Reproductive status of Onchocerca volvulus after ivermectin treatment in an ivermectin-naïve and a frequently treated population from Cameroon

Abstract

Background: For two decades, onchocerciasis control has been based on mass treatment with ivermectin (IVM), repeated annually or six-monthly. This drug kills Onchocerca volvulus microfilariae (mf) present in the skin and the eyes (microfilaricidal effect) and prevents for 3-4 months the release of new mf by adult female worms (embryostatic effect). In some Ghanaian communities, the long-term use of IVM was associated with a more rapid than expected skin repopulation by mf after treatment. Here, we assessed whether the embryostatic effect of IVM on O. volvulus has been altered following frequent treatment in Cameroonian patients.

Methodology: Onchocercal nodules were surgically removed just before (D0) and 80 days (D80) after a standard dose of IVM in two cohorts with different treatment histories: a group who had received repeated doses of IVM over 13 years, and a control group with no history of large-scale treatments. Excised nodules were digested with collagenase to isolate adult worms. Embryograms were prepared with females for the evaluation of their reproductive capacities.

Principal findings: Oocyte production was not affected by IVM. The mean number of intermediate embryos (morulae and coiled mf) decreased similarly in the two groups between D0 and D80. In contrast, an accumulation of stretched mf, either viable or degenerating, was observed at D80. However, it was observed that the increase in number of degenerating mf between D0 and D80 was much lower in the frequently treated group than in the control one (Incidence Rate Ratio: 0.25; 95% CI: 0.10-0.63; p = 0.003), which may indicate a reduced sequestration of mf in the worms from the frequently treated group.

Conclusion/significance: IVM still had an embryostatic effect on O. volvulus, but the effect was reduced in the frequently treated cohort compared with the control population.

Figure 1. Distribution of female worms according to the density of oocytes in their uteri.

A semi-quantitative approach was used to classify the density of oocytes into four categories: absence, rare (less than one PSC), few (1–10 oocytes PSC) and numerous (more than 10 oocytes PSC). The density of oocytes was assessed from 15 µl of the homogenized suspension resulting from the crushing of each female worm.

Figure 2. Mean number of embryos per female worm before and 80 days after ivermectin treatment.

These data are presented separately for all female worms (a) and only for productive female worms (b) and are compared between the control and the frequently ( = multiply) treated groups. The mean number of embryos was assessed from 15 µl of the homogenized suspension resulting from the crushing of each female worm.

Figure 3. Frequency of female worms as a function of the proportion of degenerating microfilariae in their uteri.

These frequencies were plotted before and 80 days after ivermectin treatment for the control and frequently ( = multiply) treated groups. The proportion of degenerating microfilariae was assessed from 15 µl of the homogenized suspension resulting from the crushing of each female worm.