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Merck Manual: Cestodes (Tapeworms)

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http://www.merck.com/mrkshared/mmanual/section13/chapter161/161g.jsp

 

 

Merck Manual

 

Cestodes (Tapeworms)

 

Adult cestodes are typically long, flat, segmented worms that lack a

digestive tract and absorb nutrients directly from the host's small

intestine. An adult tapeworm has three recognizable portions. The

scolex (head) functions as a holdfast organ. The neck is an

unsegmented region of high regenerative capacity. If treatment fails

to eliminate the neck and scolex, the entire worm may regenerate. The

rest of the worm consists of numerous proglottides (segments).

Segments closest to the neck are undifferentiated. As proglottides

move caudally, the sex organs are formed (tapeworms are

hermaphroditic); distal segments are gravid and contain eggs in the

uterus.

FISH TAPEWORM INFECTION

(Diphyllobothriasis)

 

Infection of the intestinal tract with Diphyllobothrium latum, which

is often asymptomatic but may cause vitamin B12 deficiency and

megaloblastic anemia.

Etiology and Pathogenesis

 

The infection occurs worldwide, but especially in cool-lake regions of

all continents, where sewage contaminates freshwater fish. Infections

in the USA occur in people who eat raw fish.

 

The adult worm inhabits the human intestinal tract. Undeveloped eggs

are released from the proglottides in the intestinal lumen and are

passed in the stool. A free-swimming ciliated larva (coracidium)

hatches from the egg in fresh water, is ingested by microcrustaceans,

and develops into a procercoid larva. Infective larvae develop in the

flesh of freshwater fish that eat infected microcrustaceans. The worms

mature in the human small intestine, begin to lay eggs in about 1 mo

(about 1 million per worm/day), and may live months or years.

Symptoms, Signs, and Diagnosis

 

Infection is usually asymptomatic, but mild GI symptoms may be noted.

Fish tapeworms take up dietary vitamin B12 from the upper small

intestine, which causes pernicious anemia in about 1% of infected

persons, especially in Scandinavians. Characteristic operculated eggs

are easily found in the stool.

Prevention and Treatment

 

All freshwater fish should be thoroughly cooked or frozen at -10° C

(14° F) for 48 hours to prevent infection. Treatment is with a single

oral dose of praziquantel, 5 to 10 mg/kg. Vitamin B12 may be needed to

correct the anemia. Certain antiparasitic drugs may have limited

availability; manufacturers can be contacted to locate suppliers.

BEEF TAPEWORM INFECTION

(Taeniasis Saginata)

 

Infection of the intestinal tract with the cestode Taenia saginata,

which is usually asymptomatic.

Etiology and Pathogenesis

 

The ribbonlike adult worm inhabits the human intestinal tract.

Egg-containing proglottides are passed in the stool and ingested by

cattle. The eggs hatch in the cattle, liberating embryonic oncospheres

that invade the intestinal wall and are carried by the bloodstream to

striated muscle, where they develop in 2 mo into a cysticercus, ie, a

small cyst that contains a single inverted scolex. Humans are infected

by eating the cysts in raw or undercooked beef. The cysticerci attach

to intestinal mucosa and mature in about 2 mo. Adult worms (usually

only 1 to 2 are present) may live several years.

 

The infection occurs worldwide but especially in cattle-raising

regions of the tropics and subtropics in Africa, the Middle East,

Eastern Europe, Mexico, and South America. Infection is uncommon in

U.S. cattle and is monitored by federal inspection.

Symptoms, Signs, and Diagnosis

 

The infection is usually asymptomatic, although epigastric pain,

diarrhea, and weight loss may occur. Passage of a motile segment often

brings an otherwise asymptomatic patient to medical attention.

 

The diagnosis is usually made by finding typical gravid proglottides

or, more rarely, the scolex in stool. Occasionally, eggs from ruptured

segments appear in feces. Eggs are indistinguishable from eggs of

other Taenia sp. The perianal area may also be examined by pressing

the sticky side of cellophane tape against the area, placing the tape

on a glass slide, and microscopically examining it for eggs. Eggs may

also be present on anal swabs.

