Cestodes: Tapeworms - General Concepts, Clinical Manifestations

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Cestodes

 

 

Donald Heyneman

 

General Concepts

Clinical Manifestations

 

Adult Tapeworms: Adult worms are found in the small intestine; these

infections are usually well tolerated or asymptomatic, but may cause

abdominal distress, dyspepsia, anorexia (or increased appetite),

nausea, localized pain, and diarrhea.

 

Larval Tapeworms: Larvae locate in extraintestinal tissues and produce

systemic infections with clinical effects related to the size, number,

and location of cysts. Taenia solium cysticercosis (infection with the

cysticercus larval stage) is often asymptomatic and chronic;

neurocysticercosis, ophthalmic cysticercosis, and subcutaneous and

muscular cysticercosis are, however, frequently reported. Echinococcus

granulosus hydatid larvae may form massive cysts in liver, lungs, and

other organs, including long bones and the central nervous system.

Structure

 

Adults, which mature sexually in the definitive or final host, are

ribbon-shaped, multisegmented, hermaphroditic flatworms; each segment

has a complete male and female reproductive system. An anterior

holdfast organ (the scolex) is followed by a germinative portion

( " neck " ) and segments at successively later stages of development.

Larvae encyst in various tissues of the intermediate host; larval

cysts contain one or many scoleces of future adult worms.

Multiplication and Life Cycle

 

The tapeworm's life cycle involves a definitive and one or more

intermediate hosts (except for the one-host cycle of Hymenolepis

nana). Each type of cycle has specialized larval forms (cysticercus,

cysticercoid; coenurus, hydatid; coracidium, procercoid, plerocercoid).

Pathogenesis

 

Pathology due to adult worms results from the physical presence and

activity of the large worms (Taenia species), occasional erosive

action (causing local inflammation) by scolex hooks (T solium, H

nana), or reduced host intake of vitamin B12 (Diphyllobothrium latum).

Allergic reactions may also be responsible for symptoms such as

headache, dizziness, inanition, and anal and nasal pruritus.

Host Defenses

 

Adult worms are probably only weakly immunogenic, although some

evidence exists for a cell-mediated host response; moderate

eosinophilia and increased IgE may occur. Hymenolepis nana, in

contrast, elicits a strong immune response when directly infected by

the eggs, since the larvae develop within the villi (see page 1105).

Larvae elicit strong immunity against reinfection that is derived from

both cell-mediated and humoral responses induced by antigenic

stimulation of tissues.

Epidemiology

 

Infective larvae are acquired by eating contaminated raw or

undercooked meat, grains, or fish. Taenia solium cysticercosis or H

nana can be transmitted in a direct cycle via ingestion of eggs from

human feces. Echinococcus eggs from dog or fox fur cause human hydatid

disease (humans are the intermediate host; canids are the definitive

hosts). Reinfection with adult tapeworms is common; second infections

with larvae are rare. T solium cysticercosis may be acquired by

autoinfection; internal autoinfection with H nana from a cysticercoid

infection is possible.

Diagnosis

 

Adult Worms: Taenia infections are diagnosed by finding gravid

segments in stool specimens; the eggs of these species are

indistinguishable. Other species are diagnosed on the basis of eggs in

stool specimens.

 

Larval Worms: Cysts in tissues may be identified in biopsy specimens,

by radiography (calcified cysts), and by computed tomography (brain

cysts). Serology (indirect hemagglutination, ELISA) is useful but of

variable sensitivity and specificity. A history of travel in endemic

areas is often of great importance.

Control

 

Meat should be cooked thoroughly or frozen at -10°C for 10 days; beef

and pork should be inspected for Taenia ( " measly meat " ); human feces

should not contaminate drinking water; sheepdogs should be treated and

should not be fed sheep viscera. Humans may be treated with

praziquantel or niclosamide.

INTRODUCTION

 

Tapeworms are ribbon-shaped multisegmented flatworms that dwell as

adults entirely in the human small intestine. The larval forms lodge

in skin, liver, muscles, the central nervous system, or any of various

other organs. Their life cycles involve a specialized pattern of

survival and transfer to specific intermediate hosts, by which they

are transferred to another human host. Each pattern is characteristic

of a given tapeworm species.

 

In general, the common gut-dwelling adult cestodes are well adapted to

the human host, induce few symptoms, and only rarely cause serious

pathology. This reality belies innumerable fearsome and largely

apocryphal stories of tapeworms stealing food and causing ravenous

hunger (far more commonly, the appetite is depressed). Larval

cestodes, however, develop in human organs or somatic tissues outside

of the gut and are therefore far more pathogenic.

