GM mosquitoes which produce a component of scorpion venom in their stomachs could fight malarial parasites, say researchers.

But other experts have warned of the potential dangers in arming insects with anything which could cause potential harm to the humans they bite.

There are up to 500m cases of malaria worldwide every year, and an estimated 1m deaths, and several teams are examining ways to use GM technology to fight the disease.

Researchers from Mexico, reports New Scientist, have found a way to alter fruit flies to produce a protein ingredient of scorpion venom in their guts.

When malarial parasites are then injected into the stomach - their natural breeding place in mosquitoes - the venom appears to stop most of them growing.

The team is confident that their results in fruit flies can be effectively reproduced in the mosquitoes that transmit malaria.

Natural competition

They hope that because an uninfected mosquito tends to live longer and reproduce more freely than one carrying the parasite, their GM version would be able to out-compete standard mosquitoes if introduced into the wild.

However, other experts are sceptical whether this could be achieved.

Professor Chris Curtis, an expert in medical entomology at the London School of Hygiene and Tropical Health, said that only a small proportion of natural mosquitoes actually carried the parasite, so the GM versions were unlikely to propel them into extinction.

Consequently, scientists had been trying to introduce "harmless" mosquitoes produced by "old-fashioned inter-breeding" for some time with little success.

He said: "The problem is always going to be how you drive these altered insects into mosquito populations worldwide."

He pointed to his work on insecticide-impregnated netting as a realistic way to tackle the mosquitoes, and called for aid to Africa to help pay for its introduction.

However, Dr Andrea Crisanti from Imperial College in London, said that it was feasible to breed hundreds of millions of GM mosquitoes in a relatively short time.

His team has managed to introduce genetic changes into Anopheles stephensi , one of the main malaria-carrying mosquitoes.

Antibody attack

It is now working on a modification that will arm malarial mosquitoes with antibodies to attack the parasites.

Another possibility, said Dr Crisanti, would be a modification in male mosquitoes which led to the death of any female offspring.

He said: "The question is, what is going to be the best thing to develop and release.

"We are certainly going to need guidelines soon on what we should and should not be doing."

On the question of adding the "venom" gene, he said: "It is possible the protein doesn't have any harmful effects on humans.

"But one has to be much more careful in what we put in the mosquito."

The curious life cycle of the parasite, called plasmodium, is one of the reasons why the disease has proved so difficult to control.

It is passed to humans through the saliva produced by the mosquito when it bites, and migrates to the liver, where it reproduces, then emerges to enter red blood cells.

It reproduces again in the red blood cells, then bursts out, causing the symptoms of fever and sweating.