Fish are being used to control malaria in India with remarkable success, according to researchers from the Indian Council for Medical Research.
Fish such as guppies eat the mosquitoes
The mosquitoes which transmit malaria have virtually been eradicated from some areas.
Scientists presented the results of several pilot projects at the Indian Science Congress in Chandigarh.
Malaria control takes up a substantial slice of India's health budget, largely through buying insecticides.
The theory is simple: find fish which like eating mosquito larvae and put them in ponds, rivers and wells where mosquitoes lay their eggs. The eggs hatch, and the fish eat the larvae.
Dr VP Sharma, a former director of India's Malaria Research Institute who now works with the Council for Medical Research, told the meeting that pilot projects in four states have met with remarkable success.
Introducing fish like guppies, he said, was one of the main reasons why the number of malaria cases each year in India was falling.
"They were more than two million," he said. "Now, actually, they have gone down to 1.8 million. The World Bank has a programme in 100 districts using the fish and it will take another five years before the real impact would be known."
Dr Sharma told the meeting that fish had virtually eliminated malaria-carrying Anopheles mosquitoes from some districts, though he cautioned that the strategy did not work everywhere.
Using fish in this way used to be a standard approach to malaria control, but when insecticides like DDT were introduced during the last century with apparently magical success, it fell into disuse.
Now mosquitoes have become resistant to many of these chemicals and fish are back on the menu.
The other attraction is cost. Supplying ponds with guppies is a cheap alternative to buying insecticides.