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Tsetse, trypanosomiasis and climate change.
What can we learn from Zimbabwe field data?
John Hargrove1, Rachid Ouifki1, Glyn Vale1,2, Steve Torr2
DST/NRF Centre of Excellence in Epidemiological Modelling and Analysis (SACEMA)
1
SACEMA (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, vectorborne 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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