Gnathostomiasis

A few days after ingestion epigastric pain, fever, vomiting, and loss of appetite resulting from migration of larvae through intestinal wall to the abdominal cavity will appear in the patient.[3] Migration in the subcutaneous tissues (under the skin) causes intermittent, migratory, painful, pruritic swellings (cutaneous larva migrans). Patches of edema appear after the above symptoms clear and are usually found on the abdomen.[3] These lesions vary in size and can be accompanied by pruritus, rash, and stabbing pain. Swellings may last for 1 to 4 weeks in a given area and then reappear in a different location.[3] Migration to other tissues (visceral larva migrans), can result in cough, hematuria, ocular (eye) involvement,[4] meningitis, encephalitis and eosinophilia. Eosinophilic myeloencephalitis may also result from invasion of the central nervous system by the larvae.[1]

Human gnathostomiasis is infection by the migrating third-stage larvae of any of five species of Gnathostoma, which is type of worm (more specifically a type of nematode). The most common cause in Asia is G. spinigerum, and the most common cause in the Americas is G. binucleatum. G. hispidium and G. doloresi occur in East and Southeast Asia; the former has also been found in Eastern Europe. G. nipponicum occurs only in Japan and China.[5][6][7] There is one unconfirmed report of G. malaysiae causing disease in humans.[8]

Life-cycle of Gnathostoma

Gnathostomiasis is transmitted by the ingestion of raw, insufficiently cooked definitive hosts such as fresh water fish, poultry, or frogs.

In Thailand and Vietnam, the most common cause appears to be consumption of undercooked Asian swamp eels (Monopterus albus, also called Fluta alba) which transmit G. spinigerum.[9][10][11][12] Monopterus albus is an invasive species in North America, but no Gnathostoma infections in humans have yet been conclusively identified in the US.[13]

The incubation period for gnathostomiasis is 3–4 weeks when the larvae begin to migrate through the subcutaneous tissue of the body.[15]

The adult parasite is reddish-brown in color and has a globular cephalic dome that is separated from the rest of the body by constriction.[15] The posterior portion of the nematode is smooth while the anterior half is covered with fine leaf-like spines.[15] The head is round and contains 4 to 8 transverse rows of hooklets that are protected by a pair of fleshy lips . The males are shorter than the females, 11–25 mm (0.43–0.98 in) compared to 25–54 mm (0.98–2.13 in) respectively.[15] Eggs are oval and have a mucoid plug at one end.[15]

Diagnosis of gnathostomiasis is possible (with microscopy) after removal of the worm. The primary form of diagnosis of gnathostomiasis is the identification of larva in the tissue.[1] Serological testing such as enzyme-linked immunosorbent assay (ELISA) or the Western blot are also reliable but may not be easily accessible in endemic areas.[1]

CT scanning or MRI can be used to help identify a soft tissue worm and when looking at CNS disease it can be used to reveal the presence of the worm.[3] The presence of haemorrhagic tracks on gradient-echo T2-weighted MRI is characteristic and possibly diagnostic.[6]

The best strategies for preventing accidental infection of humans is to educate those living in endemic areas to only consume fully cooked meat. The inability of the parasite to complete its life cycle within humans means that transmission can easily be contained by adequate preparation of meat from intermediate hosts. This is especially useful because of the difficulty and lack of feasibility inherent in eliminating all intermediate hosts of gnathostomiasis. So instead, individuals in endemic areas should avoid eating raw and undercooked meat in endemic areas, but this may be difficult in these areas. This is due to preference for dishes containing raw fish in these endemic areas.

The dish ceviche is native to Peru and a favorite of Mexico. It consists of onion, cubed fish, lime or lemon juice and Andean spices including salt and chili. The ingredients are mixed together and they are allowed to marinate several hours before being served at room temperature. Then in endemic areas in Southeast Asia there are traditional dishes associated with these areas that also include raw uncooked fish, such as koipla in Thailand, goi ca song in Vietnam, sashimi and sushi in Japan.[16]

Acknowledging these cultural traditions, individuals in these cultural can be educated on methods of adapting their food preparation activities in order to remove the larvae without greatly altering these traditional dishes. For instance, meat should be marinated in vinegar for six hours or in soy sauce for 12 hours in order to successfully kill the larvae. In areas with reliable electricity, meat can be frozen at -20 degrees Celsius for 3–5 days to achieve the same results of killing the larvae present.[14]

Surgical removal or treatment with albendazole or ivermectin is recommended. The most prescribed treatment for gnathostomiasis is surgical removal of the larvae but this is only effective when the worms are located in an accessible location.[1] In addition to surgical excision, albendazole and ivermectin have been noted in their ability to eliminate the parasite.[3] Albendazole is recommended to be administered at 400 mg daily for 21 days as an adjunct to surgical excision, while ivermectin is better tolerated as a single dose.[1] Ivermectin can also serve as a replacement for those that can't handle albendazole 200 ug/kg p.o. as a single dose.[1] However, ivermectin has been shown to be less effective than albendazole.

Endemic areas include Asia, Mexico, India and parts of South Africa.[3] Originally believed to be confined to Asia, in the 1970s gnathostomiasis was discovered in Mexico,[3] and found in Australia in 2011.[19][20] Even though it is endemic in areas of Southeast Asia and Latin America, it is an uncommon disease. However, researchers have noticed recently an increase in incidence. This disease is most common in both Thailand and Japan, but in Thailand it is responsible for most of the observed parasitic CNS infection.[14] It has long been recognised in China, but reports have only recently appeared in the English literature.[21]

The first case of Gnathostoma infection was identified by Sir Richard Owen when inspecting the stomach of a young tiger that had died at London Zoo from a ruptured aorta.[22] However it was not until 1889 that the first human case was described by Levinson when he found the Gnathostoma larva in an infested Thai woman. The lifecycle of G. spinigerum was described by Svasti Daengsvang and Chalerm Prommas from Thailand in 1933 and 1936.[23] This delay in identification of the parasite in humans is due to the fact that humans are not a definitive host for this parasite making infection from this parasite rare. Gnathostomiasis infection is rare because the parasite must be digested when it has reached its third larvae stage, providing only a short time frame in which the parasite is capable of infecting humans. It is uncommon for the larvae to penetrate the skin of individuals exposed to contaminated food or water without ingestion.[14]

• List of parasites (human)
• List of migrating cutaneous conditions

• Gnathostomiasis at CDC
• Gnathostomiasis at eMedicine