Our study has been designed to establish the proof of concept that long-lasting formulation of ivermectin using the BEPO® technology could allow the release, in a single injection, of mosquitocidal plasma concentrations of ivermectin in order to kill malaria vector mosquitoes during at least 6 months. This significant and sustained and the associated logistical advantages for mass administration, open up new perspectives for the One-Health approach and the fight against malaria in different field contexts [34].
For the two tested formulations, intra- and inter-animal variability in the pharmacokinetic profiles were observed. Fluctuation in ivermectin concentration has been observed as well in other studies [28] and may be inherent to the BEPO® formulation designs. In addition, physiological factors, like body mass index, inherent to each individual animal, may carry some degree of variability. The body fat in cattle, where ivermectin is accumulated, could act as a reservoir from which ivermectin is released in function of each individual metabolism, which differs between hosts [35]. Such hypothesis has been raised by Ouédraogo et al. [32] to explain ivermectin concentration differences between treated males and females in humans. In the same idea, species specific basal metabolic rates explain differences in ivermectin concentration profiles over time and its efficacy against parasites [36]. Recently, Tipthara et al. [37] showed that ivermectin metabolization by humans produces compounds that may extend the insecticidal action of the core molecule. Hence, metabolites differences, either quantitative or qualitative, are likely to be found as well between species and from an individual to another, and may impact efficacy.
We showed that for specific ranges of ivermectin concentrations, identical amounts seem to induce different mosquito mortality rates depending on which formulation is used. This was not expected and could be due to the formulation composition, or to the injection process. Moreover, even with the random effect integrated in the model, such difference should be taken with caution because outcomes might be influenced by the limited number of calves that we used per experimental arm. This is a true limit for characterizing the overall profile of ivermectin release, but not to conclude on the great potential of such formulation for malaria vector control. Further studies to test the long-acting ivermectin formulations, incorporating more animals per group, are definitely needed to understand as accurately as possible the PK profiles and efficacy variability according to the formulation composition and dosage.
A previous study attempted to use the same BEPO® technology to fight natural infections of zebus by Onchocerca ochengi [28]. It allowed a year-round release of microfilaricidal ivermectin concentrations. Both formulations in our study reach these concentrations as well, and as such the use of long-lasting formulations could have additional benefits and support the fight against other endo- or exo-parasitic diseases of animals, including zoonoses that are transmitted to humans [38].
With plasma concentrations reaching at least the LC50 for Anopheles species during 6 months, our formulations have the potential to help circumventing the technical and logistical gaps identified by the WHO for the use of ivermectin for mass administration to human or animals ([9]), For a One-Health approach, administering ivermectin to animals is not expected to be beneficial in all field contexts. Suitability scores computed by Imbahale et al. [39] identified the Sahelian zone as the region which would benefit the most from this approach, especially due to the zoophagic behavior of Anopheles arabiensis, one of the main Plasmodium vectors of this area.
To evaluate the potential impact of our formulations in the West African context where humans and livestock often live in close vicinity, we used a modelling approach to assess the efficacy of our formulation at killing epidemiologically relevant mosquitoes in complement to bed nets, if the cattle herds of a village are all treated. We tested different reality-based field scenarios, where the product efficacy appears to be mainly a function of the proportion of vectors that actually fed on cattle vs humans. Our model is not dynamic and mosquitoes that fall in a feeding compartment (human or cattle) will remain on it whether they are able to successfully complete blood meals or not. This is simplistic as in the field, only a proportion of the anthropophagic mosquitoes is killed by LLINs, and some are successful despite the control measures (resistant mosquitoes). Moreover, because humans are protected by the bed nets, another proportion of mosquitoes follow an uncharacterized behavior pattern and may take blood meals on alternative hosts like calves [40]. These mosquitoes would display lower Human Blood Index (HBI) values despite their innate anthropophagic preference. Our model may underestimate the formulations efficacy because it doesn’t consider these mosquitoes with opportunistic feeding behavior that change from a feeding compartment to another. Examples of such mosquitoes in western sub-Saharan Africa include An. coluzzii and An. gambiae s.s. which are the main vectors of Plasmodium. In some villages, more than 50% of An. coluzzii and An. gambiae s.s. mosquitoes could take their blood meals on animals [41], predominantly cattle, despite clear human preference (reaching 0.8–0.9 in olfactometer based lab experiments) [42]. This has been associated with a high ratio of humans protected against mosquito bites by bed net use, which diverts mosquitoes from their otherwise preferred host toward alternative ones. Taking this into account, mass treatments of cattle using our formulation should be valuable also in areas of high concern, where LLIN coverage is high, and where anthropophagic mosquitoes display resistant phenotypes that carry residual transmission. The One-health approach will capitalize on the zoophagic behavior of Plasmodium vectors whether this behavior is innate or induced by the environment. For a given area and a given time during the transmission season, evaluating the efficacy of a mass administration of ivermectin using a BEPO® formulation will necessitate beforehand assessment of the blood meals ingested by Anopheles mosquitoes. This would help compose the optimal scenario of mass treatment of an animal species versus another, depending on the frequency of blood meals taken on each.
