Gnathostoma
Gnathostoma is a genus of parasitic nematodes with multi-host life-cycles. Successive hosts include copepods, fish and amphibians, and carnivorous and omnivorous mammals. Infection with nematodes of some Gnathostoma species causes gnathostomiasis in humans.
Hosts
Gnathostoma species have life-cycles with multiple hosts. Eggs that reach fresh water develop embryos which hatch as first-stage larvae. The larvae are eaten by first intermediate hosts, copepods. The larvae molt twice in the copepods to become early third-stage larvae. The copepods are eaten by second intermediate hosts, typically fish and amphibians, where the larvae develop into advanced third-stage larvae. Second intermediate hosts may be eaten by paratenic hosts such as reptiles or birds, where the larvae stop developing but remain infectious. If an infected secondary or paratenic host is eaten by a definitive host, typically a carnivore or omnivore, the larvae migrate to the wall of the host's stomach, form a cyst, and develop into sexually mature adults. The usual final hosts for G. binucleatum and G. spinigerum are canids and felids, for G. hispidum and G. doloresi, swine, and for G. nipponicum, weasels.As of 2008, the three species of Gnathostoma found in Mexico, G. binucleatum, G. lamothei, and G. turgidum, were known to parasitize a total of 80 species of vertebrates, including 39 species of fish, 19 species of birds, eight species of reptiles and mammals, and six species of amphibians. Most host species are parasitized by only one species of Gnathostoma, but larvae of G. turgidum and G. binucleatum have both been found in a few species, including the fish Oreochromis aureus and Gobiomorus dormitor and the turtle Kinosternon integrum. Gnathostoma adults are more restricted in that only mammals serve as primary hosts. Adults of G. lamothei are found only in members of the family Procyonidae and those of G. turgidum are found only in members the family Didelphidae, while adults of G. binucleatum have been found in members of the families Canidae, Felidae, and Suidae. In Japan, 32 species of animals are known to be natural hosts of G. spinigerum.
Human infections
Gnathostoma larvae are ingested by humans in raw or under-cooked second intermediate hosts. Gnathostoma larvae do not reach sexual maturity in humans, but cause gnathostomiasis in the skin or internal organs. Six species of Gnathostoma have been identified as causing gnathostomiasis in humans: G. binucleatum, G. doloresi, G. hispidum, G. malaysiae, G. nipponicum, and G. spinigerum. Gnathostomiasis is endemic in Mainland Southeast Asia, the Philippines, and Japan, and is emerging in Mexico, Central America, and South America. Gnathostoma spinigerum is the primary cause of gnathostomiasis in Mainland Southeast Asia. G. binucleatum has been identified as the cause of cases of gnathostomiasis in Mexico and South America.Neurognathostomiasis has occurred in the USA. Gnathostoma binucleatum has not been previously reported to cause neurognathostomiasis, suggesting that G. spinigerum has been introduced to the Americas, but a survey of isolates has not confirmed this.