Ememging Pathogens Institute Special Seminar
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Tsetse, Trypanosomiasis and Climate Change; What Can We Learn From Zimbabwe Field Data?
Professor John Hargrove
DST/NRF Centre of Excellence in Epidemiological Modelling and Analysis (SACEMA)
1SACEMA (University of Stellenbosch); 2Natural Resources Institute (NRI)
Abstract: Increases in global temperatures over the past 50 – 100 years have raised questions as to the expected effect of further increases on, among many other things, vector-borne diseases in Africa. Where there have been scientific studies of the changes likely to occur for specific diseases these have been of a general nature and have relied heavily on remotely sensed climatic data. There is generally a dearth of data on the detailed effects of climate on the population dynamics of particular insect vector species or groups, and how these matters are likely to be affected by changes in climate. A striking exception to this general lack of information exists, however, in the case of the tsetse flies (Glossina spp; Glossinidae). The most complete data for species in the genera have been collected through studies on Glossina morsitans morsitans Westwood and G. pallidipes Austen, carried out more or less continuously for the past 50 years at Rekomitjie Research Station, in the Zambezi Valley of Zimbabwe. The experimental work was carried out with a view to developing new and improved methods of vector and disease control and, as a part of this process, of getting an improved general understanding of the population dynamics of tsetse and trypanosomiasis. A natural spin-off of this work has been a huge amount of data on the detailed changes in tsetse populations – both with season and over longer time periods.
Over the vast majority of the past 50 years, staff at Rekomitjie have also made routine daily notes of basic meteorological data (maximum and minimum temperature and rainfall). As a consequence we are now in a unique position to study the relationship between the dynamics of populations of two vector species - with short and long-term seasonal and longer-term climatic fluctuations that have occurred over the last 50 years.
The marshalling, cleaning, analysis, interpretation and publication of the available biological and meteorological data will be a daunting task, requiring several years of inputs from the personnel who were principally involved in the collection of these data. The present review provides preliminary analysis of the meteorological data, together with a small portion of the biological material, serves to highlight: i) the significant changes in temperature that have been observed at Rekomitjie over the past 50 years; ii) the very strong effects that temperature has on a wide variety of aspects of tsetse biology; iii) how meteorological changes are therefore likely to affect tsetse population dynamics ; iv) how extreme climatic events have already been observed to affect tsetse populations in the recent past; v) how, therefore, more frequent extreme events associated with increases in temperature might be expected to affect populations in the future; vi) the scope of the data available.
Changes in temperature are of prime importance for tsetse flies: various aspects of the population dynamics of the genus are more strongly affected by temperature than by other meteorological variables. Mean temperatures are important since they affect the general metabolic rate and thus influence such factors as the rate at which tsetse need to feed, the frequency with which they deposit their larvae, and the time spent in the pupal phase. Since the act of feeding also involves increased risk for adult tsetse it may also be expected that adult mortality will increase with mean temperature. Field observations also show that tsetse are very sensitive to high temperatures so that maximum temperatures may have an additional effect on mortality, independent of the effects of mean temperature. This is important since we may expect that a general increase in temperature will be accompanied by an increase in the frequency of extreme events. If these events are sufficiently severe they may have damaging effects on tsetse populations at particular times of the year that are so serious that they over-ride the modest effects expected given modest increases in the mean temperature. Conversely, decreasing temperatures result in a rapid increase in the pupal duration, and reduced natality.
As with all animal and plant species we may expect that the effect of climate change on tsetse will be modulated by the effects of such changes on other species and on the ecology as a whole. Since tsetse feed only on blood their populations will be affected by changes in the populations of their vertebrate hosts – and these homeotherms may be expected to have quite different responses to climate change compared with a poikilotherms such as tsetse and will, in turn, be affected by changes in vegetation consequent on climate change.
The various known direct and indirect effects of climate, particularly temperature, on tsetse population dynamics are reviewed. Seasonal and medium-term changes in tsetse populations in the Zambezi Valley of Zimbabwe – in terms of total numbers and age, species and sex distribution are analysed in terms of climatic variables. Particular attention is paid to the importance of interpretation of the data and how this is complicated by sampling bias.
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