Before the 1990s, researchers at the Rekomitjie research station in Zimbabwe’s warm, low-lying Zambezi Valley, caught more than 50 tsetse flies per head of cattle each afternoon.
But by last year, the teams frequently failed to catch a single fly.
This, says an international team of scientists behind a new study looking at the impact of climate change on sleeping sickness in Africa, suggests that rising temperatures are behind the decline of the tsetse.
The blood-feeding insect, pictured, which cannot regulate its own body temperature, transmits pathogens causing potentially fatal sleeping sickness in humans across sub-Saharan Africa. The parasites, too, cause around 1million cattle deaths a year.
But while locations such as the Zambezi Valley may soon be too hot to support tsetse populations, rising temperatures may have made some higher, cooler areas more suitable, leading to the emergence of new disease areas in the Kruger National and Hwange National parks.
These are areas where suitable hosts and habitat are abundant, says the team of researchers from the Liverpool School of Tropical Medicine, the SA Centre of Excellence for Epidemiological Modelling and Analysis (Sacema) at Stellenbosch University and the Natural Resources Institute at the University of Greenwich, in the new analysis, published in the journal PLOS Medicine last week.
“Everywhere is getting hotter, so whereas the Zambezi Valley is too hot for tsetse flies, other places where it used to be too cold now may be more suitable for them,” says Professor John Hargrove, senior research fellow at Sacema.
The parasites occurred in regions such as the Kruger in the 19th century, but the winters were too cold.
“With the massive rinderpest outbreak of the mid 1890s, when the vast majority of ungulates died, tsetse disappeared from these areas.
“But if temperatures continue to increase there is a danger they may re-emerge.”
Control measures, say the authors, may need to be adopted in case the insects reoccupy these parks, threatening cattle and humans nearby. “The only sure way of protecting both livestock and humans is to attack the fly.”
The study is based on 27 years of data - prolonged laboratory and field measures of fly densities - from Mana Pools National Park in Zimbabwe.
Since 1975, mean daily temperatures have climbed by nearly 1°C and by around 2°C in the hottest month of November.
Hargrove says the effect of recent and future climate change on the distribution of tsetse flies and other vectors, especially mosquitoes, is poorly understood.
“We know what’s happening with tsetse flies but what we don’t know is how beneficial insects are being affected by this temperature change.”