Our study is the first to compare the effect of various IVM treatment regimens on the appearance of new onchocercal nodules in human subjects exposed to transmission of O. volvulus. It demonstrates that the mean number of palpable nodules which appeared within the 3-year period of the trial was significantly lower in the individuals treated 3-monthly with IVM than in those treated annually, and that the high doses had no higher effect than standard doses in reducing the number of new nodules.
These results should be interpreted in the light of what is known on the biology of O. volvulus within the year following the infective bite. The modalities of development of O. volvulus between the penetration of the parasite into the host under the form of L3, and the time when it is found as an adult fecund stage in a nodule are not fully known. The time of the L3-L4 molt has been assessed by in vitro studies and by infecting experimentally various animals with L3s of Onchocerca sp. The time of the final molt was evaluated by following up the appearance of antibodies which were assumed to be stage-specific. It appears that, for O. volvulus, the L3-L4 molt occurs within a week of inoculation [9], that the final molt producing immature adults occurs within 3–5 months, that the adults become sexually mature at 7.5–11 months, and that the first mf produced by the mature adult female worms can be detected after an average period of 12–15 months [10]. Thus, the lifespan of the immature adult worms would range between 2.5 months (7.5 minus 5) and 8 months (11 minus 3). Given this timeframe, one can consider that all the L4s present in an individual treated 3-monthly with IVM will be exposed once (if the L4s’ lifespan is 3 months) or twice (if it is 5 months) to the drug, and that it would probably be the same for the immature worms. Conversely, only a small proportion of L3s “inoculated” to these subjects treated every 90 days would be exposed to the drug (5.6–7.8% of the L3s if the L3-L4 molt occurs 5–7 days after the infective bite). If one assumes that the appearance (or not) of new nodules reflects the effect of the drug on the parasitic stages preceding the mature adult stage, which is debatable (see below), the observed decrease of 17.7% in the number of new nodules in the subjects treated 3-monthly suggests that the prophylactic effect of IVM is not limited to the effects of the drug on the L3s or on the L3-L4 molting process, but that IVM has also a partial effect on the L4s and/or the immature adult worms.
In this study, we assumed that the effect of a drug on the L4s and immature adults can be assessed by following up the appearance of new nodules in hosts exposed to transmission of O. volvulus. This is debatable because the sites where the L4s and the immature adults live, and the modalities by which the adult female worms are finally found in a nodule are poorly known. In particular, the extent to which immature females are attracted by existing nodules or are able to create a new nodule are not known. According to Duke et al., “it is expected … that the L4 will be highly mobile and capable (by means unknown) of locating the adult sites of election or (perhaps by means of pheromones) of finding pre-existing worm bundles; and that the immature females may continue these wanderings, …, but are then likely to settle down to form nodules of their own or to join pre-existing nodules. The possibility cannot be excluded that some of the immature females may remain dormant at a prepubertal phase, situated in the connective tissues away from the nodules” [9]. Guderian et al. compared the sites of appearance of new nodules in a group of subjects from whom all the palpable nodules had been removed and a group with no nodulectomy. They concluded that “It seems likely that young, female, unencapsulated O. volvulus are attracted to existing nodules, settle down next to them and then become encapsulated themselves”[11]. However, as it is admitted (a) that the female worms, once in a nodule, stay there, and (b) that nodules form around female worms (not male worms), one may assume that the appearance of a new nodule can occur only if new females have appeared. Therefore, the lower number of new nodules recorded in the groups treated 3-monthly, when compared to the annually-treated group, results probably from an at least partial effect of ivermectin on the L4s and/or the juvenile female worms.
Specific study designs, using probably animal models, could be developed to evaluate the strength of this prophylactic effect after a single dose of IVM, which would help refine the mathematical models used to predict the impact of IVM MDA on onchocerciasis transmission intensity. Trials could also be conducted to define which treatment frequency would be required to obtain the best prophylactic effect. We found that 3-monthly treatments led to a significant reduction in the appearance of new nodules when compared to annual treatment but the difference was not very marked. Monthly treatments would probably lead to a stronger effect, as suggested by the results of the study conducted on the O. ochengi/cattle model [2]. Such monthly treatments have been used in studies evaluating their possible macrofilaricidal effect on O. volvulus [12] or their effect on Loa loa [13]. They probably cannot be applied on a large scale, but could be proposed to individuals visiting temporarily an onchocerciasis endemic area. Unlike loiasis which can be prevented using diethylcarbamazine (DEC) [14, 15], no prophylactic drug exists for onchocerciasis. Trials using DEC were conducted on chimpanzees experimentally infected with O. volvulus and on humans by looking at the effect of the drug on L3s, but the results were not conclusive [16].
A remaining question is why the difference of impact between 3-monthly and annual treatment is higher when the initial number of nodules is higher. Before the start of this study, some participants had more nodules than others. This variability can be explained by different levels of exposure to onchocerciasis transmission, but also by inter-individual heterogeneity in immunological response. Some individuals are more predisposed than other to tolerate incoming parasites, with a weaker immune response allowing more L3s’ and L4s’ developing to the adult stage, and therefore leading to more nodules. During the 3 years of the trial, it is very unlikely that these two factors changed for the participants. Then, it makes sense that the most infected people (i.e. with the highest number of nodules) at the start of the study, are also the most infected ones at the end of the trial.
Few studies tried to evaluate the impact of IVM treatment on nodules’ disappearance. Duke et al. assessed this phenomenon by comparing patients who were given IVM at 150 µg/kg 3-monthly and untreated persons. They described a higher proportion of nodules that had disappeared in the treated group but the difference was not significant [17]. Three other studies reported that nodules can disappear after repeated doses of IVM [18–20]. We did not find a difference in the number of nodules which disappeared between the treatment arms of our study, but this could be due to a lack of statistical power due to a too small sample size. Further studies have to be conducted to determine the impact of repeated doses of IVM on the nodules’ disappearance.