November 2, 2018, by Tim Radford
Parts of Africa may grow too hot for tsetse flies, farmers’ scourge and carriers of disease. But will they simply move to higher, cooler terrain?
LONDON, 2 November, 2018 – Global warming may have done one good thing for the Zambezi Valley: it may have done for the tsetse flies, with conditions soon too hot for them to breed there any longer.
That means that a blood-sucking insect responsible for perhaps a million cattle deaths a year – and that carries the parasite behind the devastating disease of human African trypanosomiasis, commonly known as sleeping sickness – could disappear from the lowlands it has plagued for centuries.
That is the good news. The bigger concern is that the same warming in tropical Africa could turn the highlands of Zimbabwe and its neighbours into a suitable breeding zone for both the disease and the creature that carries it.
For the moment, a new study in the Public Library of Science journal PLOS Medicine claims only to have established the first clear link between disease hazard and temperature.
“Tsetse disappeared from these areas and have never established
themselves again. But if temperatures continue to increase,
there is a danger that they may re-emerge”
For 27 years, scientists have kept count of tsetse fly numbers in the c in Zimbabwe. The insect feeds on wild animals, as well as domestic cattle and humans. Millions of years of evolution mean that African pests and prey have found ways to live with each other, but humans and their introduced farm animals are still vulnerable.
The researchers report that, in the last three decades, temperatures in the region have risen by 0.9°C, and the hottest month, November, is now 2°C hotter on average than in the past. The insect can reproduce and multiply only in temperatures that lie between 16°C and 32°C.
And in those same three decades, fly numbers have fallen dramatically Once, researchers could expect to go out in the afternoon in the Mana Pools National Park and find 50 flies per elephant or buffalo examined. Now, they write, they might find one tsetse fly every 10 catching sessions.
Since the vegetation and the wild animal population of the park have remained much the same, it seems likely that ambient temperature is the factor that limits tsetse fly numbers.
“If the effect at Mana Pools extends across the whole Zambezi Valley, then transmission of the trypanosomes is likely to have been greatly reduced in this warm, low-lying region”, says Dr Jennifer Lord, postdoctoral research associate at the Liverpool School of Tropical Medicine, who led the research and modelled the link between temperature and insect numbers.
But co-author John Hargrove, senior research fellow at the South African Centre of Excellence for Epidemiological Modelling and Analysis at Stellenbosch University, warns that, if so, other African parks could once again play host to the pest, including the world-famous Kruger National Park in South Africa.
The fly is now an annual hazard for an estimated 55 million cattle in sub-Saharan Africa, and is thought to cost African economies US$1 billion a year or more just in losses of meat and milk.
“Tsetse flies did occur in these areas in the 19th century, but they were always marginal because the winters there were rather too cold,” Professor Hargrove says.
“With the massive rinderpest outbreak of the middle 1890s, tsetse disappeared from these areas and have never established themselves again. But if temperatures continue to increase, there is a danger that they may re-emerge.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.