Prevention and Treatment

 

Infection may be prevented by cooking beef to a minimum of 56° C (133°

F) for 5 min. Meat inspection and adequate sanitation help to control

infection.

 

Treatment is with a single oral dose of praziquantel, 5 or 10 mg/kg.

Alternatively, a single 2-g dose of niclosamide is given as 4 tablets

(500 mg each) that are chewed one at a time and swallowed with a small

amount of water (0.5 g is the dose for children 2 to 5 yr old, 1 g for

older children). Both drugs have cure rates of about 90%. Treatment

can be considered successful when no proglottides are passed again

within 4 mo.

PORK TAPEWORM INFECTION

(Taenia Solium Infection; Cysticercosis)

 

Infection of the intestinal tract, often asymptomatic, with the adult

cestode Taenia solium; infection with larval stages may lead to

neurocysticercosis with seizures.

Etiology and Pathogenesis

 

Humans become infected by eating pork containing cysticerci. Humans

may also act as intermediate hosts and develop cysticercosis by

accidental ingestion of T. solium eggs from human excreta; or, if an

adult tapeworm is present in the intestine, regurgitation may bring

gravid proglottides from the intestine to the stomach, where

oncospheres may hatch and start migration to subcutaneous tissue,

muscle, viscera, and CNS.

 

Neurocysticercosis is a very common problem and a major cause of

epilepsy in Latin America, South Africa, and India. Infection in the

USA is rare, except in immigrants from endemic areas.

Symptoms and Signs

 

Infection with the adult worm is usually asymptomatic. Viable

cysticerci cause only a mild tissue reaction, but death of the cysts

elicits an intense tissue response; thus, symptoms often do not appear

for 4 to 5 yr after infection. Infection in the brain may provoke

severe symptoms. Symptoms result from mass effect, inflammation after

degeneration of a parasite, and obstruction of foramina and

ventricles. Patients may present with seizures, signs of increased

intracranial pressure, hydrocephalus, focal neurologic signs, altered

mental status, or aseptic meningitis. Cysticerci may also infect the

spinal cord and eye.

Diagnosis

 

The diagnosis is usually made by finding typical gravid proglottides

in stool. Eggs from ruptured segments may appear in feces or be found

on anal swabs, but they are indistinguishable from eggs of other

Taenia species. Eggs are present in <= 30% of stool samples from

patients with cysticercosis.

 

X-rays of the brain or muscle may show calcified cysts. CT or MRI is

frequently diagnostic, showing many solid nodules or cysts, calcified

cysts, ring-enhancing lesions, or hydrocephalus. A highly sensitive

and specific immunoblot assay is available.

Prevention and Treatment

 

Thoroughly cooking pork prevents infection with adult tapeworms.

Prevention of cysticercosis in endemic areas may be difficult:

infection may occur despite scrupulous personal hygiene and eating

habits. Cysticercosis is also spread by airborne eggs in contaminated

dust.

 

A person infected with T. solium should be treated promptly and

carefully to eliminate the adult worm(s). Treatment of intestinal

infection is the same as for T. saginata. The recommended drugs cause

the disintegration of proglottides, thereby releasing eggs that

theoretically could cause infection.

 

The treatment of choice for cerebral cysticercosis is praziquantel (50

mg/kg/day po for 15 days). Albendazole (15 mg/kg/day po for 28 days)

may be an equally effective and less expensive drug but is not yet

available in the USA. Not all patients respond to praziquantel, and

not all patients must be treated (cysts may already be dead, or the

inflammatory response to treatment may be worse than the disease).

Corticosteroids (such as dexamethasone 4 to 16 mg/day) and

anticonvulsants may be required to reduce the symptoms due to

degenerating cysts. Surgery may be necessary for obstructive

hydrocephalus, infection of the 4th ventricle, and spinal and ocular

cysticercosis.

