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172 CHAPTER 7

MICROSCOPIC DIAGNOSTIC

FEATURE

General Classification—Taenia spp.
(Cestode egg) Source: Centers for Disease Control and Prevention (CDC)

Organism Taenia solium and
Taenia saginata
Specimen Required Feces
Stage Egg
Size 30–43 μm for both
species when passed FIGURE 7-13 Hymenolepsis diminuta egg should be
in feces compared with Taenia spp.
Shape Concentric or almost
completely round
filaments spread through the spaces between the two mem-
Shell Yellow-brown
branes. The oncosphere or larval stage possesses 6 hooks
Other Features Shells of ova are (Figure 7-13). The adult worm ranges from 25 to 40 mm in
radially striated and length and are 1 mm wide. Remember that T. saginata does
have a 6-hooked
oncosphere in the not have hooks, a fact which provides a valuable tool for dif-
embryonated egg ferentiation between H. nana and T. saginata. H. nana has
a scolex that contains a retractable rostellum with one single
circle of between 20 and 30 hooks and has a tetrad (4) of
avoiding the feeding of animals with foods that may be suckers. The proglottids are visible macroscopically, being
contaminated (i.e., raw meat mixed with the food) are ef- approximately 2 mm wide and 1 mm long. The gravid or
fective. It has been reported that a vaccine is currently pregnant uterus of H. diminuta when filled with eggs will
being developed to protect humans against T. solium. almost completely fill the uterine cavity of the proglottid.
The uterus is usually not visible; however, as it disintegrates
to spill the ova into the area surrounding it.
HYMENOLEPSIS NANA

Hymenolepsis nana is called the dwarf tapeworm and Symptoms
is the smallest tapeworm known to infect humans. The
term may also be used somewhat synonymously with Hy- Mild gastrointestinal distress may be encountered but
menolepsis diminuta but minor differences between the the infected individuals are primarily asymptomatic.
two species exist. It is the most frequently encountered Mild diarrhea, weight loss, and abdominal cramps and
tapeworm in some areas, including the United States. The mild pain may be experienced by some. H. nana infec-
H. nana species is parasitic for both birds and mammals tions can grow worse over time because, unlike in most
such as mice and rats. Rodents are known to harbor the or- tapeworms, eggs of this species can hatch and develop
ganism and are capable of transmitting the disease, although without ever leaving the definitive host.
no intermediate host is required during its life cycle, unlike
a number of the other tapeworms that infect humans. Life Cycle

Morphology Ingestion of H. nana eggs is the most common manner in
which the infection is contracted. Larvae most often de-
Two inner membranes surround the interior of the Hy- velop in beetles and fleas where grains are contaminated
menolepis nana egg (30 to 47 μm) and two thickened poles by rat feces, although no intermediate host is required in
or areas of attachment are found from which 4 to 8 polar the life cycle of H. nana. The cysticercoid larvae mature