Tsetse fly numbers dwindle in the warming Zambezi Valley

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  • Tsetse flies carry the microorganism that causes sleeping sickness in humans and livestock, but a recent study reveals that their numbers have dropped at a site in the Zambezi Valley as temperatures have climbed.
  • Sleeping sickness, known also as trypanosomiasis, is a debilitating and potentially deadly disease to humans that also kills perhaps 1 million cattle each year.
  • The study’s authors say that the decline of the tsetse in Zimbabwe’s Zambezi Valley might be accompanied by a rise in their numbers in cooler locales where they once weren’t as prevalent.

Rising temperatures seem to have made a southern African park less hospitable to the tsetse fly (Glossina spp.), the carrier of a dangerous disease-causing microorganism, new research shows.

Named for the single-celled trypanosomes that infiltrate the bloodstream during a tsetse bite, trypanosomiasis, or sleeping sickness, kills perhaps a million cattle a year around the world. And for humans, it’s a painful disease that disrupts sleep patterns and can be deadly if left untreated.

Sleeping sickness kills around a million cattle globally each year. Image by Justinjerez (CC BY-SA 3.0), via Wikimedia Commons.

But over the past few decades, researchers in Zimbabwe’s Mana Pools National Park have noticed fewer flies. Those observations led disease ecologist Jennifer Lord of the Liverpool School of Tropical Medicine in the U.K. and her colleagues to speculate that climate change might be tinkering with the ecosystem.

Annual average temperatures are up by nearly 1 degree Celsius (1.8 degrees Fahrenheit) and by 2 degrees Celsius (3.6 degrees Fahrenheit) in the hottest month, November, at Rekomitjie Research Station in the park since the mid-1970s. Farmers haven’t worked the land since 1958, so most other conditions have been stable. At the same time, the number of flies that scientists are catching at Rekomitjie has dropped off. In 1990, they used to expect to catch about 50 tsetse flies around one cow; nowadays, they have to sample an average of 10 cows before finding just one fly.

A tsetse fly (Glossina spp.), pictured here in Burkina Faso. Image courtesy of the International Atomic Energy Agency (CC BY-SA 4.0), via Wikimedia Commons.

To test whether the climatic effect made sense, Lord and her team built a mathematical model that incorporated the impacts of temperature on tsetse fly reproduction and mortality, established in previous research. Higher temperatures speed up a fly’s metabolism, which means it has to find more animals or people to feed on. Each time a fly lands on a blood-bearing target, its chances of dying — from perhaps the swat of a hand or a tail — go up. Young pupae also tend to consume more of their fat stores when it’s warm, leaving them with less of this protection against starvation when they break out of their shells.

The results of the team’s model fit well with the trend they observed, which could have broader implications in the region. They published their results Oct. 22 in the journal PLOS Medicine.

“If the effect at Mana Pools extends across the whole of the Zambezi Valley,” Lord said in a statement, “then the transmission of trypanosomes is likely to … have been greatly reduced in this warm low-lying region.”

Tsetse fly numbers at a research site in the Zambezi Valley have declined in the past three decades as average local temperatures have risen. Image courtesy of the International Atomic Energy Agency (CC BY-SA 4.0), via Wikimedia Commons.

With few of these types of models, it’s been difficult for scientists to know what climate change might mean for the dynamics of diseases spread by insects.

“We don’t know, for example, whether the resurgence of malaria in the East African highlands in the 1990s was caused by rising temperatures or by increasing levels of drug resistance and decreasing control efforts,” John Hargrove, an epidemiologist at Stellenbosch University in South Africa, said in the statement.

Trypanosomes in a blood smear from a patient with sleeping sickness. Image courtesy of CDC/Dr. Myron G. Schultz (Public domain), via Wikimedia Commons.

And though the decline of tsetse flies might seem like a welcome reprieve from a pest that spreads a devastating disease, the authors caution that it could just point to a shift in the territory that the flies stake out.

“[Rising] temperatures may have made some higher, cooler parts of Zimbabwe more suitable for tsetse,” the authors write. This could potentially bring sleeping sickness to new locations where it was previously unknown.

Banner image of a tsetse fly courtesy of the International Atomic Energy Agency (CC BY-SA 4.0), via Wikimedia Commons

John Cannon is a Mongabay staff writer based in the Middle East. Find him on Twitter: @johnccannon

Citation

Lord, J. S., Hargrove, J. W., Torr, S. J., & Vale, G. A. (2018). Climate change and African trypanosomiasis vector populations in Zimbabwe’s Zambezi Valley: A mathematical modelling study. PLOS Medicine, 15(10), e1002675.

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