Gnathostoma spp. are a group of parasitic worms or nematodes. They pass through a series of animal hosts to complete their lifecycle.

Lifecycle

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 definitive host will release the parasites' eggs in their stool (poop) where they will hatch in freshwater. Copepods (also known as tiny "water fleas"), then eat the larvae in freshwater. In turn, animals such as freshwater fish, eels, frogs, birds, and reptiles eat the copepods. 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.

If humans eat raw or undercooked meat from one of these infected animals, they too can become infected. However, the larvae of the parasite cannot mature the adult form.

You can get Gnathostoma infection by eating undercooked or raw infected freshwater fish. You can also become infected by eating raw or undercooked infected freshwater eels, frogs, birds, and reptiles.

People might also become infected by swallowing infected copepods or through exposure to the parasite while handling uncooked tissue from animals that are infected, though this is unproven.

You can find Gnathostoma spp. throughout the world. However, infections are most commonly diagnosed in Asia, particularly Thailand, other parts of Southeast Asia, and Japan. People have reported cases in other areas, including South and Central America and Africa.

Experts believe people do not get the parasite by eating sushi in the U.S. and Western Europe. This is because there are more stringent regulations in these areas on sourcing and storage of fish eaten by humans.

Experts think the symptoms of gnathostomiasis are 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 two or three 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. This means that if you push on the swelling with a finger, you will not see an indentation left behind. The swellings often begin within three to four 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.

See your healthcare provider if you have any of the symptoms of Gnathostoma infection. Only specialty labs located outside of the United States can test your blood for Gnathostoma infection.

There are safe and effective medications to treat Gnathostoma infection. Talk to your healthcare provider about treatment.

For cutaneous symptoms of Gnathostoma infection, both albendazole and ivermectin have been shown to result in cure in several trials that were too small to firmly establish efficacy and safety of treatment.

Reported cure rates at 6 months after treatment with albendazole are >90% and after treatment with ivermectin range from 76–95.2%. Albendazole may cause outward migration of larvae. Ivermectin may cause a temporary increase of cutaneous symptoms. Two small studies in which patients were followed up for 1 year or longer found cure rates after treatment with albendazole decreased over time. Relapse is not uncommon, however, with either treatment and has been shown to occur up to 26 months after initial therapy. Monitoring for symptom recurrence is needed for all patients regardless of treatment regimen. Relapse does not necessarily require treatment with a different medication, though data on this issue also are limited.

Two different regimens of albendazole (400 mg daily for 21 days and 400 mg twice daily for 21 days) and two different regimens of ivermectin (200 mcg/kg once daily for 1 day and 200 mcg/kg once daily for 2 days) have been studied. Data are insufficient to determine which regimen is the most effective, so it would probably be prudent to use the higher dose regimen of either medication until better data are available.

Whether to treat ocular and central nervous system (CNS) Gnathostoma infection remains controversial, particularly as there are no published studies of the efficacy of albendazole or ivermectin. As albendazole may cause larvae to migrate and ivermectin may cause a disease flare, there is concern that treatment with antihelminthics could worsen a patient's neurologic status and possibly increase the risk for death or permanent neurologic deficit. There has been only one observational study of corticosteroids in patients presenting with probable Gnathostoma infection. No benefit of corticosteroids was demonstrated, possibly because it is thought that much of the damage to the CNS is caused by mechanical destruction of tissue.

At this time it is not possible to give recommendations for the treatment of CNS or ocular disease other than to provide supportive care. As gnathostomiasis has been shown to cause intracranial hemorrhage, patients with neurologic disease should be carefully monitored and plans for prompt intervention should be put into place.

Care precautions

Albendazole

Treatment in Pregnancy

Albendazole is a pregnancy category C drug. There are limited data on the use of albendazole in pregnant women. The available evidence suggests no difference in congenital abnormalities in the children of women accidentally treated with albendazole during mass drug administration (MDA) campaigns compared with those who were not. In MDA campaigns for which the World Health Organization (WHO) has determined that the benefits of treatment outweigh the risks, WHO allows use of albendazole in the 2nd and 3rd trimesters of pregnancy. However, healthcare providers should balance the risks of treatment for the fetus with the risks of disease progression in the woman in the absence of treatment.

Pregnancy Category C: Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal, or other) plus there are no controlled studies in women, or studies in women and animals are not available. Prescribe albendazole only if the potential benefits to the woman justify the potential risks to the fetus.

Treatment during lactation

Albendazole is minimally excreted in human milk. WHO has concluded that a single oral dose of albendazole can be given to lactating women.

Treatment in pediatric patients

The safety of albendazole in children less than 6 years old is not certain. Studies of the use of albendazole in children as young as one year old suggest that it is safe. According to WHO guidelines for MDA campaigns, children as young as one year of age (able to safely swallow tablets) can take albendazole. These campaigns have treated many children under six years old with albendazole, albeit at a reduced dose.

Ivermectin

Treatment in Pregnancy

Ivermectin is a pregnancy category C drug. Data on the use of ivermectin in pregnant women are limited, though the available evidence suggests no difference in congenital abnormalities in the children of women who were accidentally treated during mass drug administration (MDA) campaigns with ivermectin compared with those who were not. The World Health Organization (WHO) excludes pregnant women from MDA campaigns that use ivermectin. In a pregnant woman infected with a soil-transmitted helminth, balance the risks of treatment for the fetus with the risks of disease progression in the woman in the absence of treatment.

Pregnancy Category C: Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal, or other) plus there are no controlled studies in women, or studies in women and animals are not available. Prescribe ivermectin only if the potential benefits in the woman justify the potential risks to the fetus.

Treatment during lactation

Ivermectin is excreted in low concentrations in human milk. Treat breastfeeding women with ivermectin only when the risks of disease progression in the untreated mother outweigh the risks to the infant.

Treatment in pediatric patients

The safe use of ivermectin in children weighing less than 15 kg (33 lbs) is unclear. According to the WHO guidelines for MDA campaigns, ivermectin can safely treat children at least 90 cm (35 in) tall. The WHO growth standard curves show that 50% of boys reach this height by age 28 months and 50% of girls by age 30 months. Some MDA campaigns use ivermectin to treat younger children who are able to swallow tablets safely, albeit at a reduced dose.

The best way to not get gnathostomiasis is to not eat undercooked or raw freshwater fish, eels, frogs, birds, and reptiles. Marinating freshwater fish in lime juice, like when making 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, and wearing gloves when handling raw tissue from animals that might be infected may also reduce your risk of infection, though none of these has proven 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.