Scientists close in on eliminating deadly tsetse fly

ICIPE scientists have made a breakthrough in understanding the life cycle of the tsetse fly, raising hopes of biologically eliminating a fly that kills cattle worth $4bn a year in Sub Saharan Africa.

The discovery is good news to Kenya which has areas like the Coast chocking under tsetse fly attack. The Ministry of Agriculture estimates that about 60 percent of Coast province is infested by tsetse flies with Sabaki-Gulalu area alone, having 40 percent of its entire land under tsetse fly attack.

According to the World Health Organization, the estimated number of actual cases is 20,000 and the population at risk is approximately 70 million people in 36 African countries. The study about the blood-sucking insect that transmits African sleeping sickness in humans and nagana in animals was published on 24 April 2014 in the journal Science.

“This is a major milestone for the tsetse research community,” said Geoffrey M. Attardo, a research scientist at the Yale School of Public Health in the United States and the lead author on the paper. “Our hope is that this resource will facilitate functional research and be an ongoing contribution to the vector biology community.”

Yale School of Public Health professor Serap Aksoy helped initiate the collaborative research project in the early 2000s when she and a small group of researchers concluded that progress against the disease and new tsetse-based control opportunities could be stymied unless the biological and chemical underpinnings of the organism were completely understood. “Our hope is that tsetse research will now enjoy broader participation from the vector community and lead to improved and novel methods to eliminate disease,” Aksoy said.

Dan Masiga, head of the Molecular Biology and Bioinformatics unit at ICIPE, noted that one genetic finding that has potential for tsetse fly control is the discovery of virus DNA material that is closely related to viruses found in parasitic wasps known as Cotesia congregata, which at the larval stage feed on moths and butterfly larva as they develop into adult wasps. However, these wasps don’t normally lay their eggs in tsetse fly larvae. Normally, these parasitic wasps lay their eggs among easier to find moth and butterfly larvae.

In a blog on the website of the Wellcome Trust, an important funder of the genome work, Masiga theorizes that this DNA material might have been inserted into the tsetse fly genome at an earlier stage of the species’ development, when the fly hadn’t yet a system of hatching its eggs in the uterus.

“The tsetse fly only gives birth to well developed pupae, which within a few minutes form a hard outer shell that would be difficult for the wasp larvae to penetrate and deposit eggs,” Masiga said. This could have been an evolutionary defense against the parasitic wasps. “Cotesia congregata, the parasitic wasp, also lays eggs in the tobacco hornworm, and this knowledge has been used to develop bio-control agents against the hornworm,” he explained.

Although C. congregata isn’t present in the same areas as tsetse flies, there are very close relatives in Africa that could be responsible for the presence of the similar wasp virus material in the genome. Masiga also pointed to the discovery of olfactory receptors in the genome that trigger ‘mating deterrence’ from females as just one other example of a potential tool in tsetse control.

Tsetse flies also carry symbiotic parasitic organisms in their bodies, which could also be manipulated to reduce the reproductive cycle. One such organism is Wigglesworthia glossinidia, without which females often abort their larval offspring. Offspring that are born are immune compromised.

Although official reports of new infections in humans recently dropped below 10,000 per year, many cases especially in rural areas with limited access to health facilities go undiagnosed. According to the World Health Organization, the estimated number of actual cases is 20,000 and the population at risk is approximately 70 million people in 36 African countries.

While there are drugs to treat sleeping sickness, they are expensive, have many undesirable side effects, and are difficult to administer in wide swathes of rural Africa where the disease is most pronounced. Left untreated, sleeping sickness inevitably leads to death. Livestock farming becomes nearly impossible in areas where the tsetse fly is endemic: three million animals die per year at a cost of $US 4 billion to livestock farmers. Farmers abandon raising livestock in those areas, unable to pay for veterinary treatments when they are available.

The tsetse fly project cost approximately $US 10 million and was funded over the years from multiple public and private sources, including the Wellcome Trust, the World Health Organization Special Programme for Research and Training in Tropical Diseases (WHO-TDR), and the Ambrose Monell Foundation. The genome was sequenced in a number of centres, and assembled at the Wellcome Trust Sanger Institute.

The discovery is good news to Kenya which has areas like the Coast chocking under tsetse fly attack. The Ministry of Agriculture estimates that about 60 percent of Coast province is infested by tsetse flies with Sabaki-Gulalu area alone, having 40 percent of its entire land under tsetse fly attack.