Corresponding author. This article has been cited by other articles in PMC. Abstract The occurrence of collar spined echinostome metacercariae in freshwater snails was investigated in 6 districts of Chiang Mai Province, Thailand, from October to April A total of 2, snails that belong to 12 species were examined, and 7 snail species Clea helena, Eyriesia eyriesi, Bithynia funiculata, Bithynia siamensis siamensis, Filopaludina doliaris, Filopaludina sumatrensis polygramma, and Filopaludina martensi martensi were found infected with echinostome metacercariae. The prevalence of metacercariae was the highest in Filopaludina spp.
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Resources Causal Agents The trematode family Echinostomatidae includes numerous spiny-collared intestinal flukes known to infect humans. Infections are documented mostly from members of the genera Echinostoma E. Sporadic infections with members of other echinostomid genera including Echinoparyphium, Acanthoparyphium, Artyfechinostomum, Episthmium, Himasthla, Hypoderaeum, and Isthmiophora are known.
Life Cycle Like many trematodes, echinostomid flukes undergo a multi-host indirect life cycle. Unembryonated eggs are passed in feces of infected definitive hosts and develop in water. Miracidia usually take about 3 weeks to mature before hatching , after which they swim freely and penetrate the first intermediate host, a snail. The intramolluscan stages include a sporocyst stage , one or two generations of rediae , and cercariae , which are released from the snail.
The cercariae may encyst as metacercariae within the same first intermediate host or leave the host and penetrate a new second intermediate host. The definitive host becomes infected after eating metacercariae in infected second intermediate hosts.
Metacercariae excyst in the duodenum and adults reside in the small intestine for some species, occasionally in the bile ducts or large intestine. Hosts Many animals including birds, carnivores, rodents, and humans may serve as definitive hosts for various echinostomid species. The most frequently encountered zoonotic species, Echinostoma hortense and E.
The first intermediate host is always a snail families Planorbidae, Lymnaeidae, and Bulinidae , and the major competent intermediate hosts vary by parasite species. Also depending on species, several animals may serve as the second intermediate host, including other snails, bivalves, fish, salamanders, and tadpoles. Geographic Distribution Echinostomes occur in wildlife and domestic animals worldwide, but human cases are seen most frequently in Southeast and East Asia.
Incidence is highest in areas where undercooked or raw freshwater snails, clams, fish, or amphibians are eaten. Clinical Presentation Pathogenicity likely varies depending on the infecting species. Catarrhal inflammation often occurs due to the penetration of the sharp-spined collar into the intestinal mucosa, which creates ulcerative lesions. Peripheral eosinophilia is usually present.
Echinostomid egg in wet mounts. They have an inconspicuous operculum and the abopercular end is often thickened. The larger eggs are very similar to Fasciola and Fasciolopsis. Eggs are passed unembryonated in feces.
Figure A: Echinostomid egg in an unstained wet mount of stool. Image taken at x magnification. Echinostoma spp. Echinostomid flukes are much longer than wide and measure about 2—10 mm long by 1—2 mm wide, depending on the species. The oral sucker is surrounded by a collar of spines, the number of which varies among species. The oral and ventral suckers are located fairly close to one another.
A single ovary is situated near the large, paired testes. Adults reside in the small intestine of the definitive host. Figure A: Adult of E. Figure B: A closer view of the anterior region of the E. The following images were taken from an adult echinostomid removed from a colon polyp during routine colonoscopy. Shown here are eggs EG within the size range for Echinostoma spp. Figure B:Higher magnification of the anterior end of the specimen in Figure A.
Notice the acetabulum ventral sucker, AC. Figure C:Higher magnification of the posterior end of the specimen in Figure A.
Notice the vitelline glands VT and lobed testes TE. Intermediate hosts of Echinostomatidae Like all trematodes, echinostomids require a snail as a first intermediate host. The second intermediate host may also be a snail, sometimes the same individual snail that served as the first intermediate host. Due to the large geographic distribution of echinostomes, and the many species present, there are many species of snails that may serve as first or second intermediate hosts.
Other second intermediate hosts include bivalves, fish, salamanders, and tadpoles. Figure A: Lymnaea sp. Figure D: Viviparus sp. This snail genus has been recorded as a second intermediate host for E. Figure B: Radix sp. This snail genus has been recorded as a first intermediate host for Echinostoma Hortense and a second intermediate host for E.
Figure E: Corbicula sp. This bivalve genus has been recorded as a second intermediate host for E. Figure C: Gyraulus sp. This snail genus has been recorded as an intermediate host for E. Laboratory Diagnosis Diagnosis is based on microscopic identification of eggs in the stool. Because the eggs are large, careful measurements must be taken to avoid confusion with the eggs of Fasciola, Fasciolopsis, and Gastrodiscoides. Genus- and species-level identification cannot be done based on egg morphology and adults are needed for a definitive diagnosis.
Glossary Nearctic living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico. Palearctic living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria. For example, diseases caused by infection of filarial nematodes elephantiasis and river blindness.
Distribution[ edit ] Echinostoma revolutum is the most widely distributed species of all 20 Echinostomatidae species; it is found in Asia, Oceania, Europe, and the Americas. This parasite is predominantly found throughout North America. Two asexual generations occur in a snail or mollusk. The first snail host is penetrated by a miracidium , producing a sporocyst. Many sporocysts are produced and mother rediae emerge.