Gnathostoma FAQs


What is Gnathostoma?

Gnathostoma spp. are a group of parasitic nematodes that must pass through a series of animal hosts in order to complete their life cycle, though they sometimes infect other non-required hosts, such as humans, in which they can survive but not mature into adults. Depending on the species, adult Gnathostoma spp. can reproduce in dogs, cats, raccoons, opossums, tigers, leopards, pigs, wild boars, and weasels—these animals are called definitive hosts. The parasites’ eggs are released in the stool of the definitive host and hatch in freshwater. Larvae in freshwater are eaten by water fleas (also known as copepods), which are in turn eaten by other animals, such as freshwater fish, eels, frogs, birds, and reptiles. If humans eat raw or undercooked meat from one of these infected animals, they too can become infected, but the larvae of the parasite cannot mature the adult form. If definitive hosts eat one of these infected animals, they can become infected and the larvae can mature into the adult form and complete the life cycle.

Where is Gnathostoma found?

Gnathostoma spp. are found throughout the world, but have been reported in humans primarily in tropical and subtropical areas. Gnathostomiasis is most commonly diagnosed in Asia, particularly in Thailand, other parts of Southeast Asia, and Japan. The parasite has also been found in other areas, including South and Central America and Africa, and the diagnosis is increasingly recognized in these areas.

How do people become infected with Gnathostoma?

People become infected most commonly by eating undercooked and raw infected freshwater fish, though they can also become infected by eating raw or undercooked infected freshwater eels, frogs, birds, and reptiles. People might also become infected by swallowing infected water fleas or through exposure to the parasite while handling uncooked tissue from animals that are infected, though this has not been proven. Of note, it is felt that the parasite is not transmitted by eating sushi in the U.S. and Western Europe because typically the more expensive saltwater fish are used. Also there are more stringent regulations in those areas on sourcing and storage of fish that are to be eaten by humans.

What are the signs and symptoms of gnathostomiasis?

The symptoms of gnathostomiasis are thought to be related to the movement of the parasite through the body. When someone eats the parasite, it moves through the wall of the stomach or intestine and liver. During this early phase, many people have no symptoms or they may experience fever, excess tiredness, lack of appetite, nausea, vomiting, diarrhea, or abdominal pain. This phase may last for 2 or 3 weeks. Later, when the parasite moves under the skin, people may experience swellings under the skin that may be painful, red, or itchy. The swellings move around and typically are not pitting, which means that if you push on the swelling with a finger an indentation is not left behind. The swellings often begin within 3 to 4 weeks after ingestion of the parasite, but they can occur up to around 10 years after infection. The swellings typically last several weeks at a time. Rarely, Gnathostoma can enter other parts of the body, including the lungs, bladder, eyes, ears, and nervous system, including the brain. If the parasite enters the eye, it can result in vision loss or blindness. If the parasite enters a nerve or the spine, it usually results in severe nerve pain, followed by paralysis of the muscle controlled by the affected nerve. If the parasite enters the brain, it can result in headache, decreased consciousness, coma, and death. People who have a parasite that moves under the skin on the face are at higher risk for invasion of the brain and/or eye.

What should I do if I think I have gnathostomiasis?

See your doctor for appropriate evaluation and testing. Make sure you let him/her know if you have eaten any raw or undercooked freshwater fish, eels, frogs, reptiles or birds and if you have traveled to any of the areas where the parasite is commonly found.

How is gnathostomiasis diagnosed?

The diagnosis should be considered in someone who has swellings under the skin that move around the body and an elevated level of eosinophils in the blood. The patient should be questioned about any recent history of eating undercooked or raw freshwater fish, eels, frogs, birds, or reptiles in an area where the parasite is found. Special blood tests for Gnathostoma are available, but the blood must be sent to specialty labs outside the United States.

Is medication available to treat gnathostomiasis?

There are 2 antiparasitic medications that have been used successfully in patients with gnathostomiasis affecting the skin. The medications are albendazole and ivermectin. See your doctor for appropriate diagnosis and treatment.

How can I prevent gnathostomiasis?

Don’t eat undercooked or raw freshwater fish, eels, frogs, birds, and reptiles, particularly if you are in an area of the world where the parasite is commonly found. Marinating freshwater fish in lime juice, as is done in ceviche, does not kill the parasite. Avoiding contaminated freshwater in areas where the parasite is commonly found, washing your hands with soap and warm water before and after preparing food, or wearing gloves when handling raw tissue from animals that might be infected may also reduce your risk of infection, though none of these has be proven to be effective.

The FDA recommends the following for fish preparation or storage to kill parasites.

  • Cooking
    • Cook fish adequately (to an internal temperature of at least 145° F [~63° C]).
  • Freezing (Fish)
    • At -4°F (-20°C) or below for 7 days (total time), or
    • At -31°F (-35°C) or below until solid, and storing at -31°F (-35°C) or below for 15 hours, or
    • At -31°F (-35°C) or below until solid and storing at -4°F (-20°C) or below for 24 hours.

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Can I infect others?

No, the parasite that causes gnathostomiasis is not spread from person to person.

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This information is not meant to be used for self-diagnosis or as a substitute for consultation with a health care provider. If you have any questions about the parasites described above or think that you may have a parasitic infection, consult a health care provider.

Page last reviewed: September 17, 2020