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168 CHAPTER 7

MICROSCOPIC DIAGNOSTIC

FEATURE

General Classification—Cestode (fish
tapeworm egg) Source: Centers for Disease Control and Prevention (CDC)

Organism Diphyllobothrium
latum
Specimen Required Feces
Stage Egg
Size The egg ranges from
58–76 μm 3 40–50 μm FIGURE 7-8 Gravid proglottids of a cestode
Shape Broad and oval fertil-
ized egg
alimentary tract requires nutritive substances to enter the
Infertile eggs are
longer, 80–90 μm in tapeworm across the tegument (membranous structure
length resembling skin that covers the segments). This struc-
tural arrangement is well adapted for transport functions
Shell Thick, yellow-brown,
and mammillated because it is covered with numerous microvilli (hair-like
projections) resembling those lining the lumen (open
Other Features Unfertilized eggs
show disorganized area of a tube-like structure) of the mammalian intestine.
internal contents These villi serve to increase the surface area of the lumen
Egg is unembryonated and provide for better absorption. The excretory system
when passed and is is a specialized excretory cell type called a flame cell that
operculated functions in a similar manner to both kidneys and anus
Egg has small, in mammals.
knoblike protuberance Of the two species in humans, Taenia saginata, or
opposite the opercu- the beef tapeworm, is the larger of the two and T. solium,
lum at times, but size the pork tapeworm, use cattle and pigs as their respective
and lack of “shoulders” intermediate hosts. The scientific study of the taeniid
is used to differentiate
the egg from that of tapeworms of humans can be traced to the late seven-
P. westermanni teenth century and the observations of Edward Tyson, a
British physician and scientist of anatomy who studied
the tapeworms of humans, dogs, and other animals. Tyson
proper disposal of human wastes, which might contami- was apparently the first person to recognize the “head”
nate the waters and provide eggs to infect the copepods. (scolex) of a tapeworm, and descriptions of the anatomy
and physiology of the adult tapeworms led to the current
TAENIA SAGINATA/TAENIA body of knowledge regarding the biological life cycle of the
taeniid tapeworms of humans (Cox, 2002). By the time of
SOLIUM
Tyson’s contributions, it was quite evident that there were
The adult tapeworm has three distinct regions. Follow- basic differences between the broad fish tapeworm and
ing the scolex, there is a short “neck” region followed by other tapeworms now categorized as taeniids, but the dif-
any number of segments, called proglottids (Figure 7-8). ferentiation between T. solium and T. saginata was not
Another characteristic feature of an adult tapeworm is fully discovered until later.
the absence of a digestive tract, which is ironic because It was not until some years later that observations
the adult worms inhabit the small intestine, a princi- led to the belief that there were actually two species of
pal digestive system of the human host. This lack of an Taenia responsible for infections by what was originally