A two-year study in eight Ethiopian villages found that a single dose of an antibiotic is not enough to end infections that cause trachoma, the world's leading preventable cause of blindness.

A previous study in one Tanzanian village had suggested a single mass distribution of the same antibiotic could be enough to bring infection rates close to zero.

But the new study found infection slowly returned over two years.

"I think it's actually good news," said study co-author Dr. Tom Lietman of the University of California San Francisco. "While it comes back, it comes back very slowly."

The findings suggest that twice-yearly antibiotic treatment with azithromycin could eliminate infection within a few years, Lietman said. The study appears in Wednesday's Journal of the American Medical Association.

Trachoma is caused by a strain of bacteria called chlamydia trachomatis, which spreads to the eyes from fingers, clothing or, some researchers think, from flies.

Young children have the highest rates of infection. Blindness develops over decades through repeated infections and scarring of the inside of the eyelid.

Trachoma is not a sexually transmitted disease, although the strain that causes it is related to chlamydia, Lietman said.

In the new study, the average infection rate among young children was 43 percent before treatment. Two months after treatment, the rate was 5 percent, but it climbed to 11 percent after two years.

The International Trachoma Initiative supplied the antibiotic through donations from drug maker Pfizer.

Trachoma afflicts an estimated 84 million people worldwide, mostly in poor regions of Africa, Asia, the Middle East, Latin America and Australia, according to the World Health Organization. About 8 million people currently are blind because of it.

Dr. Silvio Mariotti of the WHO said even multiple treatments with antibiotics will not eliminate trachoma-caused blindness unless drugs are accompanied by surgery for advanced stages of the disease and improved hygiene, water quality and sanitation.

"Antibiotics are not the way ahead, when used alone," Mariotti said in an e-mail.