Researchers from France, Cameroon, and England compared the effect of standard and high doses of ivermectin on adult worms of Onchocerca volvulus.
At present, control of onchocerciasis depends almost entirely on yearly treatments with 150 µg/kg ivermectin.
A total of 657 patients who had onchocerciasis were included in the trial.
They were randomly allocated to 150 µg/kg ivermectin yearly (reference group), 150 µg/kg every 3 months, 400 then 800 µg/kg yearly, or 400 then 800 µg/kg every 3 months.
Skin snip samples were taken from every patient before, and 3 and 4 years after the first dose. At the same time, one subcutaneous O. volvulus nodule was excised and was examined histologically.
The primary outcome was the vital status of the female worms.
Nodules were obtained from 511 of the patients.
| Ivermectin every 3 months more effective than yearly treatment.
After 3 years of treatment, more female worms had died in the groups treated every 3 months than in the reference group (odds ratio = 1·84 for 150 µg/kg and 2·17 for high doses).
The researchers also found that female worms were less fertile in these groups than in the reference group (0·24 and 0·14, respectively).
No difference was recorded between groups treated yearly.
Unexpected side effects consisted of mild, temporary, subjective visual changes in patients on high-dose regimens.
Dr Jacques Gardon, of the Pasteur Institute of Guyane, Cayenne, France, concluded on behalf of fellow authors, "Treatment with ivermectin every 3 months could greatly reduce the number of female worms, acute itching, and skin lesions.
"It could also lower transmission of O. volvulus and change the duration of control programs," he concluded.