Abstract: The main effects of ivermectin (IVM) on Onchocerca volvulus are a microfilaricidal effect, leading to a rapid decrease in the microfilarial loads, and a temporary blockage of the release of microfilariae (mf) by the adult worms. The mf loads re-increase slowly from 3 months after treatment. While reports from Ghana suggest that this re-increase is more rapid in patients who have received many IVM doses, the long term impact of repeated IVM treatments on the dynamics of mf loads is still unclear. To evaluate this, mf loads have been monitored in 1994, 1997 and 2007 in a cohort of Cameroonian subjects. They all had received either 4 annual or 13 3-monthly IVM doses between 1994 and 1997 (during a closely monitored trial), and were proposed the drug during the subsequent years as part of annual community-directed treatments with IVM (CDTI). In 2007, information was collected from each individual about the date of their last dose. Mf loads measured in 1994 (before the 1st dose), 1997 (one year after the 4th dose), and 2007 (10 months after the last round of CDTI) in a group of 32 patients treated annually between 1994 and 1997 and who had actually taken IVM in 2006, were 137.7, 15.2 and 15.4 mf/mg, respectively. Thirty-eight other individuals had been treated either annually or three-monthly between 1994 and 1997, and had not participated in the CDTIs organised in 2006; in these patients, the mf loads measured in 1994 and 2007 were 99.5 and 47.3 mf/mg, respectively. When extending the non compliance period to more than 3 years, the mean mf load in 2007 was 109.2 mf/mg (14 subjects). 105 skin-snip positive individuals from the 1994 cohort were retreated in 2007 after the parasitological examination, and then followed up at 15, 80 and 180 days after treatment. Mf loads recorded on D15 showed that the microfilaricidal effect of IVM was similar to that classically reported. However, the mf prevalence observed at D80 (41%) and D180 (77%) and the significant mf loads observed at D180 (13% of individuals present with >10mf/mg) reflect the persisting reproductive capacities of some adult worms. These observations suggest that despite 13 years of intervening treatments halting IVM treatment could result in a rapid return to the initial hyperendemic state. Embryograms and genotyping of parasites collected as part of this study are underway to assess whether the repopulation rates of skin by mf observed in some individuals reflects an emerging resistance of O. volvulus to IVM.