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Hymenolepiasis

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[Hymenolepis diminuta] [Hymenolepis nana]

Causal Agents

Hymenolepiasis is caused by two cestodes (tapeworm) species, Hymenolepis nana (the dwarf tapeworm, adults measuring 15 to 40 mm in length) and Hymenolepis diminuta (rat tapeworm, adults measuring 20 to 60 cm in length). Hymenolepis diminuta is a cestode of rodents infrequently seen in humans and frequently found in rodents.

Life Cycles

lifecycle

Eggs of Hymenolepis nana are immediately infective when passed with the stool and cannot survive more than 10 days in the external environment The number 1. When eggs are ingested by an arthropod intermediate host The number 2 (various species of beetles and fleas may serve as intermediate hosts), they develop into cysticercoids, which can infect humans or rodents upon ingestion The number 3 and develop into adults in the small intestine. A morphologically identical variant, H. nana var. fraterna, infects rodents and uses arthropods as intermediate hosts. When eggs are ingested The number 4 (in contaminated food or water or from hands contaminated with feces), the oncospheres contained in the eggs are released. The oncospheres (hexacanth larvae) penetrate the intestinal villus and develop into cysticercoid larvae The number 5. Upon rupture of the villus, the cysticercoids return to the intestinal lumen, evaginate their scoleces The number 6, attach to the intestinal mucosa and develop into adults that reside in the ileal portion of the small intestine producing gravid proglottids The number 7. Eggs are passed in the stool when released from proglottids through its genital atrium or when proglottids disintegrate in the small intestine The number 8. An alternate mode of infection consists of internal autoinfection, where the eggs release their hexacanth embryo, which penetrates the villus continuing the infective cycle without passage through the external environment The number 9. The life span of adult worms is 4 to 6 weeks, but internal autoinfection allows the infection to persist for years.

lifecycle

Eggs of Hymenolepis diminuta are passed out in the feces of the infected definitive host (rodents, man) The number 1. The mature eggs are ingested by an intermediate host (various arthropod adults or larvae) The number 2, and oncospheres are released from the eggs and penetrate the intestinal wall of the host The number 3, which develop into cysticercoid larvae. Species from the genus Tribolium are common intermediate hosts for H. diminuta. The cysticercoid larvae persist through the arthropod’s morphogenesis to adulthood. H. diminuta infection is acquired by the mammalian host after ingestion of an intermediate host carrying the cysticercoid larvae The number 4. Humans can be accidentally infected through the ingestion of insects in precooked cereals, or other food items, and directly from the environment (e.g., oral exploration of the environment by children). After ingestion, the tissue of the infected arthropod is digested releasing the cysticercoid larvae in the stomach and small intestine. Eversion of the scoleces The number 5 occurs shortly after the cysticercoid larvae are released. Using the four suckers on the scolex, the parasite attaches to the small intestine wall. Maturation of the parasites occurs within 20 days and the adult worms can reach an average of 30 cm in length The number 6. Eggs are released in the small intestine from gravid proglottids The number 7 that disintegrate after breaking off from the adult worms. The eggs are expelled to the environment in the mammalian host’s feces The number 1.

Geographic Distribution

Hymenolepis nana is the most common cause of all cestode infections, and is encountered worldwide. In temperate areas its incidence is higher in children and institutionalized groups. Hymenolepis diminuta, while less frequent, has been reported from various areas of the world.

Clinical Presentation

Hymenolepis nana and H. diminuta infections are most often asymptomatic. Heavy infections with H. nana can cause weakness, headaches, anorexia, abdominal pain, and diarrhea.

Hymenolepis diminuta eggs in wet mounts.

 

Eggs of Hymenolepis diminuta. These eggs are round or slightly oval, size 70 – 85 µm X 60 – 80 µm, with a striated outer membrane and a thin inner membrane. The space between the membranes is smooth or faintly granular. The oncosphere has six hooks. There are no polar filaments extending into the space between the oncosphere and the outer shell.
Figure A: Egg of H. diminuta in a wet mount stained with iodine. Four of the hooks are visible at this level of focus. Image courtesy of the Georgia Department of Public Health.
Figure E: Egg of H. diminuta in an unstained wet mount of concentrated stool. Image taken at 400x magnification.
Figure B: Egg of H. diminuta in a wet mount stained with iodine. Four of the hooks are visible at this level of focus.
Figure F: Egg of H. diminuta in an unstained wet mount of concentrated stool. Image taken at 400x magnification.
Figure C: Eggs of H. diminuta in an unstained wet mount of concentrated stool. Image taken at 200x magnification.
Figure D: Higher magnification (400x) of one of the eggs in Figure C.
Hymenolepis nana egg in wet mounts.

