Onchocerca volvulus parasitic worms. Light micrograph of a section through a skin nodule of a patient with onchocerciasis or river blindness (Onchocerca worm infestation). A longitudinal section of an adult worm is seen across centre, surrounded by connective tissue. 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. Magnification unknown.