Loa loa worm, computer illustration. The Loa loa worm is a parasitic nematode that lives under human skin, causing a form of filariasis called loiasis. The worms are spread by bloodsucking Chrysops flies in tropical Africa, and reach 3-7cm when mature. They can sometimes be seen migrating across the surface of the eyes, where they cause irritation but do no serious harm. The symptoms of loiasis are swollen, painful and itchy areas of skin called calabar swellings. As in this case, diagnosis may be made by taking a blood sample. Illustration shows diagnostic morphological features of Loa loa, it is sheathed, and has nuclei extending to the tip of the tail.