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).
Molecular Biology and Genetics
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Human echinostomiasis is an intestinal parasitic disease caused by one of at least sixteen trematode flukes from the genus Echinostoma. Found largely in southeast Asia and the Far East, mainly in cosmopolitan areas. It has extensive vitelline glands for egg yolk production and tandem, oval testes. Echinostomiasis is transmitted through the ingestion of one of several possible intermediate hosts, which could include snails or other mollusks, certain freshwater fish, crustaceans or amphibians. These flukes are of moderate size, about 2 mm, and are distinguished by an oral sucker surrounded by a characteristic collar of spines.
Adults are in the small intestines of vertebrate definitive hosts. Eggs are released through the feces and embryonated. The egg becomes a miracidium with an operculum, which penetrates a snail, the first intermediate host. Upon penetration, it becomes a mother sporocyst, producing many mother rediae. Each of these mother redia produce many daughter rediae, which each produce many free-swimming cercariae. Each of these cercaria encysts in a freshwater mollusc, the second intermediate host, becoming a metacercaria. These mollusc are eaten by a vertebrate, the definitive host.
Upon infection of the human host, the worms aggregate in the small intestine where they may cause no symptoms, mild symptoms, or severe symptoms in rare cases, depending on the number of worms present. Diarrhea is a result of heavy infections. Effective drugs for treatment do exist, but the disease still remains a problem in endemic areas.
Prevention and control is possible through measures such as health education; altered eating habits to exclude ingestion of raw fish, mollusks and other sources of the disease; and removing of wastewater and industrial discharge that may be home to the parasite.
See also[edit source | edit]
References[edit source | edit]
- Rudolphi K. (1809). Entoz. Hist. Nat. 2(1): 38.
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