ONCHOCERCIASIS is the medical name for a horrible parasitic disease, but in the villages of West Africa it is more starkly known as river blindness. Among some of the most destitute communities in the world, the disease thrives. It is borne by flies that breed in rivers and transmit the larvae of parasitic worms to humans. When the parasite invades the eyes, it causes total and permanent blindness. In some villages, 15 percent of the population and a much higher percentage of older adults have lost their sight.

In 1974, a consortium of governments, including the United States, and a group of international organizations began the Onchocerciasis Control Project, which operates in about a dozen West African countries where the disease is most prevalent. In an effort to eradicate the fly that carries the disease, the project has financed an effective program of spraying river areas with an agent almost identical to the spray used on gypsy moths here. But now there is heartening news of a drug -- ivermectin -- that in clinical trials has been extremely effective in treating river blindness. The developer and manufacturer of the drug, Merck & Co. of Rahway, N.J., has announced that it will provide free of charge all the doses needed to eradicate the disease.

Ivermectin destroys the larvae that cause the disease. It was first developed for use in combating parasites in livestock -- Merck makes money on this product -- but later experiments revealed its effectiveness in treating onchocerciasis in humans. The drug is produced in the form of a pill that has to be taken only once a year. If all the tens of millions of people in the affected areas were to take the drug, the disease would be wiped out. That is more easily said than done, however, since medical delivery systems in West Africa leave much to be desired. Officials at Merck decided that none of the available financial resources should be used to buy the drug, but instead should be directed at distribution and development of health facilities and personnel.

Merck's decision surprised and greatly pleased international public health experts. The spraying program will continue, of course. But in the long run, distribution of the drug could conquer a disease that, according to the World Health Organization, has been an untreatable "scourge of humanity since recorded history" and is now the world's leading cause of blindness.