Zambezi Valley may soon be too hot for tsetse flies

The Zambezi Valley in Zimbabwe may soon become too hot to support tsetse fly populations, researchers have warned.

By Staff Reporter

A new study, based on 27 years of data from Mana Pools National Park in Zimbabwe, suggests that temperature increases over the last three decades have already caused major declines in the population of tsetse flies.

According to a statement released by Stellenbosch University in South Africa yesterday, the analysis, published in the PLOS Medicine Journal this week, provides a first step in linking temperature to the risk of sleeping sickness in Africa.

“Since the 1990s, catches of tsetse flies from cattle in the park declined from more than 50 flies per animal per catching session in 1990, to less than one fly per 10 catching sessions in 2017. Since 1975, mean daily temperatures have risen by nearly 1°C and by around 2°C in the hottest month of November,” the study established.

It said researchers from the Liverpool School of Tropical Medicine (LSTM), the South African Centre of Excellence for Epidemiological Modelling and Analysis (SACEMA) at Stellenbosch University, and the Natural Resources Institute at the University of Greenwich, developed a mathematical model, which showed that recent increases in temperature could account for the simultaneous decline of tsetse.

“If the effect at Mana Pools extends across the whole of the Zambezi Valley, then transmission of trypanosomes is likely to have been greatly reduced in this warm low-lying region,” Jennifer Lord, lead author and post-doctoral fellow at LSTM, says.

Stellenbosch University said while this would be good news for the disease situation in the Zambezi Valley, rising temperatures may have made some higher, cooler parts of Zimbabwe, more suitable for the flies.

Professor John Hargrove, senior research fellow at SACEMA, said the effect of recent and future climate change on the distribution of tsetse flies and other vectors, particularly mosquitoes, is poorly understood.

“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,” Hargrove said.

“In general, the ways in which climate change will affect the spread of infectious diseases in sub-Saharan Africa is poorly understood because of sparse empirical evidence.”

However, Stellenbosch University said work on tsetse and trypanosomiasis carried out at Rekomitjie Research Station at Mana Pools over the past 59 years has produced long-term data sets for both vector abundance and climate change.

It said unlike mammals and birds, insects such as tsetse flies cannot regulate their own body temperatures, and their development and mortality rates are, therefore, strongly influenced by environmental temperatures
“Pupae cannot survive at sustained temperatures below 16°C or above 32°C. In addition, tsetse populations can become established in an area only if there are sufficient numbers of host animals and suitable vegetation to support tsetse,” Hargrove explained.

He warned that Hwange National Park in Zimbabwe and Kruger National Park in South Africa were examples of areas where suitable hosts and habitat for tsetse are abundant.

Hargrove said while prophylactic drugs can protect livestock from the tsetse, no such drugs were available for humans, with the only sure way of protecting both livestock and humans being to attack the fly.