Spermless mosquitoes could help halt malaria spread

Releasing genetically modified, spermless male mosquitoes into the wild could in the future help prevent malaria transmission and reduce the chances of large outbreaks of the killer disease, British scientists said on Monday.

Researchers from Imperial College London sterilized male mosquitoes by genetically modifying them to neutralize a gene required for sperm production.

In a study to see how these mosquitoes would fare when trying to get a mate, they found that female mosquitoes cannot tell if the males they mate with are fertile, or spermless and therefore unable to fertilize the females' eggs.

The researchers said findings suggest that in future it
might be possible to control the size of the malaria-carrying mosquito population by introducing a genetic change that makes males sterile. Female mosquitoes would then unknowingly mate with the modified males. The female insect only mates once in her life before laying a batch of eggs. So, a female who mates with a sperm-less male subsequently produces eggs that are unfertilized 'blanks'. These eggs never hatch into future generations of malaria-carrying mosquitoes

Malaria is a mosquito-borne disease that affects up to 300 million people and kills nearly 800,000 every year. Its threat is greatest in Africa, where the World Health Organization says a child dies of malaria about every 45 seconds.

Malaria is transmitted exclusively through the bites of Anopheles mosquitoes. The intensity of transmission depends on factors related to the parasite, the vector, the human host, and the environment.

About 20 different Anopheles species are locally important around the world. All of the important vector species bite at night. The mosquitoes breed in shallow collections of freshwater like puddles, ricefields, and hoofprints.

Transmission is more intense in places where the mosquito is relatively long-lived (so that the parasite has time to complete its development inside the mosquito) and where it prefers to bite humans rather than other animals. For example, the long lifespan and strong human-biting habit of the African vector species is the underlying reason why more than 85% of the world's malaria deaths are in Africa.

WHO Malaria Key facts

  • Malaria is a life-threatening disease caused by parasites that are transmitted to people through the bites of infected mosquitoes.
  • In 2008, malaria caused nearly one million deaths, mostly among African children.
  • Malaria is preventable and curable.
  • Malaria can decrease gross domestic product by as much as 1.3% in countries with high disease rates.
  • Non-immune travelers from malaria-free areas are very vulnerable to the disease when they get infected.
Public health experts are working toward the eventual global eradication of malaria, but progress is slow and there is a constant need for better and cheaper ways to get there. (Malaria distribution map, courtesy of WHO)

"In the fight against malaria, many hope that the ability to genetically control the mosquito vector will one day be a key part of our armory," said Flaminia Catteruccia from Imperial's life sciences department, who led the study. This is especially key in areas where the mosquito passes on a malaria germ that is resistant to the malaria drugs used typically today.

But she added that for these currently theoretical control ideas to work in practice, scientists have to establish whether the insects would continue to mate as normal, unaware that their sexual mechanisms had been tampered with.

After mating for the first and only time in her life, the female mosquito goes through certain physiological changes, then eats a meal of blood, and lays a batch of eggs.

In this research, Catteruccia's team found that this behavior pattern was the same whether or not the mating had produced fertilized eggs that could hatch into mosquito larvae.

Using Anopheles gambiae mosquitoes -- the species primarily responsible for malaria spread in Africa -- the team created sperm-less males by injecting ordinary mosquito eggs with a protein that disrupts the development of their testes and prevents them from producing sperm as adults.

Crucially, this did not interfere with any other sexual function or behavior in either the female or the male, they explained in their study, published in the journal Proceedings of the National Academy of Sciences.

The scientists said they were also encouraged to find that after mating with a sperm-less male, the female made no attempt to find another mate -- and so effectively missed out on the opportunity to reproduce and pass on her genes.

This was contrary to what they had expected based on studies of other species such as fruit flies, where females tend to mate with more than one male to ensure their eggs are fertilized.

Another group of British scientists said last year they had created genetically sterile Aedes aegypti mosquitoes which early field trials suggested could be used to halt the rapid spread of another infectious disease, dengue fever.


Reuters,"Spermless mosquitoes could help halt malaria spread ",accessed August 9, 2011

Daily Mail, "Spermless mosquitoes may be the key to controlling malaria, scientists discover", accessed August 9, 2011

WHO, "Malaria Fact Sheet", accessed August 9, 2011


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