The figure shows a typical Hymenolepis nana egg found in feces from a 2-year-old child with diarrhea. An oncosphere containing the hexacanth embryo with three pairs of hooks and two polar thickenings can be observed. The polar filaments are present in the space between the oncosphere and the eggshell; those filaments are difficult to see in this picture, however.
The feces were collected in a sterile wide-mouth plastic bottle and prepared using the formalin-ethyl acetate sedimentation procedure; the sediment was analyzed using light microscopy. The egg was photographed with a digital camera (Leica) using an oil immersion objective (1,000x).
H. nana, known as the dwarf tapeworm, is the smallest of the adult human tapeworms. It is normally a parasite of mice, in which the life cycle involves various beetles as intermediate hosts. In humans, transmission is usually accomplished by direct ingestion of infective eggs containing oncospheres. Also humans may acquire the tapeworms by accidental ingestion of infected beetles, often occurring in dry cereals. H. nana has a worldwide geographic distribution but is the most common tapeworm infection of humans in the United States.
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