|Adult dwarf tapeworm|
(Bilharz, 1851) Ransom, 1901
Dwarf tapeworm (Hymenolepis nana, also known as Rodentolepis nana, Vampirolepis nana, Hymenolepis fraterna, and Taenia nana) is a cosmopolitan species though most common in temperate zones, and is one of the most common cestodes (a type of intestinal worm or helminth) infecting humans, especially children.
As its name implies (Latin: 'nanos' – dwarf), it is a small species, seldom exceeding 40 mm long and 1 mm wide. The scolex bears a retractable rostellum armed with a single circle of 20 to 30 hooks. The scolex also has four suckers, or a tetrad. The neck is long and slender, and the segments are wider than long. Genital pores are unilateral, and each mature segment contains three testes. After apolysis, gravid segments disintegrate, releasing eggs, which measure 30 to 47 µm in diameter. The oncosphere is covered with a thin, hyaline, outer membrane and an inner, thick membrane with polar thickenings that bear several filaments. The heavy embryophores that give taeniid eggs their characteristic striated appearance are lacking in this and the other families of tapeworms infecting humans. The rostellum remains invaginated in the apex of the organ. Rostellar hooklets are shaped like tuning forks. The neck is long and slender, the region of growth. The strobila starts with short, narrow proglottids, followed with mature ones.
Infection is acquired most commonly from eggs in the feces of another infected individual, which are transferred in food, by contamination. Eggs hatch in the duodenum, releasing oncospheres, which penetrate the mucosa and come to lie in lymph channels of the villi. An oncosphere develops into a cysticercoid which has a tail and a well-formed scolex. It is made of longitudinal fibers and is spade-shaped with the rest of the worm still inside the cyst. In five to six days, cysticercoids emerge into the lumen of the small intestine, where they attach and mature.
The direct lifecycle is doubtless a recent modification of the ancestral two-host lifecycle found in other species of hymenolepidids, because cysticercoids of H. nana can still develop normally within larval fleas and beetles. One reason for facultative nature of the lifecycle is that H. nana cysticercoids can develop at higher temperatures than can those of the other hymenolepidids. Direct contaminative infection by eggs is probably the most common route in human cases, but accidental ingestion of an infected grain beetle or flea cannot be ruled out. The direct infectiousness of the eggs frees the parasite from its former dependence upon an insect intermediate host, making rapid infection and person-to-person spread possible. The short lifespan and rapid course of development also facilitate the spread and ready availability of this worm.
H. nana, like all tapeworms, contains both male and female reproductive structures in each proglottid. This means that the dwarf tapeworm, like other tapeworms is hermaphroditic. Each segment contains three testes and a single ovary. When a proglottid becomes old and unable to absorb any more nutrition, it is released and is passed through the host's digestive tract. This gravid proglottid contains the fertilized eggs, which are sometimes expelled with the feces. However, most of the time, the egg may also settle in the microvilli of the small intestine, hatch, and the larvae can develop to sexual maturity without ever leaving the host.
The dwarf tapeworm, like all other tapeworms, lacks a digestive system and feeds by absorption on nutrients in the intestinal lumen. They have nonspecific carbohydrate requirements and they seem to absorb whatever is being passed through the intestine at that time. When it becomes an adult, it attaches to the intestinal walls with its suckers and toothed rostellum and has its segments reaching out into the intestinal space to absorb food.
In 1887, Grassi demonstrated that transmission from rat to rat did not require an intermediate host. Later, in 1921, Saeki demonstrated a direct cycle of transmission of H. nana in humans, transmission without an intermediate host. In addition to the direct cycle, Nicholl and Minchin demonstrated that fleas can serve as intermediate hosts between humans.
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