Hymenolepis nana
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p682-684
2025-10-29
66
GENERAL CHARACTERISTICS
Hymenolepis nana, also known as the dwarf tapeworm, is very small in comparison to other tapeworms. The organ ism may reach up to 4 cm in length. The proglottid contains a scolex with a short-armed rostellum. It is the most common tapeworm with worldwide distribution. An intermediate host is not required, thus making person to-person spread possible. An adult dwarf tapeworm can live within the host for approximately 4 to 6 weeks.
EPIDEMIOLOGY
H. nana is generally found in children. Although it is most prevalent in the southern United States, it has a wide distribution, particularly in crowded areas. It is more frequent in populations living in conditions of poverty or poor hygiene, in day care centers, and in persons living in institutional settings or prisons.
PATHOGENESIS AND SPECTRUM OF DISEASE
H. nana has an unusual life cycle; ingestion of the egg can lead to the development of the adult worm in humans, thus bypassing the need for an intermediate host (Figure 1). Humans can serve as both intermediate and definitive hosts. Infection occurs by accidentally ingesting dwarf tapeworm eggs. This happens most commonly through direct fecal-oral transmission or accidental ingestion of an infected arthropod. The worm resides within the upper ileum of the intestinal tract. Once infected, the dwarf tapeworm may reproduce inside the body, thus causing autoinfection. Autoinfection is essentially a reinfection or constant reproduction of the para site within the host. Massive infection with several thousand worms may follow autoinfection, resulting in hyperinfection. Hyperinfction refers to a large parasitic burden within the host. Autoinfection appears to initiate a cellular and humoral immune response. The immune response will provide the host with some protective immunity. Most patients are asymptomatic. Symptomatic patients may experience weight loss, nausea, weakness, loss of appetite, diarrhea, and abdominal discomfort. Young children, especially those with a heavy infection, may develop headache, itchy bottom, or difficulty sleeping. Dwarf tapeworm infection may be misdiagnosed as pinworm infection.
Fig1. Life cycle of Hymenolepis nana.
LABORATORY DIAGNOSIS
Adult worms and proglottids are rarely seen in stool specimens. Diagnosis is typically through the identification of eggs in stool specimens. Eggs are characterized by the presence of a thin shell enclosing an embryo (oncosphere) with six hooklets contained within two layers of membrane. The eggs are spheroidal, pale, and thin-shelled (30 to 47 µm in diameter). The eggs of H. nana and Hymenolepis diminuta are very similar. However, H. nana eggs are smaller and have polar filaments present in the space between the oncospheres and the eggshell. The egg morphology is easily distinguishable in fresh or formalin-fixed fecal samples. It is important to note that eggs are infectious and therefore unpreserved specimens should be handled carefully. Concentration techniques and repeated examinations will increase the likelihood of detecting light infections. Some patients may demonstrate a low-grade eosinophilia.
ANTIPARASITIC SUSCEPTIBILITY TESTING AND THERAPY
Praziquantel remains the therapy of choice. Niclosamide is also effective and can be repeated with reinfection. Human adults living in endemic areas are provided some immunity as a result of their cellular and humoral immunologic responses.
PREVENTION
Good hygiene is the best method for control and prevention. Preventing fecal contamination of food and water is the first line of defense. General sanitation measures, along with rodent control, are helpful in controlling the f lea population.
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