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Epidemiology of Nematodes, Cestodes, and Trematodes 125

environmental conditions. The general term in use where
Source: Centers for Disease Control and Prevention (CDC) in developing countries, for obvious reasons such as the
an infection by tapeworms has occurred is taeniasis.
However, the incidence of a cestode infection is higher

lack of sanitation and of food safety laws for protection
of human health. The infection rate is lowest in North
America, at approximately 1 infected person per 1,000
in most of North America. But the infection rate may be
as high as 10 percent of the population in the poorest

worm shows a higher rate of incidence than any of the
others. But this is dependent on dietary habits because
pigs are a main source of meat almost universally and
FIGURE 5-5 Embryonated egg is indistinguishable and least developed parts of the world. The pork tape-
between the Ancylostoma duodenale or Necator may be raised in close proximity to dwellings in most of
americanus hookworm the population centers of the world.

impossible to differentiate by examining the eggs of the Taenia solium, T. saginata
two species (Figure 5-5). (Taeniasis)

CESTODES (TAPEWORMS) Unlike those of a number of other species of parasites,
the eggs (ova) of the two species of the genus Taenia
The most important cestodes of clinical interest and that are virtually indistinguishable from each other. But
are pathogenic to man are Taenia solium (pork tape- fortunately for the medical laboratory professional or the
worm), T. saginata (beef tapeworm), Diphyllobothrium parasitologist, identification of the two species is made
latum (fish or broad tapeworm), Hymenolepis nana possible by observing certain characteristic features of
(dwarf tapeworm), and Echinococcus granulosus and each of the adult forms and is accomplished as follows.
E. multilocularis (hydatid). As is readily apparent from Gravid proglottids (pregnant) are longer than they are
the names of some of them, these parasites are chiefly con- wide in the two species. But T. solium and T. saginata
tracted through the diet and from contact with animals differ in the number of primary lateral uterine branches
and their excrement. Infective larvae are able to infect the as T. solium has only 7 to 13 lateral branches, whereas
host by eating contaminated raw or undercooked meat, T. saginata possesses 12 to 30 lateral branches of the
fish, or grains, depending upon the species of the parasite proglottid. Due to a slight overlap in the numbers of
in question. Taenia solium cysticercosis or Hymenolepsis lateral branches, it is often necessary to examine several
nana can be transmitted in a direct cycle via ingestion of representative segments before arriving at a definitive
eggs from human feces. Echinococcus eggs primarily from identification. Adults of Taenia spp. can reach a length of
dog or fox hair cause human hydatid disease with hu- 2 to 8 meters, but the hooked scolex, located at the head
mans as the intermediate host and with members of the region, is only 1 to 2 millimeters in diameter. Humans
canine family (dog and fox) as the definitive hosts. Re- are the only definitive hosts in the life cycle of these two
infection with adult tapeworms is common but second tapeworm species (Figure 5-6).
infections with larvae are rare, which may be due to an
immune response to the larvae stages of the parasite. Life Cycle of Tapeworms

Epidemiology of Cestodal Larval cysts of the tapeworm (cysticerci) are ingested with
Infections poorly cooked infected meat that contains these cysts and
may lead to a disease generally called cysticercosis. Upon
Epidemiologically, cestodes commonly found in humans ingestion, the larvae escape the cysts and pass to the small
have a worldwide distribution regardless of climate and intestine where they attach to the mucosa by the scolex