Page 471 - Review of Medical Microbiology and Immunology ( PDFDrive )
P. 471
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460
PART VI Parasitology
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mebooksfree.com mebooksfree.com mebooksfree.com by other tapeworms, where only one adult worm is present, mebooksfree.com
FIGURE 54–8
Echinococcus granulosus. Life cycle. Center and left side of figure describes the natural cycle of E. granulosus within dogs
(top half) and sheep (bottom half). Dogs are the definitive hosts and contain the adult tapeworm in the intestines. Sheep are an important
intermediate host and ingest the eggs in dog feces. Hydatid cysts containing larvae form in the sheep. Humans are accidental intermediate
hosts when they ingest food contaminated with dog feces containing the eggs (#2 at blue arrow at right). Eggs hatch oncosphere embryos in
human intestine (#3 in human figure). Hydatid cysts form primarily in the liver, lung, brain, and bone (#4 in human figure). (Source: Dr. Alexander
J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)
2. Hymenolepis nana
Hymenolepis nana (dwarf tapeworm) is the most fre-
Infection causes little damage, and most patients are
quently found tapeworm in the United States. It is only 3 to
asymptomatic. The organism is found worldwide, com-
5 cm long and is different from other tapeworms because many H. nana worms (sometimes hundreds) are found.
monly in the tropics. In the United States, it is most preva-
its eggs are directly infectious for humans (i.e., ingested
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mebooksfree.com mebooksfree.com mebooksfree.com the membrane of the six-hooked larva and the outer shell. mebooksfree.com
lent in the southeastern states, usually in children. Diagnosis
eggs can develop into adult worms without an intermediate
is based on finding eggs in stools. The characteristic feature
host). Within the duodenum, the eggs hatch and differenti-
of H. nana eggs is the 8 to 10 polar filaments lying between
ate into cysticercoid larvae and then into adult worms.
Gravid proglottids detach, disintegrate, and release fertil-
The treatment is praziquantel. Prevention consists of good
ized eggs. The eggs either pass in the stool or can reinfect
personal hygiene and avoidance of fecal contamination of
the small intestine (autoinfection). In contrast to infection
food and water.
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