The pork tapeworm Taenia
solium and the beef tapeworm T saginata are
the most common tapeworms of humans. The diseases associated with
infection by these organisms have been known since ancient times,
being found wherever insufficiently cooked pork or beef is eaten.
Human infection with the pork tapeworm is uncommon in the United
States and Canada, although larval infection (ie, cysticercosis)
of swine may still occur. In many areas of the world, especially
Mexico and parts of South and Central America, Africa, southeastern
Europe, India, and China, infection with T solium is
relatively common. Human infection with the larval stage of T
solium (Cysticercus cellulosae), or cysticercosis,
is found wherever adult T solium infection is common.1T
saginata infection occurs among those who eat raw or insufficiently
cooked beef. Human infection with larval T saginata (Cysticercus
bovis) almost never occurs.
Humans are the mandatory definitive hosts who disseminate infection
to porcine or bovine intermediate hosts. Transmission to swine usually
occurs through contaminated soil, where gravid proglottids are deposited
with human feces. Eggs can survive for weeks in moist soil. In cattle,
grazing lands, water, or cattle feed that is contaminated with infected
human feces are sources of infection. Intrauterine infection of calves
has been reported.
Adult worms live in the upper small intestine, with T
solium measuring 2 to 8 m and T saginata measuring
3 to 10 m. The scolex of the pork tapeworm is distinguished by a
crown or rostellum with a double row of hooklets. The scolex of T
saginata is without hooks. The gravid uterus holds thousands
of eggs, each with a mature 6-hooked (ie, hexacanth) embryo. Eggs
are 30 to 40 μm in diameter and similar in both human Taenia species.
If the eggs are ingested by a suitable intermediate host such as
swine (T solium) or cattle (T saginata),
the embryo is liberated, penetrating the intestinal wall and disseminating
via the bloodstream. The embryo of T solium may
invade all tissues of the body and develops into a cysticercus or bladder
worm. Cysticerci are ellipsoidal, white, translucent cysts into
which the scolex is inverted.
When infected meat is eaten, the cysticercus is activated by
gastric juices and bile, which stimulate evagination of the scolex.
The scolex attaches to the jejunal wall, and the embryo becomes
a mature tapeworm in 10 to 12 weeks for T saginata and
5 to 12 weeks for T solium. In humans, the eggs
of T solium are ingested, and the larval stage may
develop in every tissue of the body, a condition known as cysticercosis
cellulosae. In tissue, the larvae cause an inflammatory
infiltrate of eosinophils, plasma cells, neutrophils, and lymphocytes,
with eventual necrosis and fibrosis and subsequent calcification
of the parasite.
Infection with the adult T solium or T
saginata is either asymptomatic or associated with only