Genus of filariid nematodes, which are transmitted by biting midges ( Culicoides ) to horses (e.g., Onchocerca cervicalis: male 7–10 cm long, female 50–70 cm): O. reticulata (male 15–20 cm long, female 75 cm) and by simuliids to cattle (e.g., O. gutturosa: male 2.7 cm long, female 36–40 cm long, O. lienalis: male 2.5 cm long, female up to 85 cm). In humans O. volvulus occurs and is transmitted by simuliids. In all cases the unsheathed L1 microfilariae ( O. volvulus , Fig. 2, Microfilariae) occur in the skin. The vectors transmit the L3 during the biting process. The worm then reaches maturity in about 10–16 months. Nematodes, Filariidae.