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Untitled Document

Presentation of Quiz #20

A 28-year-old postdoctoral student presented to his physician with complaints of recurrent abdominal cramps. He had grown up in Italy, come to the United States for graduate studies, and returned home each year for the holidays. Over a period of several years he had complained of recurrent abdominal cramps; however, the causes were thought to be changes in diet, as well as the usual stress associated with graduate school. During the past few months, the symptoms had become worse and he occasionally had diarrhea, as well. There was some speculation that he might have an ulcer.

On presentation, the patient appeared in no particular distress, and the examination was unremarkable. The one exception was some slight discomfort on palpation of the abdomen.

A gastrointestinal series was normal; however, stool exams revealed the following:

This object measured about 30 by 47 microns and was easily seen in the saline wet preparation examination. Note the suggestion of filaments that lie between the eggshell and the embryo.

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Answer and Discussion of Quiz #20

The image presented in Diagnostic Quiz #20 is the following:

  1. This image is a Hymenolepis nana egg; these eggs generally measure 30 to 47 microns in diameter. They round to oval, contain a six-hooked oncosphere (embryo), and have polar filaments that lie between the eggshell and the developing embryo.

Comment: 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. When young children have heavy infections, they may have loose stools or even diarrhea containing mucus. Persistent, diffuse abdominal pain seems to be the most common symptom.

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 (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. The adult worm is rarely seen in the stool; they measure up to 40 mm long. This life cycle involves transition from the egg to the adult worm within the human; no intermediate host is required. Although accidental ingestion of the insect intermediate host can result in development of the adult worms, this mode of infection is probably rare. There is evidence that auto- or hyperinfection occurs when eggs spontaneously hatch in the small intestine and initiate a new life cycle. Although adult worms live for only about a year, some patients can carry a large worm burden for many years, probably due to the autoinfection capability within the life cycle.

Diagnosis: Since the adult worm or proglottids are rarely seen in the stool, the diagnosis is based on recovery and identification of the characteristic eggs. They are most easily seen and identified from fresh specimens or those preserved in formalin-based fixatives. The thin-shelled eggs also tend to collapse on the permanent stained smear and may be difficult to identify in this type of preparation (see images below). Eggs with the characteristic thin shell, six-hooked oncosphere, and polar filaments are diagnostic. Remember, the eggs are infectious, and unpreserved stool specimens should be handled with caution.

Epidemiology and Control: H. nana is the only human tapeworm in which an intermediate host is not required 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.

Hymenolepis nana, Key Points - Laboratory Diagnosis

  1. Adult worms or proglottids are rarely seen in the stool.
  2. Eggs with the characteristic thin shell, six-hooked oncosphere, and polar filaments are diagnostic.
  3. Egg morphology is more easily seen in fresh specimens or those preserved in formalin-based fixatives.
  4. The eggs are infectious, and unpreserved stool specimens should be handled with caution.

Treatment: Niclosamide has been widely used for years. However, the use of praziquantel in a single oral dose is apparently more effective and kills the cysticercoid as well as the adult worm. The recommended dose for adults and children is 25 mg/kg in a single dose.

Two images of Hymenolepis nanaeggs from a routine stool examination (permanent stained smear - trichrome stain). Note that the eggshells are somewhat shrunk and distorted; the oncosphere stains quite dark and none of the internal structures (hooks) can be seen. Also, it is unlikely that the polar filaments can be seen in this type of preparation.

The top image shows a diagram of the developing cysticercoid and adult worm, while the image on the bottom is a tissue biopsy preparation showing the developing cysticercoids.


  1. Garcia, LS, 2016. Diagnostic Medical Parasitology, 6th Ed., ASM Press, Washington, DC.
  2. Garcia, L.S. 2009. Practical Guide to Diagnostic Parasitology, 2nd Ed., ASM Press, Washington, D.C.


Each Quiz has a two section format: the first section will present the Quiz topic and the second section will provide a discussion of the answer and/or various options in response to the Quiz situation presented to the user. In some situations, there may be more than one correct response.

The content within this site is made possible through the extensive contribution of Lynne S. Garcia, M.S., MT(ASCP), CLS(NCA), BLM(AAB), F(AAM), Director, Consultantation and Training Services (Diagnostic Medical Parasitology and Health Care Administration). For additional information, she can be contacted at LynneGarcia2@verizon.net.

Reference: Garcia, L.S. 2015. Diagnostic Medical Parasitology, 6th Ed., ASM Press, Washington, D.C.

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