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Onchocerca volvulus parasitic worm, computer illustration. O. volvulus is the causative agent of onchocerciasis or river blindness (Onchocerca worm infestation). An infestation begins when a host is bitten by a black fly (Simulium sp.) carrying the worm larvae. One or more adult worms develop at the site of the bite and a nodule of fibrous tissue forms around them due to the host's immune response. An adult worm produces thousands of microfilariae, which are taken up by black flies to continue transmission of the parasite. Illustration shows diagnostic morphological features of O. volvulus, there is no sheath and no nuclei in the tip of the tail.