Page 194 - parasitology for medical and clinical laboratoryprofessionals

174 CHAPTER 7

are significantly larger than those of H. nana at 30 to
MICROSCOPIC DIAGNOSTIC 47 μm. Eggs of H. diminuta should be compared with
FEATURE those of the Taenia spp. when identifying the ova of
H. diminuta (refer to Figure 7-13).

General Classification—Cestode (dwarf
tapeworm egg) Symptoms

Organism Hymenolepis nana Mild gastrointestinal distress may be encountered but
Specimen Required Feces the infected individuals are most often asymptomatic.
Symptoms are generally mild with occasional bouts of
Stage Egg
nausea. Minimal abdominal cramps and pain may be ex-
Size Eggs are grayish and
may range from perienced by some.
30–47 μm
Shape Round to oval Life Cycle
Shell Contains an Rats are the definitive hosts for H. diminuta. Eggs
envelope-like center
with two thickenings are passed in rat or mouse (sometimes other rodents
at opposite ends of are involved) feces that may contaminate food sources.
the structure The organisms are perhaps eaten by insects such as
Other Features Four to eight filaments grain beetles and fleas, among others, and therefore
extending from the become the intermediate host. The chance ingestion of
polar thicken ings; these insects by man from cereals and grains may lead to
three pairs of human infections as well as infections of other rats that
hooklets are also ingest a beetle or flea containing the infective stage of the
visible
organism. The adult worm inhabits the human intestine
and the life cycle is similar to that of H. nana, where
the cysticercoid larvae mature and develop in the small
intestine and the scolex attaches to the intestinal mucosa.
The secondary host is the beetle, particularly those who The scolex grows and reproduces, and then discharges
consume rat feces. gravid proglottids that rupture and are evacuated in the
feces. Humans and other hosts that feed on contaminated
Morphology food products or contaminated household items become
infected when cysticercoid larvae again develop and eggs
One difference between the two species lies in the fact are discharged. In addition to the eggs being defecated,
that the adult worms are slightly larger on average than the eggs may hatch inside the intestine and reinfect the
H. nana, averaging 20 to 60 mm in length. The rostel- host, a process that is termed autoinfection.
lum of Hymenolepis diminuta is small and resembles
that of H. nana except it lacks the hooks found in Laboratory Diagnosis
H. nana. Proglottids of both H. diminuta and H. nana
are similar in all aspects of morphology and cannot be Adult worms and proglottids are seldom observed in
differentiated by the proglottids alone, as the proglot- stool specimens. Diagnosis is made by the recovery of
tids of each contain a uterus consisting of a saclike struc- the characteristic eggs from fecal specimens.
ture virtually full of eggs. Again, the uterus, when fully H. diminuta larvae develop in the definitive host,
gravid, will disintegrate and spill the ova. The eggs of the rat, and an intermediate host may be the grain beetle
H. diminuta and H. nana are similar in appearance ex- or flea. Cysticercoid larvae may mature in the intermedi-
cept that the polar filaments are absent in H. dimunita. ate host and are then ingested by rats or humans who are
In addition, the eggs of H. diminuta at 50 to 75 μm eating the cereal grains.