 

Eggs of Hymenolepis nana. These eggs are oval and smaller than those of H. diminuta, with a size range of 30 to 50 µm. On the inner membrane are two poles, from which 4-8 polar filaments spread out between the two membranes. The oncosphere has six hooks.
Figure A: Egg of H. nana in an unstained wet mount. Note the presence of hooks in the oncosphere and polar filaments within the space between the oncosphere and outer shell.
Figure E: Egg of H. nana in an unstained wet mount. In this image, the polar filaments in the space between the oncosphere and outer shell are clearly visible.
Figure B: Egg of H. nana in an unstained wet mount. Note the presence of hooks in the oncosphere and polar filaments within the space between the oncosphere and outer shell.
Figure F: Egg of H. nana in an unstained wet mount. In this image, the polar filaments in the space between the oncosphere and outer shell are clearly visible.
Figure C: Egg of H. nana in an unstained wet mount.
Figure D: Egg of H. nana in an unstained formalin ethyl acetate (FEA) wet mount. In this image, four of the hooks in the oncosphere are clearly visible. Image courtesy of the Oregon State Public Health Laboratory.
Hymenolepis nana eggs, zinc PVA trichrome stain.

 

Eggs of Hymenolepis nana. These eggs are oval and smaller than those of H. diminuta, with a size range of 30 to 50 µm.  On the inner membrane are two poles, from which 4-8 polar filaments spread out between the two membranes. The oncosphere has six hooks.
Figure A: Egg of H. nana in a trichrome-stained stool specimen. Although trichrome is not the preferred method for observing helminth eggs, they can be detected this way. The eggs are distorted, probably due to the zinc polyvinyl alcohol (PVA) used for preserving specimens for trichrome stain. Images courtesy of the Oregon State Public Health Laboratory.
Figure B: Egg of H. nana in a trichrome-stained stool specimen. Although trichrome is not the preferred method for observing helminth eggs, they can be detected this way. The eggs are distorted, probably due to the zinc polyvinyl alcohol (PVA) used for preserving specimens for trichrome stain. Images courtesy of the Oregon State Public Health Laboratory.
Hymenolepis proglottids.

 

Proglottids of Hymenolepis spp. Proglottids of Hymenolepis spp. are craspedote; i.e. they overlap.
Figure A: Cross-sections of mature proglottids of H. nana stained with hematoxylin and eosin (H&E), taken at 100x. Note the craspedote (overlapping) proglottids.
Figure B: Higher magnification of eggs within the proglottid in Figure A, taken at 400x.
Figure C: Higher magnification of the eggs in Figures A and B, taken at 1000x, oil. Hooks do not stain with H&E but are refractile and may be visible in stained specimens with proper adjustment of the microscope. Polar filaments are visible in the egg in the upper right quadrant of the image.
Figure D: Proglottids of H. diminuta stained with carmine. Notice the craspedote form of the proglottids.
Hymenolepis nana adults.
Figure A: Three adult specimens of H. nana. Image courtesy of the Georgia Department of Public Health.
Figure B: Scolex of H. nana in an unstained wet mount of stool. Image courtesy of Dr. David Bruckner.
Figure C: Higher magnification of the scolex in Figure B. In this image, two of the suckers and the rostellar hooks are clearly visible.
Intermediate hosts of Hymenolepis spp.

 

Arthropods, especially beetles, serve as intermediate hosts for Hymenolepis spp. The arthropod intermediate host is required for H. diminuta, but not H. nana, and humans can become infected with the latter by direct ingestion of eggs. Within the arthropod host, the eggs develop into cysticeroids, which can infect the mammalian host upon ingestion and develop into adults in the small intestine.
Figure A: Tribolium confusum, a common intermediate host for Hymenolepis spp. Tribolium and related genera breed in cereals, grains, and grain-based snack foods and are easily ingested by humans and rodents. Since these food products are usually not heated prior to consumption, cysticeroids within the beetles remain viable and infective. Image courtesy of Parasite and Diseases Image Library, Australia.
Figure B: Tribolium castaneum, another beetle commonly found in grain products that may serve as an intermediate host for Hymenolepis spp. Image courtesy of Parasite and Diseases Image Library, Australia.

Diagnostic Findings

The diagnosis depends on the demonstration of eggs in stool specimens. Concentration techniques and repeated examinations will increase the likelihood of detecting light infections.

Morphologic comparison with other intestinal parasites

Treatment Information

Treatment information for hymenolepiasis can be found at: https://www.cdc.gov/hymenolepis/hcp/clinical-care/index.html

DPDx is an educational resource designed for health professionals and laboratory scientists. For an overview including prevention, control, and treatment visit www.cdc.gov/parasites/.