Dynamics of <em>Onchocerca volvulus</em> Microfilarial Densities after Ivermectin Treatment in an Ivermectin-naïve and a Multiply Treated Population from Cameroon

<div><p>Background/Objective</p><p>Ivermectin has been the keystone of onchocerciasis control for the last 25 years. Sub-optimal responses to the drug have been reported in Ghanaian communities under long-term treatment. We assessed, in two Cameroonian foci, whether the microfilaricidal and/or embryostatic effects of ivermectin on <i>Onchocerca volvulus</i> have been altered after several years of drug pressure.</p> <p>Methods</p><p>We compared the dynamics of <i>O. volvulus</i> skin microfilarial densities after ivermectin treatment in two cohorts with contrasting exposure to this drug: one received repeated treatment for 13 years whereas the other had no history of large-scale treatments (referred to as controls). Microfilarial densities were assessed 15, 80 and 180 days after ivermectin in 122 multiply treated and 127 ivermectin-naïve individuals. Comparisons were adjusted for individual factors related to microfilarial density: age and number of nodules.</p> <p>Findings</p><p>Two weeks post ivermectin, microfilarial density dropped equally (98% reduction) in the ivermectin-naïve and multiply treated groups. Between 15 and 180 days post ivermectin, the proportion of individuals with skin microfilariae doubled (from 30.8% to 67.8%) in controls and quadrupled (from 19.8% to 76.9%) in multiply treated individuals but the mean densities remained low in both sites. In fact, between 15 and 80 days, the repopulation rate was significantly higher in the multiply treated individuals than in the controls but no such difference was demonstrated when extending the follow-up to 180 days. The repopulation rate by microfilariae was associated with host factors: negatively with age and positively with the number of nodules.</p> <p>Conclusion</p><p>These observations may indicate that the worms from the multi-treated area recover mf productivity earlier but would be less productive than the worms from the ivermectin-naïve area between 80 and 180 days after ivermectin. Moreover, they do not support the operation of a strong cumulative effect of repeated treatments on the fecundity of female worms as previously described.</p> </div

Ivermectin efficacy against <i>Onchocerca volvulus</i> in ivermectin-naïve (○) and multiply treated subjects (Δ).

<p>Microfilarial density was assessed before and 15, 80 and 180 days after ivermectin in two groups of subjects with different exposure to ivermectin. Bars indicate the median and inter-quartile range of microfilarial density, including zeros.</p

Post-ivermectin microfilarial densities of the 10% less densely infected subjects before treatment.

<p>Post-ivermectin microfilarial densities of the 10% less densely infected subjects before treatment.</p

Evolution of <i>Onchocerca volvulus</i> microfilarial density (mf/mg) between 15 and 80 days after ivermectin treatment.

<p>Microfilarial density at D80 is plotted against density at D15 (scatterplots), for each individual, in ivermectin-naïve and multiply treated subjects. Distribution of individual variation between 15 and 80 days post-treatment is also given (histograms) for the ivermectin-naïve and multiply treated subjects (positive difference means an increase between D15 and D80). Lines in scatterplots represent slope = 1, i.e no change in microfilarial density. X-axis below histograms indicates central values of 2 mf/mg width bins (e.g. 0 indicates microfilarial density between −1 and 1).</p

Evolution of <i>Onchocerca volvulus</i> microfilarial density (mf/mg) between 80 and 180 days after ivermectin treatment.

<p>Microfilarial density at D180 is plotted against density at D80 (scatterplots), for each individual, in ivermectin-naïve and multiply treated subjects. Distribution of individual variation between 80 and 180 days post-treatment is also given (histograms) for the ivermectin-naïve and multiply treated subjects (positive difference means an increase between D80 and D180). Lines in scatterplots represent slope = 1, i.e no change in microfilarial density. X-axis below histograms indicates central values of 2 mf/mg width bins (e.g. 0 indicates microfilarial density between −1 and 1).</p

Observed (grey bars) and predicted (empty bars) distributions of microfilarial density in multiply treated subjects.

<p>Observed (grey bars) and predicted (empty bars) distributions of microfilarial density in multiply treated subjects.</p

Observed and predicted mean microfilarial density in ivermectin naïve (diamonds) and multiply treated (squares) groups.

<p>Filled symbols represent observed means and open symbols represent zero-inflated negative binomial predictions of the mean. Bars indicate 95% confidence intervals for both observations and predictions.</p

Zero-inflated negative binomial regression of post ivermectin microfilarial density in ivermectin-naïve and multiply treated subjects.

*<p><a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0002084#s3" target="_blank">Results</a> are presented as incidence rate ratios (IRR) for the negative binomial model. For quantitative variables (e.g. age), IRR is the ratio of mean <i>O. volvulus</i> microfilarial density when explanatory factor is [x+1] to mean microfilarial density when explanatory factor is [x] (holding all other variables constant). For qualitative variables (e.g. study group) the IRR is the ratio of mean microfilarial density for that category compared to the baseline category.</p>¶<p>Baselines for categorical variables are Date of follow-up = D15 and Study group = ivermectin-naïve.</p

<i>Onchocerca volvulus</i> microfilarial densities (mf/mg) in ivermectin-naïve and multiply treated subjects before and after ivermectin.

*<p>arithmetic mean of microfilarial densities (mf/mg), including zero counts.</p>‡<p>standard deviation.</p>§<p>VMR = variance to mean ratio.</p>#<p>distributions of microfilarial densities were compared between the two groups using Kolmogorov-Smirnov test.</p

Composition of the nodules and distribution of female worms according to their uterine contents.

<p>The evaluation of the uterine content was done from 15 µl of the homogenized suspension resulting from the crushing of each female worm. We expressed the numbers of embryos using this volume (15 µl) as arbitrary unit; sd: standard deviation; mf: microfilariae; max: maximum.</p