Echinostoma trematode flatworms have a worldwide distribution. Around 10 species have been reported to infect humans, including E. hortense, E. macrorchis, E. revolutum, E. ilocanum, E. perfoliatum, E. malayanum, and E. echinatum (work by Detwiler et al. 2010, however, suggests that a number of putative Echinostoma species may in fact represent complexes of cryptic species). Human infections are seen most frequently in Southeast Asia (e.g., Korea, the Philippines, Indonesia, Malaysia, and Thailand) and in other areas where undercooked or raw freshwater snails, clams, and fish are eaten. Heavy infection may result in diarrhea, anorexia, and abdominal discomfort. (Centers for Disease Control Parasites and Health Website; Macpherson 2005)
Many animals may serve as definitive hosts for various Echinostoma species, including aquatic birds, carnivores, rodents, and humans (a definitive host is the host in which the adult parasites occur). Unembryonated eggs are passed in feces and develop in the water. On average, the ciliated miracidium larva takes around 10 days to mature before hatching and penetrating the first intermediate host, a snail. Several genera of snails may serve as the first intermediate host. The intramolluscan stages include a sporocyst, one or two generations of rediae, and cercariae. The cercariae may encyst as metacercariae within the same first intermediate host or leave the host and penetrate a new second intermediate host. Depending on the species, a range of animals may serve as the second intermediate host, including other snails, bivalve mollusks, fish, and tadpoles. The definitive host becomes infected after eating a second intermediate host. Metacercariae excyst in the duodenum and adults reside in the small intestine of the definitive host. (Centers for Disease Control Parasites and Health Website)
Echinostoma trivolvis is one of the trematode species of dominant importance with respect to North American larval anuran (frog and toad) communities. Signs of infection in amphibians include edema, growth inhibition, and mortality. These symptoms are far less dramatic than those seen in frogs infected by the better known trematode Ribeiroia ondatrae, which has drastic effects on amphibian limb development (e.g., extra or missing limbs, misshapen eyes or tails, skin lesions, and whole body deformities). Echinostoma trivolvis occurs in lentic (still water) aquatic environments and infects the intestines of a range of vertebrate hosts, typically aquatic or semi-aquatic birds and mammals. Its complex life cycle requires three different hosts: a planorbid snail (Planorbella trivolvis), a larval amphibian or fish, and a bird or mammal. Szuroczki and Richardson (2009) reviewed the biology of E. trivolvis. (Szuroczki and Richardson 2009 and references therein) Some Echinostoma species may contribute to local amphibian declines in some contexts (Holland et al. 2007; Detwiler et al. 2010 and references therein).
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