The avermectins are a group of naturally occurring compounds, which have strong insecticidal and anti-worm properties. One of the family, ivermectin, was a breakthrough discovery in 1987 in the treatment of river blindness (onchocerciasis), one of the neglected tropical diseases that wreck the lives of many millions of poor people in the developing world and are rarely a goal for drug developers because there is no market in wealthy countries.

The earlier drugs for river blindness had serious side-effects and were not as effective as ivermectin, which kills the larval Onchocerca volvulus worms – microfilariae – that live in the subcutaneous tissue of an infected person.

It does not kill the adult worms so it has to be repeated once a year for up to 18 years to cure somebody. It is an oral pill, so easy to take, however, and the manufacturer, Merck, has promised to donate the tablets for free for as long as necessary to eliminate the disease.

The disease causes intense itching, skin discolouration, rashes, and eye disease, which often leads to blindness. It is spread by infected black flies which breed in rivers.

The World Health Organisation says the drug has “transformed the lives of millions of people suffering from onchocerciasis” since 1987.