An infection of the subcutaneous and deeper tissues by a large nematode. A blister appears, usually on a lower extremity (especially the foot) when the gravid, 60–100 cm long adult female worm is ready to discharge its larvae. Burning and itching of the skin in the area of the lesion, and frequently fever, nausea, vomiting, diarrhea, dyspnea, generalized urticaria and eosinophilia, may accompany or precede vesicle formation. After the vesicle ruptures, the worm discharges larvae whenever the infected part is immersed in fresh water. The prognosis is good unless bacterial infection of the lesion occurs; such secondary infections may produce arthritis, synovitis, ankylosis and contractures of the involved limb, and may be life-threatening. Tetanus infections may occur via the site of the lesion.

Diagnosis is made by visual recognition of the adult worm protruding from a skin lesion, or by microscopic identification of larvae.