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Causal Agent:

The nematode (roundworm) Gnathostoma spinigerum and Gnathostoma hispidum, whichinfects vertebrate animals. Human gnathostomiasis is due to migrating immature worms.

Life Cycle:

Adapted from a drawing provided by Dr. Sylvia Paz Díaz Camacho, Universidade Autónoma de Sinaloa, Mexico.

In the natural definitive host (pigs, cats, dogs, wild animals) the adult worms reside in atumor which 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 bya 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 ingestedby 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 eatingundercooked fish or poultry containing third-stage larvae, or reportedly by drinking water

containing infective second-stage larvae in Cyclops .

Geographic Distribution:Asia, especially Thailand and Japan; recently emerged as an important human parasite inMexico.

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Clinical Features:The clinical manifestations in human gnathostomiasis are caused by migration of theimmature worms (L3s). Migration in the subcutaneous tissues causes intermittent,migratory, painful, pruritic swellings (cutaneous larva migrans). Migration to other tissues(visceral larva migrans), can result in cough, hematuria, and ocular involvement, with themost serious manifestations eosinophilic meningitis with myeloencephalitis. Higheosinophilia is present.

Laboratory Diagnosis:Removal and identification of the worm is both diagnostic and therapeutic.

Microscopy

Treatment:Surgical removal or treatment with albendazole* or ivermectin* is recommended. For

additional information, see the recommendations in The Medical Letter (Drugs forParasitic Infections).

* This drug is approved by the FDA, but considered investigational for this purpose.

Microscopy

Humans serve only as paratenic hosts for Gnathostoma spp. Identification of gnathostomiasis is achieved by serology or microscopic observation of the larval worms intissue sections. Diagnostic characters for Gnathostoma include the presence of large lateralchords, multinucleate intestinal cells (some species), presence of pigmented granularmaterial in the intestinal cells, and the presence of spines on the cuticle, especially near the

anterior end of the worm.

A B

A: Hematoxylin and eosin (H&E) stained cross-section of Gnathostoma sp., taken from asubcutaneous nodule above the right breast of a patient, showing the esophagus. Note the

presence of cuticular spines (arrow). Image courtesy of Diagnostix Pathology LaboratoriesLTD, Canossa Hospital, Hong Kong, China.B: Another H&E-stained cross-section of Gnathostoma sp., taken of the same specimen inFigure A, showing the intestinal cells and characteristic large lateral chords (LC). Note themultinucleate intestinal cells and the presence of pigmented granular material in theintestinal cells.