A common arthritis drug may prove an effective weapon against parasitic organisms responsible for dysentery, research suggests.
Auranofin, marketed as Ridaura, contains a medical form of gold and combats rheumatoid arthritis by reducing inflammation.
The new studies show that it could also be used to treat amoebic dysentery, and possibly Giardia.
Both infections cause dysentery symptoms which include stomach cramps and severe diarrhoea.
The amoebic dysentery organism Entamoeba histolytica, which also causes liver abscesses, is responsible for more than 70,000 deaths worldwide each year, mostly in developing countries.
Scientists in the US discovered that auranofin targets a protective enzyme the amoeba needs for its survival.
In infected mice and hamsters, the drug greatly decreased the number of parasites, damage from inflammation, and the size of liver abscesses.
Laboratory tests indicated that auranofin was 10 times more potent than the current standard treatment for dysentery, the antibiotic metronidazole.
This suggests it could be used at a low dose and on a one-time or limited basis.
"This new use of an old drug represents a promising therapy for a major health threat," said lead researcher Professor Sharon Reed, from the University of California at San Diego.
Her team has applied for permission to start clinical trials, using auranofin to treat patients both with amoebic dysentery and Giardia.
The findings are published in the latest issue of the journal Nature Medicine.
The fact that auranofin is already proven to be safe will cut the time and cost needed to develop it as a dysentery treatment.
Co-author Professor James McKerrow, from the University of California at San Francisco Sadler Centre for Drug Discovery, said: "When we're looking for new treatments for the developing world, we start with drugs that have already been approved.
"If we can find an approved drug that happens to kill these organisms, we've leapfrogged the development process that goes into assessing whether they are safe, which also makes them affordable throughout the world."
Colleague Dr Anjan Debnath, also from the University of California at San Francisco, said: "This is a drug that you can find in every country.
"Based on the dosage we're seeing in the lab, this treatment could be sold at about 2.50 dollars (£1.58) per dose, or lower. That cost saving could make a big difference to the people who need it the most."
In 2009, a decision was taken by auranofin's manufacturer to stop marketing the drug in the UK due to falling sales.