Viagra could offer an unlikely treatment for controlling the spread of malaria, a study has shown.

The impotency drug has an effect on red blood cells that reduces transmission of the malaria parasite Plasmodium from humans to mosquitoes.

The surprise discovery could lead to a new way of protecting populations from the infection that kills a million people each year, mostly children under the age of five.

Current treatments target an immature form of the parasite that invades red blood cells and causes symptoms but ignore a later stage that passes from a human host to a biting mosquito.

Preventing the human to mosquito transmission can interrupt the parasite's life cycle and help to prevent the disease spreading.

The scientists, led by Ghania Ramdani from the University of Paris Descartes, wrote in the journal Public Library of Science Pathogens: "Our observations provide an opportunistic approach towards the discovery of new malaria transmission-blocking drugs, by taking advantage of the wealth of clinical data available for sildenafil (Viagra).

"This strategy .. opens new avenues towards the design of novel interventions to halt the spread of malaria to humans."

Viagra was found to facilitate the elimination of infected red blood cells by the spleen by making the parasites within them them "stiffer".

Normally the spleen filters out rigid old or abnormal blood cells, but allows healthy flexible cells to remain in the circulation. Cells containing the "stiffened" parasites became less flexible and therefore more likely to be trapped by the spleen.

In this way, parasite-infected blood cells were removed from the body before they could be transmitted to blood-sucking mosquitoes.