickness in humans. Results suggested that in many situations the use of tiny targets at treble the normal density for a third of the normal time could be the most cost-effective strategy for rapidly reducing or eliminating tsetse populations, so helping with disease control. In deciding whether to aim for local control or elimination, and in planning the operations, it would be highly advantageous to distinguish those parts of the tsetse infestation that support self-sustaining populations, and those containing populations that cannot be sustained unless supplemented by immigrants. Sorts of information that can help to assess the type of sustainability in field habitats are identified. These findings can assist the planning of any method of tsetse control used against any species of tsetse, including those important as vectors of livestock disease.
  • Thumbnail Image
    Item
    Tsetse control and Gambian sleeping sickness; implications for control strategy
    (Public Library of Science, 2015-08) Tirados, Inaki; Esterhuizen, Johan; Mangwiro, T. N. Clement; Vale, Glyn A.; Hastings, Ian; Solano, Philippe; Lehane, Michael J.; Torr, Steve J.
    Background: Gambian sleeping sickness (human African trypanosomiasis, HAT) outbreaks are brought under control by case detection and treatment although it is recognised that this typically only reaches about 75% of the population. Vector control is capable of completely interrupting HAT transmission but is not used because it is considered too expensive and difficult to organise in resource-poor settings. We conducted a full scale field trial of a refined vector control technology to determine its utility in control of Gambian HAT. Methods and Findings: The major vector of Gambian HAT is the tsetse fly Glossina fuscipes which lives in the humid zone immediately adjacent to water bodies. From a series of preliminary trials we determined the number of tiny targets required to reduce G. fuscipes populations by more than 90%. Using these data for model calibration we predicted we needed a target density of 20 per linear km of river in riverine savannah to achieve >90% tsetse control. We then carried out a full scale, 500 km2 field trial covering two HAT foci in Northern Uganda to determine the efficacy of tiny targets (overall target density 5.7/km2). In 12 months, tsetse populations declined by more than 90%. As a guide we used a published HAT transmission model and calculated that a 72% reduction in tsetse population is required to stop transmission in those settings. Interpretation: The Ugandan census suggests population density in the HAT foci is approximately 500 per km2. The estimated cost for a single round of active case detection (excluding treatment), covering 80% of the population, is US$433,333 (WHO figures). One year of vector control organised within the country, which can completely stop HAT transmission, would cost US$42,700. The case for adding this method of vector control to case detection and treatment is strong. We outline how such a component could be organised. Author Summary: Sleeping sickness is controlled by case detection and treatment but this often only reaches less than 75% of the population. Vector control is capable of completely interrupting HAT transmission but is not used because of expense. We conducted a full scale field trial of a refined vector control technology. From preliminary trials we determined the number of insecticidal tiny targets required to control tsetse populations by more than 90%. We then carried out a full scale, 500 km2 field trial covering two HAT foci in Northern Uganda (overall target density 5.7/km2). In 12 months tsetse populations declined by more than 90%. A mathematical model suggested that a 72% reduction in tsetse population is required to stop transmission in those settings. The Ugandan census suggests population density in the HAT foci is approximately 500 per km2. The estimated cost for a single round of active case detection (excluding treatment), covering 80% of the population, is US$433,333 (WHO figures). One year of vector control organised within country, which can completely stop HAT transmission, would cost US$42,700. The case for adding this new method of vector control to case detection and treatment is strong. We outline how such a component could be organised.