A number of species of Gnathostoma, which are parasitic nematodes of carnivores, can occasionally infect humans. Most cases have been seen in Southeast Asia, but the disease has also been described in many other areas. Eggs shed in the feces of mammals are ingested by marine crustaceans, which are then ingested by fish, frogs, snakes, or mammals. Larvae then encyst in muscles. Human infection follows eating undercooked fish, shellfish, chicken, or pork and can also be transmitted by ingesting copepods in contaminated water. After ingestion, larvae cannot complete development in humans, but rather migrate through tissues (eFigure 35–43).
Life cycle of Gnathostoma spinigerum. In the natural definitive host (pigs, cats, dogs, wild animals), the adult worms reside in a tumor that they induce in the gastric wall. They deposit eggs that are unembryonated when passed in the feces . Eggs become embryonated in water, and eggs release first-stage larvae. If ingested by a small crustacean (Cyclops, first intermediate host), the first-stage larvae develop into second-stage larvae. Following ingestion of the Cyclops by a fish, frog, or snake (second intermediate host), the second-stage larvae migrate into the flesh and develop into third-stage larvae. When the second intermediate host is ingested by a definitive host, the third-stage larvae develop into adult parasites in the stomach wall. Alternatively, the second intermediate host may be ingested by the paratenic host (animals such as birds, snakes, and frogs) in which the third-stage larvae do not develop further but remain infective to the next predator. Humans become infected by eating undercooked fish or poultry containing third-stage larvae, or reportedly by drinking water containing infective second-stage larvae in Cyclops. (Adapted from a drawing provided by Dr. Sylvia Paz Díaz Camacho, Universidade Autónoma de Sinaloa, Mexico. Content source: Global Health, Division of Parasitic Diseases and Malaria, CDC.)
Acute gastrointestinal symptoms, including nausea, vomiting, abdominal pain, and fever, may develop soon after infection and persist for 2–3 weeks. The disease may then progress to findings consistent with cutaneous or visceral larva migrans. Migratory subcutaneous erythematous swellings may be painful or pruritic. Migrating larvae may also invade other tissues, leading to findings in the eyes, lungs, intestines, and elsewhere (eFigure 35–44). The most serious complications are due to invasion of the CNS, leading to eosinophilic meningoencephalitis and other serious findings. Although a less common cause of eosinophilic meningitis than A cantonensis infection, gnathostomiasis tends to be more severe. Severe pain due to migration through spinal roots and focal neurologic findings may be seen. Symptoms are highly variable over time. CSF eosinophilic pleocytosis and peripheral eosinophilia are seen.
Gnathostomiasis of the right eyelid. (Reproduced, with permission, from Goldsmith R, Heyneman D [editors]. Tropical Medicine and Parasitology. Originally published by Appleton & Lange. Copyright © 1989 by The McGraw-Hill Companies, Inc.)