Parasitos helmintos

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  • Publicado : 2 de septiembre de 2012
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Causal Agents:
The trematodes Fasciola hepatica (the sheep liver fluke) and Fasciola gigantica, parasites of herbivores that can infect humans accidentally.
Life Cycle:

Immature eggs are discharged in the biliary ducts and in the stool .  Eggs become embryonated in water , eggs release miracidia , which invade a suitable snail intermediate host , including the genera Galba,Fossaria and Pseudosuccinea.  In the snail the parasites undergo several developmental stages (sporocysts , rediae , and cercariae ).  The cercariae are released from the snail  and encyst as metacercariae on aquatic vegetation or other surfaces.  Mammals acquire the infection by eating vegetation containing metacercariae.  Humans can become infected by ingesting metacercariae-containing freshwater plants,especially watercress .  After ingestion, the metacercariae excyst in the duodenum  and migrate through the intestinal wall, the peritoneal cavity, and the liver parenchyma into the biliary ducts, where they develop into adults .  In humans, maturation from metacercariae into adult flukes takes approximately 3 to 4 months.  The adult flukes (Fasciola hepatica: up to 30 mm by 13 mm; F. gigantica: up to 75mm) reside in the large biliary ducts of the mammalian host.  Fasciola hepatica infect various animal species, mostly herbivores.
Geographic Distribution:
Fascioliasis occurs worldwide.  Human infections with F. hepatica are found in areas where sheep and cattle are raised, and where humans consume raw watercress, including Europe, the Middle East, and Asia.  Infections with F. gigantica havebeen reported, more rarely, in Asia, Africa, and Hawaii.
Laboratory Diagnosis:
Microscopic identification of eggs is useful in the chronic (adult) stage.  Eggs can be recovered in the stools or in material obtained by duodenal or biliary drainage.  They are morphologically indistinguishable from those of Fasciolopsis buski.  False fascioliasis (pseudofascioliasis) refers to the presence of eggs inthe stool resulting not from an actual infection but from recent ingestion of infected livers containing eggs.  This situation (with its potential for misdiagnosis) can be avoided by having the patient follow a liver-free diet several days before a repeat stool examination.  Antibody detection tests are useful especially in the early invasive stages, when the eggs are not yet apparent in thestools, or in ectopic fascioliasis.
H no embrionado

Ruminants are the usual definitive hosts for Dicrocoelium dendricitum, although other herbivorous animals, carnivores, and humans can serve as definitive hosts.  Embryonated eggs are shed in feces .  The eggs are ingested by a snail .  Many species of snail may serve as the first intermediate host, including Zebrina spp. and Cionella spp.  Whenthe miracidia hatch , they migrate through the gut wall and settle into the adjacent vascular connective tissue, where they become mother sporocysts .  The sporocysts migrate to the digestive gland where they give rise to several daughter sporocysts.  Inside each daughter sporocyst, cercariae are produced .  The cercariae migrate to the respiration chamber where they are shed in slime ball from thesnail .  After a slime ball is ingested by an ant, the cercariae become free in the intestine and migrate to the hemocoel where they become metacercariae .  Many ants may serve as the second intermediate host, especially members of the genus, Formica.  After an ant is eaten by the definitive host , the metacercariae excyst in the small intestine.  The worms migrate to the bile duct where theymature into adults .  Humans can serve as definitive hosts after accidentally ingesting infected ants .
Geographic Distribution:
Europe, northern Asia, America and northern Africa.
Laboratory Diagnosis:
Microscopic identification of eggs in the stool or duodenal fluid.  If eggs are found only in stool, it could represent spurious passage following the ingestion of infected animal liver. ...
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