A major
scientific breakthrough has yield a powerful genetic tool that could one day
eliminate sleeping sickness from sub-Saharan Africa. According to a study
published in the journal science, Scientists backed by UN have been able to
completely map the genome of tsetse fly,
the blood-sucking insect that transmits African sleeping sickness in humans and
nagana in animals.
The breakthrough has brought hope to
70 million people who at a risk because of the insect in the continent. It is
also a relief to livestock farmers since the practice was becoming 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.
There is
no sleeping sickness vaccine and drugs to treat sleeping sickness, they are
expensive, let alone having 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.
“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.
According to Dan Masiga, head of the
Molecular Biology and Bioinformatics unit at icipe, 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 biocontrol 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.
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.