HYDATID DISEASE

(Echinococcus granulosus Infection; Echinococcosis)

 

Infection with larvae of Echinococcus granulosus, which may cause

cysts in the liver and other organs.

Etiology and Pathogenesis

 

Eggs from the feces of dogs, wolves, and other canines are ingested by

herbivore animals (such as sheep or moose) or humans. The oncospheres

penetrate the intestinal wall, migrate via the circulation, and lodge

in the liver, lungs, and, less frequently, in the brain, bone, and

other organs. The larva develops slowly (usually over many years) into

a large unilocular, fluid-filled bladder--the hydatid cyst. Brood

capsules sprout from these cysts; within these capsules are numerous

small infective scolices. Large cysts may contain several liters of

highly antigenic hydatid fluid as well as millions of scolices.

Daughter cysts sometimes form within or outside primary cysts.

 

The disease is common in sheep-raising areas of the Mediterranean,

Middle East, Australia, New Zealand, South Africa, and South America.

Foci also exist in regions of Canada, Alaska, and California.

Symptoms and Signs

 

Most infections are acquired in childhood, but except when cysts are

in vital organs, clinical signs may not appear for decades. Signs and

symptoms resemble those of a space-occupying tumor. Most cysts are

found in the liver, where they eventually produce abdominal pain or a

palpable mass. Jaundice may occur if the bile duct is obstructed.

Rupture into the bile duct, abdominal or peritoneal cavity, or lung

may produce fever, urticaria, or a serious anaphylactic reaction.

Released scolices may produce metastatic infection. Pulmonary cysts

are usually discovered on routine chest x-ray. Some rupture, causing

cough, chest pain, and hemoptysis. Eosinophilia may be present.

Diagnosis

 

CT and ultrasound scans may be pathognomonic if daughter cysts are

present, but simple hydatid cysts are difficult to differentiate from

a simple epithelial cyst. The presence in cyst fluid of hydatid sand

(debris of old scolices and hooklets) is diagnostic. Chest x-ray may

show a round, often irregular, pulmonary mass of uniform density. The

Casoni skin test is often positive but lacks sensitivity and

specificity. Serologic tests are often positive. Detection of

antibodies to the echinococcal arc 5 antigen or demonstration of

typical bands by immunoblot are highly specific.

Prevention and Treatment

 

Infection can be prevented by not feeding infected game to dogs and

not allowing dogs to feed on sheep carcasses and offal. Dogs in

sheep-raising areas should be dewormed repeatedly.

 

Careful surgery, sometimes via laparoscopy, is the treatment of

choice. Some centers are now performing percutaneous aspiration under

CT guidance followed by instillation of a scolecoidal agent and

reaspiration (the PAIR technique). Albendazole (400 mg bid po for 28

days, 15 mg/kg/day in children) may suppress the growth or kill cysts

and is indicated for inoperable cases or for cases of intraoperative

cyst spillage to prevent metastatic infections.

ALVEOLAR HYDATID DISEASE

(Multilocular Echinococcosis)

 

Infection with Echinococcus multilocularis.

 

The adult worms are found in foxes, and the hydatid larvae in small

wild rodents. Infected dogs are the main link to occasional human

infection. The life cycle of the organism is similar to E. granulosus.

 

The larvae form irregular alveolar cysts that usually contain no

scolices. The germinative tissue and brood capsules spread rapidly and

produce spongy tumors that are difficult or impossible to treat

surgically. The cysts are found mainly in the liver but can

metastasize to the lungs, lymph nodes, and other tissues.

 

E. multilocularis is present mainly in Central Europe, Alaska, Canada,

and Siberia. The range of natural infection in the USA extends from

Wyoming and the Dakotas to Indiana and Ohio.

 

Symptoms, signs, and diagnosis are similar to hydatid disease (see

above). Prognosis is poor, unless the entire larval mass can be

removed. High-dose mebendazole or albendazole may suppress the growth

of the parasite. Concomitant interferon-gamma may enhance the efficacy

of these drugs. Liver transplantation has been lifesaving in a small

number of patients.

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