Another slow-release subcutaneously implantable ivermectin formulation was designed and tested against Anopheles by Chaccour et al. in cattle and rabbits [43, 44]. Its release duration (6 months) and ivermectin plasma concentrations were equivalent to ours. However, the solid units required surgical incisions to be implanted in animals and retrieved.
Among all the formulations that have been tested so far, ours offers the unique advantage of being injected in a single shot and of resorbing progressively while releasing active concentrations of ivermectin for a duration that could largely encompass the Plasmodium transmission season. The LC50 values found in our study were in the same range of previous in vivo experiments reports [32, 45, 46], and are compatible with ivermectin therapeutic effects for the treatment of parasitic diseases in humans and animals [38]. For these reasons, the concept of an injectable long-acting formulation of ivermectin using BEPO® technology appears to be an effective to combat Malaria transmission through a mass administration scenario. Indeed, multiple dosing programs seem hardly compatible with field logistical requirements.
Whatever the formulation, administering ivermectin to animals (and/or humans) will require further studies to consider the risk of ivermectin resistance in the populations of helminths that are classically treated using ivermectin [47], but also, in Anopheles populations [46]. The selection pressure exerted by ivermectin will dramatically increase if the approach is deployed on a mass scale, and the emergence of ivermectin resistance would likely be only a matter of time, as was the case for all other insecticidal compounds widely used to date. Ideally, mitigation strategies based on careful monitoring of existing or future markers should be proposed and implemented together with the approach, in all targeted field contexts. The threat of selecting the most anthropophagous mosquitoes by targeting peridomestic animals and zoophagic behaviors has to be assessed as well. Moreover, mitigation strategies should also explore the environmental toxicity issues that would ensue with the release of ivermectin in the excrements of treated hosts. These could have potential dramatic consequences on non-targeted terrestrial or aquatic fauna, especially for numerous species of dung degrading insects that are crucial for soil fertilization [48]. In addition, ivermectin has been reported to be phytotoxic as well [49]. Today, the need for environmentally sustainable solutions must be borne in mind, even if ivermectin seems to be a promising solution to increase the health of both humans and animals by killing malaria mosquitoes and parasites responsible for human and livestock diseases that impair local development. Ongoing studies on already marketed BEPO® ivermectin formulations attempt to measure the ivermectin amounts released in the feces of treated cattle to evaluate toxicity on reference non-targeted coprophagic species. These studies should give initial findings on the associated risks in the fields [50]. Treatments and mitigation measures should be defined and developed with the help and expertise of local herders and peasants, which would support their full participation to the overall approach.
Aside from Plasmodium-bearing mosquitoes, other insects of medical and veterinary importance are sensitive to ivermectin as well [38]. Novel ivermectin formulations like the one in our study could serve as broader control tools by targeting other neglected tropical diseases of zoonotic origin. In theory, this innovative tool could definitely be integrated within a One-Health approach, with benefits for both animals and humans. Because resistance risks are potent, the use of other endectocides is a possible mitigation strategy to address ivermectin resistance appearance in both helminths and Anopheles.