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CHAPTER 55
Cestodes
Cestodes are long, ribbon-like helminths that have gained the common appellation of
tapeworm from their superficial resemblance to sewing tape. Their appearance, number,
and exaggerated reputation for inducing weight loss have made them the best known of
the intestinal worms. In some parts of the world, indigenous populations take purgatives
monthly to rid themselves of this, the largest and most repulsive of the intestinal
parasites.
GENERAL FEATURES
I. PARASITOLOGY
A. MORPHOLOGY
1. Divided into scolex, neck, and segmented body parts
2. Individual segments, or proglottids, are generated one at a time to form the chainlike body
3. Each proglottid a hermaphroditic unit releasing eggs via rupture or through
uterine pore
4. When segment reaches gravidity, it releases its eggs by rupturing, disintegrating,
or passing them through its uterine pore
B. LIFE CYCLE
1. Eggs of Taenia must be ingested by intermediate host
2. Infectious cysts of Taenia form in tissues of intermediate
3. Definitive host ingests cysts in flesh of intermediate hosts to yield adult intestinal
worms
4. D. latum requires two intermediates—a copepod and a freshwater fish—to
complete cycle
II. CLINICAL DISEASE
1. Clinical effects depend on whether humans are definitive hosts or intermediate
hosts
2. As definitive host the adult worm is confined to the lumen of the gut, and the
consequences of the infection are typically minor
3. When intermediate host larval development produces tissue invasion and
frequently serious disease
BEEF TAPEWORM: Taenia saginata
I. PARASITOLOGY
1.
2.
3.
4.
5.
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
II. BEEF TAPEWORM DISEASE
A. EPIDEMIOLOGY
1. Sanitary disposal of human feces and federal inspection of meat interrupt
transmission of T. saginata
2. Where sanitary facilities are less comprehensive and undercooked or raw beef is
eaten, T. saginata is highly prevalent
III. BEEF TAPEWORM DISEASE: CLINICAL
ASPECTS
A. MANIFESTATIONS
1. Clinical symptoms usually mild
2. Most infected patients are asymptomatic and become aware of the infection only
through the spontaneous passage of proglottids
B. DIAGNOSIS
1. The diagnosis is made by finding eggs or proglottids
2. Adhesive cellophane tape technique and stool examination detect eggs and
proglottids
C. TREATMENT AND PREVENTION
1. The drugs of choice are praziquantel or niclosamide, which act directly on the
worm
2. Sewage disposal, meat inspection, and adequate cooking
PORK TAPEWORM: Taenia solium
I. PARASITOLOGY
1. T. solium strobila shorter than in T. saginata
2. Eggs infective to swine and to humans
3. Tissue cysticerci develop in humans and swine
II. PORK TAPEWORM DISEASE
A. EPIDEMIOLOGY
1. Particularly common in south and southeast Asia, Africa, Latin America, and
Eastern Europe
2. T. solium rarely found in United States
III. PORK TAPEWORM DISEASE: CLINICAL
ASPECTS
A. MANIFESTATIONS
1.
2.
3.
4.
Major clinical manifestations caused by reaction to cysticerci
Meningoencephalitic syndrome with eosinophilia produced by CNS invasion
Multiple small cysts formed
Focal neurologic signs and epilepsy related to cysts
B. DIAGNOSIS
1. Presence of adult worm diagnosed from proglottids
2. Biopsy required for cysticerci
C. TREATMENT
1. Patients with parenchymal lesions usually respond to prolonged treatment with
praziquantel or albendazole
2. Surgery occasionally needed for cysticercosis
FISH TAPEWORM: Diphyllobothrium latum
I. PARASITOLOGY
1. D. latum has broad proglottids
2. Eggs release motile coracidia in water
3. Ingested by crustacean and fish intermediates where larvae migrate to
musculature
4. Humans infected by ingesting inadequately cooked fish
II. FISH TAPEWORM DISEASE
A. EPIDEMIOLOGY
1. Worldwide distribution
2. Worm found in Alaska, midwestern states, and Florida
3. Eating raw fish increases risk
A. FISH TAPEWORM DISEASE: CLINICAL ASPECTS
A. MANIFESTATIONS
1. Occasional intestinal obstruction
2. Vitamin B12 deficiency related to consumption by worm
B. DIAGNOSIS
1. D. latum produces large numbers of ova
2. Eggs demonstrated in stool
C. TREATMENT AND PREVENTION
1. Treatment is carried out as described for T. saginata infections
2. Fish rendered noninfectious at –10°C for 48 hours
HYMENOLEPIS
1. “Dwarf tapeworm” is the only tapeworm that can be transmitted directly from
human to human
2. Endemic areas include parts of Asia, Europe, Central and South America and
Africa
3. Transmission is fecal-oral, either directly or via contaminated food
4. Most persons are asymptomatic, but heavy worm burdens may be associated with
diarrhea, abdominal cramping and anorexia
ECHINOCOCCUS GRANULOSUS
I. PARASITOLOGY
1. Adult in small intestine of canines
2. Herbivores and humans serve as intermediates
3. Larvae penetrate to portal or systemic circulation
4. Cysts and daughter cysts develop in tissues
5. Cycle completed with ingestion of cysts by canine
II. ECHINOCOCCOSIS
A. EPIDEMIOLOGY
1. Pastoral infections maintained by allowing dogs to feed on sheep viscera
2. Hand-to-mouth infection of humans by dog contact
3. Sylvatic cycle in Alaska and western Canada
III. ECHINOCOCCOSIS: CLINICAL ASPECTS
A. MANIFESTATIONS
1.
2.
3.
4.
Disease caused by mechanical effects of cysts after many years
Pulmonary cysts predominate in sylvatic disease, hepatic in pastoral
Cysts may attain large size
Rupture leads to hypersensitivity manifestations and dissemination
B. DIAGNOSIS
1. Radiologic and scanning appearance characteristic
2. Serologic diagnosis important but needs improved sensitivity
C. TREATMENT AND PREVENTION
1. Treatment may include careful aspiration with concomitant albendazole
2. Infected dogs should be wormed, and infected carcasses and offal burned or
buried
ECHINOCOCCUS MULTILOCULARIS
1. Adult worms are found in the gut of foxes and, to a lesser extent, coyotes
2. Domestic dogs may acquire adult tapeworms by killing and ingesting these larvalinfected sylvatic rodents
3. Humans are infected with larval forms through the ingestion of eggs passed in the
feces of their domestic dogs
4. Larvae bud externally; produce multilocular cysts
5. The clinical course characterized by epigastric pain; obstructive jaundice; and,
less frequently, metastasis to the lung and brain