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Hymenolepis nana (Pathogen – Intestinal Cestode)

Organism:
Hymenolepis nana has been called the dwarf tapeworm and has a worldwide distribution. The fact that an intermediate host is not required in the life cycle was determined in the late 1800s. For this reason, H. nana has been considered to be the most common tapeworm throughout the world. The infection is most commonly seen in children, although adults are also infected. 

H. nana scolex  H. nana egg   H. nana development in GI tract

Life Cycle:
Infection is usually acquired by the ingestion of H. nana eggs, primarily from human stool. The eggs hatch in the stomach or small intestine, and the liberated larvae, or oncospheres, penetrate the villi in the upper small intestine. The larvae develop into the cysticercoid stage in the tissue and migrate back into the lumen of the small intestine, where they attach to the mucosa. The adult worms mature within several weeks. They are very small compared with eggs of the Taenia species and measure up to 40 mm long. The more worms present, the shorter the total length of each worm.  Although accidental ingestion of the insect intermediate host can result in development of the adult worms, this mode of infection is probably not common.

Acquired:
Infection in humans is acquired through ingestion of infective eggs or (less commonly) cysts (immature tapeworm larvae, cysticercoid) from infected grain beetles.

Epidemiology:
Worldwide, primarily human to human transmission

Clinical Features:
An infection with H. nana may cause no symptoms even with a heavy worm burden. Some patients complain of headache, dizziness, anorexia, abdominal pain, diarrhea, or possibly irritability. Some patients may have a low‑grade eosinophilia of 5% or more.  Heavy human infection can be attributed to internal autoinfection in which the eggs hatch in the intestine and follow the normal life cycle to the adult worm. This autoinfection feature of the life cycle can lead to complications in the compromised patient.  Heavy human infection can be attributed to internal autoinfection in which the eggs hatch in the intestine and follow the normal life cycle to the adult worm. This autoinfection feature of the life cycle can lead to complications in the compromised patient.

Clinical Specimen:
Stool:  The standard O&P examination is the recommended procedure for recovery and identification of H. nana eggs in stool specimens, primarily from the wet preparation examination of the concentration sediment.
Adult worms:  Adult worms are rarely seen. 

Laboratory Diagnosis:
Stool:  The standard O&P examination is the recommended procedure for recovery and identification of H. nana eggs in stool specimens, primarily from the wet preparation examination of the concentration sediment. The eggs are most easily seen on a direct wet smear or a wet preparation of the concentration sediment.
Adult worms The scolex has four suckers and a short rostellum with hooks. The adult worm is rarely seen in the stool.

Organism Description:
Egg:  Since the eggs of H. nana and H. diminuta look very much alike, identification to the visual identification of the polar filaments seen in H. nana eggs. The eggs are round to oval with a thin shell and measure 30 to 47 µm in diameter. The oncosphere has two polar thickenings from which arise polar filaments that lie between the oncosphere and the shell.
Adult worm:  The scolex has four suckers and a short rostellum with hooks. The worms are very small compared with worms of the Taenia species and measure up to 40 mm long. The more worms present, the shorter the total length of each worm.  The adult worm is rarely seen in the stool.

Laboratory Report:
Hymenolepis nana eggs present.

Proficiency Testing:
In Proficiency Testing specimens, the eggs may not always have hooklets visible within the oncosphere.  You may have to look very carefully for the presence of the polar filaments.  These filaments and hooklets will be more easily seen in stool specimens that have not been stored for a long time in formalin.

Treatment:
Garcia, L.S. 2007.  Diagnostic Medical Parasitology, 5th ed., ASM Press, Washington, D.C.

Control:
H. nana is the only human tapeworm in which the intermediate host is not necessary and transmission is from person to person. Children are usually infected more often than adults. Since infection is from person to person via the eggs, good personal hygiene is an important preventive measure. Infection from rats and mice is always a possibility, as is the accidental ingestion of infective insect intermediate hosts.