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. 2018 Oct 22;15(10):e1002675.
doi: 10.1371/journal.pmed.1002675. eCollection 2018 Oct.

Climate change and African trypanosomiasis vector populations in Zimbabwe's Zambezi Valley: A mathematical modelling study

Affiliations

Climate change and African trypanosomiasis vector populations in Zimbabwe's Zambezi Valley: A mathematical modelling study

Jennifer S Lord et al. PLoS Med. .

Abstract

Background: Quantifying the effects of climate change on the entomological and epidemiological components of vector-borne diseases is an essential part of climate change research, but evidence for such effects remains scant, and predictions rely largely on extrapolation of statistical correlations. We aimed to develop a mechanistic model to test whether recent increases in temperature in the Mana Pools National Park of the Zambezi Valley of Zimbabwe could account for the simultaneous decline of tsetse flies, the vectors of human and animal trypanosomiasis.

Methods and findings: The model we developed incorporates the effects of temperature on mortality, larviposition, and emergence rates and is fitted to a 27-year time series of tsetse caught from cattle. These catches declined from an average of c. 50 flies per animal per afternoon in 1990 to c. 0.1 in 2017. Since 1975, mean daily temperatures have risen by c. 0.9°C and temperatures in the hottest month of November by c. 2°C. Although our model provided a good fit to the data, it cannot predict whether or when extinction will occur.

Conclusions: The model suggests that the increase in temperature may explain the observed collapse in tsetse abundance and provides a first step in linking temperature to trypanosomiasis risk. If the effect at Mana Pools extends across the whole of the Zambezi Valley, then transmission of trypanosomes is likely to have been greatly reduced in this warm low-lying region. Conversely, rising temperatures may have made some higher, cooler, parts of Zimbabwe more suitable for tsetse and led to the emergence of new disease foci.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Temperature at Rekomitjie.
(a) Monthly mean temperatures. Horizontal line at 30°C highlights the increase in the number of consecutive years during the hot-dry seasons in which mean monthly temperatures have exceeded this level. (b) Five-year running mean monthly temperature (°C) anomalies relative to 1960–1990 reference period.
Fig 2
Fig 2. Increases in mean daily temperature between 1975 and 2017 calculated for each month of the year.
Estimated using time series linear regression. Segments are 95% prediction intervals. All months except January and April had a statistically significant (p < 0.05) increasing trend between 1975 and 2017.
Fig 3
Fig 3. Fitted temperature-dependent functions.
(a) Adult female mortality rate per day: points—published estimates from mark-recapture experiments on Antelope Island, Zimbabwe [30]; line—fitted temperature-dependent adult mortality function (Eq 1). (b) Pupal mortality rate per day: points—published estimates from laboratory experiments [27]; line—fitted temperature-dependent pupal mortality function (Eq 2). (c) Pupal emergence rate per day: points—published estimates from laboratory experiments; line—Eq 3 fitted as described in [26]. (d) Larviposition rate per day: points—data from published field experiments [28]; lines—Eq 4 fitted as described in [30]. See Table 1 for fitted parameter estimates of the mortality functions.
Fig 4
Fig 4. Observed (points) and modelled (line) changes in numbers of G. pallidipes females caught between 1960 and 2017.
Data, on log base 2 scale, from 1990 to 2017, are average numbers caught by hand net, per afternoon, using an ox-bait. Fitted parameters are provided in Table 1.

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