Onchocerciasis is caused by the filarial nematode Onchocerca volvulus and occurs mainly in sub-Saharan Africa with approximately 21 million people being infected.
The disease is transmitted by blood-feeding black flies (Simulium), which transmit the infective larvae. As the transmitting Simulium flies only occur along fast-flowing rivers, the disease is restricted to those areas and is also called river blindness. Adult worms reside in subcutaneous nodules and release their progeny, the microfilariae, which are also found in the subcutaneous tissue. The death of microfilariae, either drug-induced or naturally over time, can lead to severe dermatitis. Furthermore, microfilariae may also migrate through the eye and their death can lead to keratitis, vision loss and blindness. Approximately 30-50% of onchocerciasis patients suffer from dermatitis and almost one million people have onchocerciasis-caused vision loss.
Due to the risk of developing blindness following microfilariae-killing DEC treatment, the triple therapy (ivermectin, DEC and albendazole) used for mass drug administration against lymphatic filariasis is not given in areas co-endemic for onchocerciasis. The current ivermectin treatment used for mass drug administration against onchocerciasis does not kill the adult worms and has to be given once to twice per year for the reproductive live span of the adult filariae (up to 15 years). Anti-Wolbachial therapy targeting the filarial Wolbachia endosymbionts with doxycycline can kill the adult filariae safely and is recommended by the WHO for individual therapy, but 5- to 6-week-long daily treatment and contraindications for children, pregnant and breast-feeding women prevent its broader use. A specific macrofilaricidal drug such as oxfendazole that allows short treatment regimens would facilitate the elimination of onchocerciasis.