Gnathostoma spp. are spirurid nematodes characterized by the presence of a prominent cephalic bulb and body spines, and are typically associated with carnivorous mammal definitive hosts. Humans are accidental hosts; the only forms found in humans are larvae or immature adults that never reach reproductive maturity. Most human infections are caused by G. spinigerum; other species confirmed to be zoonotic include G. hispidum, G. doloresi, G. binucleatum, and G. nipponicum. Two unconfirmed human cases of G. malaysiae infection have been reported from Myanmar.
In definitive hosts, adult worms of most Gnathostoma spp reside in a tumor-like mass in the gastric wall; adult worms of some species are found in the esophagus or kidney. Adults mate and produce unembryonated eggs, which pass through a small opening in the tumor-like mass and ultimately into the feces . Eggs become embryonated in water, and eggs release sheathed first-stage larvae (L1) . Freshwater copepods, which serve as first intermediate hosts, ingest the free-swimming L1, and the larvae molt twice to become early third-stage larvae (EL3) . Following ingestion of the copepod by a suitable second intermediate host, the EL3 migrate into the tissues of the host and develop further into advanced L3 larvae (AL3) . When the second intermediate host is ingested by a definitive host, the AL3 develop into adult parasites in the gastric wall . Alternatively, the second intermediate host may be ingested by a paratenic host, in which the AL3 do not develop further but remain infective . Humans become infected by eating raw or undercooked meat of second intermediate or paratenic hosts containing AL3. In the human host, AL3 migrate in various tissues and may develop into immature adults but never achieve reproductive maturity; they may range in size from 2 mm to about 2 cm depending on the species and the extent of development . Whether humans can become infected by drinking water that contains infected copepods is not clear.
Life cycle image and information courtesy of DPDx.