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DNA experts aim to swat dreaded tsetse fly

Reuters

An international team of scientists has deciphered the genetic code of the tsetse fly, the bloodsucking insect that spreads deadly African sleeping sickness, with the hope that its biological secrets can be exploited to eradicate the malady.

The findings, announced Thursday, were the culmination of a multimillion-dollar, decadelong effort involving more than 140 scientists from 78 research institutions in 18 countries.

The fly’s bite carries a parasitic micro-organism that causes sleeping sickness in people in sub-Saharan Africa and a form of the disease in animals that can devastate livestock herds.

Sequencing the tsetse fly’s genome exposed the molecular underpinnings of its weird biology. It gives live birth to young rather than laying eggs like other insects; it nourishes larvae inside the uterus with a form of milk; it is oddly attracted to the colors blue and black; and it feeds exclusively on blood.

The scientists expressed optimism that the genetic blueprint could lead to new ways to combat the tsetse fly, like a chemical that could interfere with its reproduction or ways to improve existing traps used to kill it.

“Like any such discoveries, there will be new leads that we might not see now. I am, however, optimistic that unique aspects of tsetse fly biology will lead to new methods to fight the disease,” said one of the researchers, Daniel Masiga of the International Center of Insect Physiology and Ecology in Kenya.

“If you could come up with a tsetse-specific reproductive inhibitor that has no mammalian toxicity, that would be ideal,” said another researcher, Geoffrey Attardo of the Yale School of Public Health.

The tsetse fly genome was double the size of a fruit fly’s but only one-tenth as big as a human’s genome. It has about 12,000 genes and 366 million letters of genetic code.

The tsetse fly has brought misery to humans and animals for eons. They have existed far longer than people; a tsetse fly fossil found in Colorado dates back about 34 million years. African sleeping sickness, also known as trypanosomiasis, is a widespread tropical disease throughout sub-Saharan Africa that is fatal if not treated.

Its form in animals is called nagana. It has caused billions of dollars in economic damage and has forced farmers to rear hardier but scrawnier cattle that provide less meat and milk but can better withstand the parasite, said tropical disease researcher Matthew Berriman of the Wellcome Trust Sanger Institute in Britain.

The fly is not born with the parasite but ingests it when it bites an infected person or animal to eat blood. It spreads the parasite through saliva when it bites another victim.

In its advanced stages, sleeping sickness targets the central nervous system, causing alteration of the biological clock (circadian rhythm), changes in personality, confusion, slurred speech, seizures and difficulty in walking and talking.

“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,” said John Reeder, who heads World Health Organization’s program for research and training in tropical diseases.

In recent years, public health efforts have cut the number of cases and deaths. The World Health Organization said it considers the disease to be “entering into a phase of elimination.” The WHO says 5,967 cases were reported last year, compared with 26,574 in 2000.

Disease prevention has focused on reducing fly populations. Experts think a preventive vaccine is unlikely because of the way the parasite evades the mammalian immune system. Sleeping sickness causes far fewer infections and deaths than the mosquito-borne tropical diseases malaria and dengue. In mosquitoes, only females feed on blood, using its protein for egg development. Both sexes of tsetse flies eat blood.

Experts say tsetse flies may be easier to target than mosquitoes. For one thing, female mosquitoes can lay more than 100 eggs at a time, while tsetse flies multiply fairly slowly as they give birth to only one larva per reproductive cycle.

The study was published in the journal Science, with accompanying research appearing in other journals.