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Transmission of Gnathostomiasis

            Gnathostomiasis transmission depends on the acquisition of third-stage larval worms by humans (Crowley and Kim 1995). Consumption of undercooked or raw fish, poultry, or pork is the primary mode of infection. Rare cases have shown that third-stage larval worms can penetrate skin and occupy hosts by this mechanisms (Rusnak and Lucey 1993). Prenatal transmission of gnathostomiasis is also possible and can lead to significant birth defects (Ibid). Although ingestion is the most common way people acquire gnathostomiasis, it is not the only way to develop the parasitic disease.

Click on picture for enlargement.

            The transmission of gnathostomiasis is intricately tied to the nematode's life cycle. Gnathostoma are not able to fully mature in humans, but certain other mammals are necessary for them to complete their life cycle (Taniguchi et al. 1991). G. spinigerum is the best understood species of Gnathostoma. Its definitive hosts are dogs, wild and domestic cats, tigers, leopards, lions, minks, opossums, raccoons, and otters (Rusnak and Lucey 1993). These organisms harbor adult worms in the walls of their stomachs that release eggs into their digestive tracts. These eggs are released in the animal's feces but require water to develop further. Copepods (Cyclops) living in freshwater ingest the first-stage larva and serve as first intermediate hosts. Within the gut of Cyclops, the larva develop to their early-third-stage. These copepods may be eaten by second intermediate hosts or directly by definitive hosts. A wide variety of organisms, including fish, birds, and mammals, serve as second intermediate hosts to Gnathostoma. The worms lodge into the gastric wall only in definitive hosts; in second intermediate hosts and humans, they lodge into other tissues and do not develop to their adult forms (Ibid).

A diagram of worms encysted into the gastric wall of a definitive host. Eggs are released 

from the pore of the cyst (the arrow) (Rusnak and Lucey 1993).

            Humans can acquire larval worms by ingesting definitive hosts (though these animals are not commonly eaten in most areas of the world), second intermediate hosts, or first intermediate hosts. In Mexico, gnathostomasis incidence is strongly connected to a famous traditional dish called ceviche (Ogata et al. 1998).  Since humans are "dead-end hosts," the larvae are unable to mature to adulthood. Thus, they migrate throughout the body and can encyst in any tissue (Taniguchi et al. 1991). This, in part, explains both why gnathostomiasis is an uncommon parasitic disease and also why there is tremendous breadth in the types of tissues affected by it in clinical cases.

Click here for a list of the many second intermediate hosts of G. spinigerum.

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