Gnathostoma sp. nematode parasites are found in carnivores all over the world. Occasionally they infect humans — the most common species in humans is Gnathostoma spinigerum, a parasite native to Southeast Asia and Japan. Recently a few cases have been reported from southern Africa.
The Life Cycle of Gnathostoma spinigerum
The life cycle of G. spinigerum is very similar to that of the anisakid nematodes of marine mammals except that the definitive host (the host in which the adult worm resides) is a land mammal:
- Carnivores harbour the adult worms in the stomach tissue. Eggs released into stomach contents are carried out in the feces.
- When eggs enter fresh water, larvae develop and hatch, becoming free-swimming organisms and part of the food available to copepods.
- Ingested by a copepod — the first intermediate host — a larva continues to develop and is infective for fish, amphibians, reptiles, birds and other animals.
- When an infected copepod is eaten by a fish — the second intermediate host — the larva is freed. It migrates into the muscle tissue of the fish where it undergoes final larval development.
- Maturation to adult worms depends on the definitive host eating the fish and the dormant larva along with it. Freed from the fish’s tissue, larvae penetrate the stomach wall, mature, and mate, releasing eggs that are passed in the feces.
Humans usually acquire this parasite by eating raw fish, frogs, snakes, domestic fowl or other meat containing the larvae; however, it may also be possible to catch it by swallowing the infected copepod in unfiltered water, or through larval invasion of the skin while handling raw meat and fish.
Symptoms of Gnathostoma Infection – Larva Migrans
Gnathostoma infection, or gnathostomiasis, is a zoonosis — a disease of animals accidentally occurring in a human. Unable to mature to an adult worm in the wrong host, the larvae are doomed to wander in the human body with alarming and sometimes fatal consequences for the host. When parasitic larvae move about the condition is called larva migrans — cutaneous larva migrans if the worm larva is moving in the skin, visceral larva migrans if the parasite moves through internal organs. Gnathostoma spinigerum is usually a skin parasite in humans but it can cause visceral larval migrans as well:
- There may be abdominal pain as well as vomiting and a loss of appetite as the larva breaks through the stomach or intestinal wall.
- The first visible sign of Gnathostoma infection is often a red raised itchy, and often painful lump somewhere on the body.
- Typically, the lump moves, leaving an inflamed track behind it, or disappears only to reappear somewhere else.
- The larva may move about randomly, visiting the chest, back, limbs, face, and sometimes submerging to travel through the internal organs.
- Occasionally, gnathostomiasis resolves suddenly with the larva exiting the body through the skin.
- Gnathostoma infection can be fatal if the larva damages vital organs or invades the brain. Symptoms of nervous system damage include seizures, paralysis, and coma.
In humans gnathostomiasis is primarily a parasitic disease of those who eat raw or undercooked fish and meat. The majority of infections have been acquired in Southeast Asia and Japan. Infections caused by Gnathostoma sp. have also been reported from South America, where people enjoy the raw fish dish ceviche.
Treatment of Gnathostoma Infection
Once treatable only with surgical removal of the larva, this infection has occasionally been treated successfully with drugs. Authorities now recommend the same modern antiparasitic drugs used for other parasitic nematodes such as hookworm and Onchocerca volvulus, the cause of river blindness.
Sources
“Drugs for Parasitic Infections.” Treatment Guidelines from The Medical Letter. Vol 5(Suppl), 2007.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
“Gnathostomisasis Acquired by British Tourists in Botswana.” Herman, Joanna S., Emma C. Wall, Christoffer van Tulleken et al. Emerging Infectious Diseases 2009; 15(4): 594-597.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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