CESTODES
CHAPTER 55
867
serves as the intermediate host (eg, for Echinococcus granulosus), larval development produces tissue invasion and frequently serious disease. The capacity of H nana and T solium
to use humans as both primary and intermediate hosts is unique.
BEEF TAPEWORM
TAENIA SAGINATA: PARASITOLOGY
T saginata inhabits the human jejunum, where it may live for up to 25 years and grow to a
maximum length of 10 m. Its 1-mm scolex lacks hooklets but possesses the four sucking
discs typical of most cestodes (Figure 55–1A). The creamy white strobila consists of 1000
to 2000 individual proglottids. The terminal segments are longer (20 mm) than they are
wide (5 mm) and contain a large uterus with 15 to 20 lateral branches; these characteristics are useful in differentiating them from those of the closely related pork tapeworm,
T solium. When fully gravid, strings of six to nine terminal proglottids, each containing
approximately 100,000 eggs, break free from the remainder of the strobila. These muscular segments may crawl unassisted through the anal canal or be passed intact with the
stool. Proglottids reaching the soil eventually disintegrate, releasing their distinctive eggs.
These eggs are 30 to 40 μm in diameter, spherical, and possess a thick, radially striated
shell (Figure 55–1B). In appropriate environments, the hexacanth embryo may survive
for months. If ingested by cattle or certain other herbivores, the embryo is released, penetrates the intestinal wall, and is carried by the vascular system to the striated muscles of
the tongue, diaphragm, and hindquarters. Here it is transformed into a white, ovoid (5 by
10 mm) cysticercus (Cysticercus bovis). When present in large numbers, cysticerci impart
a spotted or “measly” appearance to the flesh. Humans are infected when they ingest
inadequately cooked meat containing these larval forms.
T saginata inhabits human jejunum
Gravid proglottids passed in stool
Eggs ingested by herbivore
intermediates
Cysticerci in bovine striated muscle
Humans infected by eating
inadequately cooked infected meat
BEEF TAPEWORM DISEASE
EPIDEMIOLOGY
In the United States, sanitary disposal of human feces and federal inspection of meat
have nearly interrupted transmission of T saginata. At present, less than 1% of examined
carcasses are infected. Nevertheless, bovine cysticercosis is still a significant problem in
the southwestern area of the country, where cattle become infected in feedlots or while
pastured on land irrigated with sewage or worked by infected laborers without access to
sanitary facilities. Shipment of infected carcasses can result in human infection in other
areas of the United States. In countries where sanitary facilities are less comprehensive and
undercooked or raw beef is eaten, T saginata is highly prevalent. Examples include Kenya,
Ethiopia, the Middle East, the former Yugoslavia, and parts of the former Soviet Union and
South America.
Indigenously acquired disease rare
in United States
BEEF TAPEWORM DISEASE: CLINICAL ASPECTS
MANIFESTATIONS
Most persons infected with beef tapeworm are asymptomatic and become aware of the
infection only through the spontaneous passage of proglottids. The proglottids may be
observed on the surface of the stool or appear in the underclothing or bed sheets of the
alarmed host. Passage may occur very irregularly and can be precipitated by excessive alcohol consumption. Some patients report epigastric discomfort, nausea, irritability (particularly after passage of segments), diarrhea, and weight loss. Occasionally, the proglottids may
obstruct the appendix, biliary duct, or pancreatic duct.
Ryan_CH55_p865-878.indd 867
Clinical symptoms usually mild
9/18/09 10:14:27 PM
868
PART V
PATHOGENIC PARASITES
2.
1.
Suckers
Nerve cord
Uterus and ova
Genital pore
Neck
Excretory canal
1 mm
1 mm
A
3.
Rostellum
4.
Hooklets
Suckers
Genital pore
Uterus and ova
1 mm
1 mm
5.
Egg shell
Hooklet
Hexacanth larvum
FIGURE 55–1. Tapeworm
structures. A. Taenia saginata.
B. Taenia solium. (1, 3) scolices; (2, 4)
gravid proglottids; (5) ova (indistinguishable between species).
Ryan_CH55_p865-878.indd 868
10 μm
B
9/18/09 10:14:28 PM
CESTODES
CHAPTER 55
869
DIAGNOSIS
The diagnosis of beef tapeworm disease is made by finding eggs or proglottids in the stool.
Eggs may also be distributed on the perianal area secondary to rupture of proglottids during anal passage. The adhesive cellophane tape technique described for pinworm can be
used to recover the worms from this area. With this procedure, 85% to 95% of infections
are detected, in contrast to only 50% to 75% by stool examination. Because the eggs of
T solium and T saginata are morphologically identical, it is necessary to examine a proglottid to identify the species correctly.
Adhesive cellophane tape
technique and stool examination
detect eggs and proglottids
TREATMENT AND PREVENTION
The drugs of choice are praziquantel or niclosamide, which act directly on the worm. Both
are highly effective in single-dose oral preparations. Ultimately, control is best effected
through the sanitary disposal of human feces. Meat inspection is helpful; the cysticerci are
readily visible. In areas where the infection is common, thorough cooking is the most practical method of control. Internal temperatures of 56°C or more for 5 minutes or longer
destroy the cysticerci. Salting or freezing for 1 week at –15°C or less is also effective.
Sewage disposal, meat inspection,
and adequate cooking
PORK TAPEWORM
TAENIA SOLIUM: PARASITOLOGY
Like the beef tapeworm, which it closely resembles, T solium inhabits the human jejunum,
where it may survive for decades. It can be distinguished from its close relative only by
careful scrutiny of the scolex and proglottids; T solium possesses a rostellum armed with
a double row of hooklets (Figure 55–1B3). The strobila is generally smaller than that of
T saginata, seldom exceeding 5 mm in length or containing more than 1000 proglottids.
Gravid segments measure 6 by 12 mm and thus appear less elongated than those of the
bovine parasite (Figure 55–1B4). Typically, the uterus has only 8 to 12 lateral branches.
Although the eggs appear morphologically identical to those of T saginata, they are infective only to swine and, perhaps reflecting a genetic proximity we would prefer to overlook,
humans. Both pigs and people become intermediate hosts when they ingest food contaminated with viable eggs (Figure 55–2). Some authorities have suggested that humans may
be autoinfected when gravid proglottids are carried backward into the stomach during the
act of vomiting, initiating the release of the contained eggs. It seems more likely that
autoinfection results from the transport of the eggs from the perianal area to the mouth on
contaminated fingers.
Regardless of the route, an egg reaching the stomach of an appropriate intermediate host
hatches, releasing the hexacanth embryo. The embryo penetrates the intestinal wall and
may be carried by the lymphohematogenous system to any of the tissues of the body. Here
it develops into a 1-cm, white, opalescent cysticercus over 3 to 4 months (Figure 55–3).
The cysticercus may remain viable for up to 5 years, eventually infecting humans when they
ingest undercooked and “measly” flesh. The scolex everts, attaches itself to the mucosa, and
develops into a new adult worm, thereby completing the cycle.
T solium strobila shorter than in
T saginata
Eggs infective to swine and to
humans
Tissue cysticerci develop in humans
and swine
PORK TAPEWORM DISEASE
EPIDEMIOLOGY
Although infected swine are still occasionally found in the United States, most human
disease is found in immigrants from endemic areas. Although pork tapeworm disease is
widely distributed throughout the world, it is particularly common in south and southeast
Asia, Africa, Latin America, and Eastern Europe.
Ryan_CH55_p865-878.indd 869
T solium rarely found in United
States
9/18/09 10:14:28 PM