Scientists have welcomed the development of genome sequence data on the tsetse fly, the vector responsible for the transmission of human African trypanosomiasis (HAT), commonly known as sleeping sickness. They say it could be instrumental in devising strategies to eradicate the fly and reduce deaths and the spread of other diseases associated with it.
“The genome data could ultimately advance knowledge on the biology of the tsetse fly and the trypanosome parasite it carries. Aspects of its biology may offer some vulnerabilities, such as the rearing of live young inside pregnant females, the dependence of the fly on bacteria that live inside its cells and its unusual prey-finding behavior,” Mathew Berriman, group leader, Parasite Genomics with Wellcome Trust Sanger Institute, told IRIN by email.
“The protein involved in sensing light, smell and taste have been found opening the door for refinement of traps. Also in the genome we find evidence of viruses that are associated with parasitic wasps - this highlights the possibility that a natural predator of tsetse exists in the wild; if it could be found, it could be utilized for biological control.”
Serap Aksoy from the University of Yale who co-authored the study, told IRIN: “The African trypanosomiasis affects thousands of people in sub-Saharan Africa. The absence of a genome-wide map of tsetse biology was a major hindrance for identifying vulnerabilities.”
She added: “This community of researchers across Africa, Europe, North America and Asia has created a valuable research tool for tackling the devastating spread of sleeping sickness.”
The researchers also found a set of visual and odour proteins that seem to drive the fly’s key behavioural responses, such as in searching for hosts or for mates.
The analysis of the genome will help in understand the basic biology of the fly.
‘‘By identifying the genes that make proteins associated with vision or smell, it allows us to use this information to better understand what sights or smells might attract or repel tsetse flies from traps. We can then use this information to make more efficient ways to control tsetse fly populations,’’ Geoffrey Attardo, the lead researcher with Yale School of Public Health, told IRIN by email.
Tsetse flies have a highly unusual biology. Unlike other flies which lay eggs, they give birth to a single live larva which is then nursed into a full grown fly by feeding on the mother’s milk glands.
Wellcome Trust Sanger Institute said in a statement: “This disease-spreading fly has developed unique and unusual biological methods to source and infect its prey. Its advanced sensory system allows different tsetse fly species to track down potential hosts either through smell or by sight…
“This study lays out a list of parts responsible for the key processes and opens new doors to design prevention strategies to reduce the number of deaths and illnesses associated with human African trypanosomiasis and other diseases spread by the tsetse fly.”
According to the researchers, the genome sequencing has helped to reveal “the fly's special repertoire of proteins for procuring, filtering, and packaging the blood and for viviparity [retention and growth of the fertilized egg within the maternal body until the young animal, as a larva or newborn, is capable of independent existence] and the expression of analogs of mammalian milk proteins.”
Critical proteins identified
“Proteins have been identified that are critical for feeding unborn larvae - interfering with this process would break the life cycle,” Wellcome Trust Sanger Institute’s Berriman told IRIN.
The study was conducted by a team of 146 scientists from 78 research institutes across 18 countries. They analysed the genome of the tsetse fly and its 12,000 genes.
Beyond disease control, the genome is an important resource for evolutionary biology.
‘‘The evolution of these amazing adaptations can now be examined on a genomic level relative to other related insects (such as the fruit fly Drosophila) for which genomic information is also available. Insights gained from such comparisons allow us to understand how such dramatic changes develop at the genetic level in related organisms,’’ said Attardo.
Other than sleeping sickness, the tsetse fly is also responsible for nagana disease (also known as nagana pest or animal African trypanosomiasis) in livestock.
The UN World Health Organization (WHO) says “sleeping sickness threatens millions of people in 36 countries in sub-Saharan Africa.”
Many of the affected populations “live in remote areas with limited access to adequate health services, which complicates the surveillance and therefore the diagnosis and treatment of cases. In addition, displacement of populations, war and poverty are important factors that facilitate transmission.”
The disease attacks the central nervous system, hence causing severe neurological disorders or even death if left untreated.
The researchers hope the new revelations on the fly’s genome will lead to the development of repellants or insecticides.
In many parts of Africa including Ethiopia, different strategies are currently being employed to control sleeping sickness.
‘‘These include, control of the fly through insecticide-treated blue traps and release of very dominant and competent laboratory-reared sterile males to the tsetse fly habitat,’’ Aysheshm Kassahun, a researcher at Addis Ababa University, told IRIN by email.
In 2009, WHO set up a specimen bank to help researchers to facilitate the development of new and affordable diagnostic tools. It contains samples of blood, serum, cerebrospinal fluid, saliva and urine from patients infected with both forms of the disease as well as samples from uninfected people from areas where the disease is endemic.