World Environment News

FEATURE - Mosquito diet pill seen as West Nile weapon

Date: 04-Sep-02
Country: USA
Author: Jane Sutton

The larvicide, which could hit the commercial market next year under the brand name Skeetercide, holds promise as a powerful and environmentally safe new weapon against mosquitoes that spread a host of illnesses such as West Nile, dengue fever and malaria. It will be tested soon in salt ponds in Key West.

"It's a perfect diet pill," said Dov Borovsky, the biochemist who developed it at the University of Florida's Medical Entomology Laboratory in Vero Beach. "In four to six days they're all going to starve to death."

Epidemiologists have long considered killing mosquitoes in the larval stage the best way to halt the spread of mosquito-borne diseases such as West Nile. The virus, found in birds and spread by mosquitoes, has killed 31 people in the United States so far this year.

The insects lay their eggs in water - along the edges of ponds, in pet water dishes, in old tyres or anything else that collects rainwater.

There they hatch into larvae, emerging in a few days as adults. That's where the trouble starts. The males feed on nectar and are harmless to humans. But the females must sip blood in order to produce more eggs and continue the species.

With every blood meal, they ingest whatever parasites are in the blood of their victims, be it birds, horses, reptiles or humans. And with every subsequent meal they pass on the micro-organisms that can sicken whoever or whatever they bite.

Killing larvae in the water is considered most efficient because once the adults start flying around: "It's a shot in the dark," Borovsky said.

And all the pesticides used against adult mosquitoes are neurotoxins. Though regulators insist they're safe, residents of neighbourhoods targeted for spraying sometimes balk and health officials consider them a weapon of last resort.

DIGESTIVE SYSTEM SWITCHED OFF

The University of Florida researchers were looking for a mosquito birth control pill when they discovered the hormone trypsin modulating oostatic factor, or TMOF.

It is produced in mosquito ovaries and they thought it worked by halting the development of the eggs. They extracted it from thousands of mosquito ovaries, synthesised it in the lab and injected it back into female mosquitoes.

When they later cut the mosquitoes open, they found their bellies full of blood, which would normally be digested quickly to nourish developing eggs. They realised what they had found instead was something that switched off the digestive system.

"The female has a natural hormone that controls the digestion of blood. Without the blood they cannot produce eggs ...hence they become sterile," Borovsky said.

The hormone had the same effect on mosquito larvae, switching off the digestive system and starving them to death.

Borovsky said the hormone kills all varieties of mosquitoes they've tested it on - there are some 3,000 species. It works in salt and clear water, does not harm the environment or other species, and is safe enough to use in drinking water containers, added Borovsky and the biologists developing it for commercial use.

"Each mosquito has lots of this hormone. Bats, birds already get it naturally when they eat mosquitoes," he said.

Borovsky's research drew widespread attention about five years ago when he isolated the gene that makes the hormone and spliced it into a common algae called chlorella, creating mosquito-killing pond scum.

In adapting the discovery for practical use, he found that transplanting the gene into yeast worked better. Yeast reproduces much more quickly than chlorella, taking less time to crank out sufficient quantities of the hormone.

And, Borovsky said: "Mosquito larvae love yeast."

The next step was formulating the dried yeast so it would work effectively in the water. Too powdery and it would blow away, too heavy and it would sink out of the mosquito larvae's reach.

The researchers hit the right formulation by binding it with cellulose particles to produce granules that are

Reuters
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