Page 190 - parasitology for medical and clinical laboratoryprofessionals

170 CHAPTER 7

Source: Centers for Disease Control and Prevention (CDC) Source: Centers for Disease Control and Prevention (CDC)

FIGURE 7-11 Structural morphology of an adult FIGURE 7-12 Structural details of a Taenia sp. ovum
cestode, Echinococcus granulosus

The coracidium develops into a procercoid stage in
percentages of eosinophils may also occur, a condition its micro-crustacean copepod, which serves as its
that is not in itself diagnostic of any particular parasite. first immediate host. Later development resulting in a
If the cysticerci migrate to the brain, however, headaches plerocercoid larva occurs in its next intermediate host,
and seizures may result, and will possibly lead to a life- which is a vertebrate. The plerocercoid larva develops
threatening medical condition. into an adult worm in the definitive or final host. The on-
cosphere of cyclophyllidean tapeworms, depending on
Life Cycle the species, develops into a cysticercus larva, cysticercoid
larva, coenurus larva, or hydatid larva (cyst) in specific
Cestodes are hermaphroditic, with each proglottid pos- intermediate hosts. These larvae, in turn, become adults
sessing both male and female reproductive systems in the definitive host. Remember that the eggs of the two
similar to those found in flukes. Proglottids nearest the Taenia spp. species of T. solium and T. saginata are not
scolex are immature but the proglottids near the caudal possible to differentiate from each other, or from other
end are called gravid (pregnant) proglottids and may con- members of the taeniid family, Taeniidae. The eggs mea-
tain numerous eggs in the uterus of each segment. How- sure 30 to 35 μm in diameter and are radially striated.
ever, tapeworms also differ from flukes in the manner The internal oncosphere reveals 6 hooks (Figure 7-12).
by which eggs are deposited. Eggs of pseudophyllidean
tapeworms exit through a uterine pore in the center of Disease Transmission
the ventral surface rather than through a genital atrium,
a structure also found in flukes. In the cyclophyllidean Human infection with either T. saginata or T. solium
tapeworm the female system includes a uterus without a results from eating uncooked or poorly cooked beef or
uterine pore. Therefore, the cyclophyllidean eggs are re- pork, in the order presented. The cysticercus larvae of
leased only when the tapeworms shed gravid proglottids the parasite are contained in the meat. Cows and pigs
into the intestine. Some proglottids disintegrate and re- as intermediate hosts eat contaminated food containing
lease eggs that are evacuated in the feces, whereas other eggs, and become infected when the oncosphere hatches
proglottids are passed intact. from the egg and develops into a cysticercus larva in the
The eggs of pseudophyllidean tapeworms are oper- tissues of the animal.
culated (lid or covering), but those of cyclophyllidean
species are not, so this is a valuable characteristic for Laboratory Diagnosis
identifying eggs of cestodal species. All tapeworm eggs
contain at some stage of development an embryo or on- The recovery of gravid proglottids and/or eggs from
cosphere. The oncosphere of pseudophyllidean tape- human fecal specimens following the rupture of the pro-
worms is ciliated externally and is called a coracidium. glottids is the most effective method of identification.