There are 12 recognised species of Gnathostoma. Human gnathosomiasis is principally caused by Gnathostoma spinigerum, but disease has been associated with G. hispidum, G. nipponicum and G. doloresi, although these are much less frequent
(Nawa, 1991; Sato et al., 1992). The adult worm is rust-coloured, 2-3 cm long. It has a bulbous head with 4-8 rows of hooklets. The L3 form generally has four rows of hooklets, while the adult has eight (see Figure 19c.5). The anterior part of the worm is covered with rows of cuticular spines. There are four fluid-filled muscular
Fig. 19c.5 Head of L3 larva of Gnathostoma spinigerum sacs opening into the cephalic bulb and these are thought to contract and expand the bulb, enabling the worm to migrate through the tissues. Within the definitive host adult worms reach a length of 2-3 cm but migrating worms found in humans and other hosts tend to be smaller and less mature. The eggs are oval, with a mucoid plug, and measure 40-70 p,m. The eggs are found in the definitive host faeces but not usually in the stools of humans.
Dogs, cats, feral canids and felids, mink, racoons and otters are among the organisms which are definitive hosts for G. spinigerum (Nawa, 1991) (Figure 19c.6). In these hosts the adult worms live
in a tumour-like structure within the gastric wall, discharging eggs into the stomach. In the environment, Lj larvae hatch within 7 days and these are ingested by the first intermediate host, a crustacean of the genus Cyclops, in which they undergo L2 and L3 development. Cyclops is ingested by the second intermediate host, which includes fish, frogs, snakes, domestic chickens and domestic pigs. The larvae penetrate the gastric wall of this host and migrate to the muscles, where they mature into advanced L3 larvae and then encyst. When the definitive host eats this second intermediate host, the larvae excyst in the stomach, penetrate the gastric wall and migrate to the liver and subsequently through the connective tissues and muscles. After 4 weeks the larval forms migrate back to the gastric wall, where they enter externally to produce the tumour-like structure that connects with the gastric lumen. Up to 8 months is required for maturation into adults. When the adults mate and pass eggs through the aperture into the gastric lumen, egg excretion continues for up to a year after ingestion of the third stage larvae.
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