Development of a preliminary in vitro drug screening assay based on a newly established culturing system for pre-adult fifth-stage Onchocerca volvulus worms

Abstract

BackgroundThe human filarial parasite Onchocerca volvulus is the causative agent of onchocerciasis (river blindness). It causes blindness in 270,000 individuals with an additional 6.5 million suffering from severe skin pathologies. Current international control programs focus on the reduction of microfilaridermia by annually administering ivermectin for more than 20 years with the ultimate goal of blocking of transmission. The adult worms of O. volvulus can live within nodules for over 15 years and actively release microfilariae for the majority of their lifespan. Therefore, protracted treatment courses of ivermectin are required to block transmission and eventually eliminate the disease. To shorten the time to elimination of this disease, drugs that successfully target macrofilariae (adult parasites) are needed. Unfortunately, there is no small animal model for the infection that could be used for discovery and screening of drugs against adult O. volvulus parasites. Here, we present an in vitro culturing system that supports the growth and development of O. volvulus young adult worms from the third-stage (L3) infective stage.Methodology/Principal findingsIn this study we optimized the culturing system by testing several monolayer cell lines to support worm growth and development. We have shown that the optimized culturing system allows for the growth of the L3 worms to L5 and that the L5 mature into young adult worms. Moreover, these young O. volvulus worms were used in preliminary assays to test putative macrofilaricidal drugs and FDA-approved repurposed drugs.ConclusionThe culture system we have established for O. volvulus young adult worms offers a promising new platform to advance drug discovery against the human filarial parasite, O. volvulus and thus supports the continuous pursuit for effective macrofilaricidal drugs. However, this in vitro culturing system will have to be further validated for reproducibility before it can be rolled out as a drug screen for decision making in macrofilaricide drug development programs.

Individuals living in an onchocerciasis focus and treated three-monthly with ivermectin develop fewer new onchocercal nodules than individuals treated annually

“…The time of the final molt was evaluated by following up the appearance of antibodies, which were assumed to be stage-specific, in a simian model, and by an in vitro culturing system. It appears that, for O. volvulus, the L3-L4 molt starts 2-3 days after inoculation [11], that the final molt occurs between 1.5 and 2.5 months [12], 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 [13]. Thus, the lifespan of the immature adult worms would range between five months (7.5 minus 2.5) and 9.5 months (11 minus 1.5).…”

Onchocerca jakutensis ocular infection in Poland: a new vector-borne human health risk?

“…Therefore, molecular identification is necessary to establish the correct diagnosis. In ocular onchocerciasis, surgical removal of the worm decreases eye complications [18].…”

Subjects living in an onchocerciasis focus and treated 3-monthly with ivermectin develop fewer new onchocercal nodules than subjects treated annually

“…The time of the final molt was evaluated by following up the appearance of antibodies, which were assumed to be stage-specific, in a simian model, and by an in vitro culturing system. It appears that, for O. volvulus, the L3-L4 molt starts 2-3 days after inoculation [11], that the final molt occurs between 1.5 and 2.5 months [12], 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 [13]. Thus, the lifespan of the immature adult worms would range between 5 months (7.5 minus 2.5) and 9.5 months (11 minus 1.5).…”