 

The adult cestodes elicit little host inflammatory or immune response

in contrast to the strong responses elicited by the larval stages in

tissues. Adult cestodes are often acquired by ingestion of meat from

intermediate hosts. Extraintestinal infection with larvae results from

ingestion of eggs of fecal origin. Diagnosis of infection with adult

cestodes is based on identification of eggs and segments

(proglottides) in feces. Larval infections are more difficult to

assess; serology and biopsy are helpful. Control depends on

sanitation, personal hygiene, and thorough cooking of meat and fish.

Taenia saginata, The Beef Tapeworm

Clinical Manifestations

 

The clinical manifestations of infection with adult T saginata

tapeworms are confined to occasional nausea or vomiting, appetite

loss, epigastric or umbilical pain, and weight loss.

 

Moderate eosinophilia may develop. A disturbing manifestation of T

saginata infection is the active crawling of the muscular segments out

of the anus. Rarely, intestinal perforation may occur from the scolex

of Taenia, or proglottides may be vomited and then aspirated.

Structure

 

Adults are ribbonlike, flattened, segmented, hermaphroditic flatworms

5 to 10 m long, consisting of scolex, neck, and immature, mature, and

ripe segments in linear sequence. The distinctive morphologic and

physiologic properties of the adult tapeworm reflect on the one hand

their remarkable specialization for survival in the vertebrate

intestine, and on the other hand their massive reproductive powers

which are made possible by the multiple sexual units, the proglottides

or segments. This ensures the worm species against the enormous rate

of loss of the segments or eggs passed in the feces, with only the

most remote probability of any one egg succeeding in reaching an

intermediate host and being transferred to another human. The terminal

one-third to one-half of the worm's length consists of gravid

(egg-filled) segments. These segments are muscular and can crawl

caterpillar-fashion through the anal sphincter to the outside

environmentwhich renders them available to their herbivore

intermediate hosts.

 

The larval cyst of T saginatathe cysticercusis a pea-sized,

fluid-filled cyst, which develops in the muscles of the intermediate

host. Within the cyst is a single inverted scolex, formed from a

germinative portion of the inner cyst wall (Fig. 89-1).

 

FIGURE 89-1 Larval types found in the taeniid tapeworms. (From Muller

R: Worms and Disease: A manual of Medical Helminthology. William

Heinemann Medical Books, London, 1975, with permission.)

Multiplication and Life Cycle

 

Figure 89-2 illustrates the life cycle of T saginata. Gravid segments

break off from the worm and are carried in the fecal bolus or by their

own crawling activity to the soil. The segments move away from the

bolus and adhere to grass. If ingested by a bovine intermediate host,

the segments are digested open in the gut, each releasing 50,000 to

100,000 eggs. The eggs hatch, each releasing a six-hooked larva, the

oncosphere (also called the hexacanth), which penetrates the gut wall

and reaches the muscles via the circulation. There the oncosphere

fills with fluid and develops into the 8-mm cysticercus. If a human

eats raw or undercooked infected beef, the cysticercus is digested

free and inverts the scolex, which attaches to the wall of the small

intestine and begins to bud off the long chain of segments. In about 3

months the worm reaches 4-5 m in length and gravid segments begin to

pass through the anus. The worm is long-lived, surviving 5 to 20 years

or more.

 

FIGURE 89-2 Life cycles of Taenia solium and T saginata. (a) The port

(pig) tapeworm (outer developmental cycle); (b) the beef (cattle)

tapeworm (inner developmental cycle).

 

A. Final host: humans only. Tapeworm in the small intestine

 

1. Tapeworm with its head (see B-5 below)

 

1a. Mature segment of T solium

 

1b. Mature segment of T saginata

 

2. Tapeworm egg (= embryophore with six-hooked larva; the eggs of

Taenia species cannot be morphologically differentiated)

 

B. Intermediate host

 

a. Pig; humans rarely (Ba1) cysticercosis)

 

b. Cattle

 

3-4. Cysticercus (measle) in different stages of evagination of

the scolex

 

3a. Cysticerus cellulosae of T solium (with its crown of

hooklets and four suckers commencing evagination

 

3b. Cysticercus bovis of T saginata (with four suckers only)

 

4. Evaginated cysticerus stage of T solium (a) and T saginata (b)

 

5. Head:

 

a. of the pork tapeworm (with a crown of hooklets)

 

b. of the beef tapeworm (without a crown of hooklets)

 

I-III. Phases of the movements of freshly detached tapeworm segments

 

(From Piekarski G: Medical Parasitology. Springer-Verlag, Heidelberg,

1989, with permission.)

Pathogenesis

 

Rare intestinal blockage or penetration have been reported, but

pathology is usually inconsequentialalthough the psychological

distress at passing motile segments may be extreme.

Host Defenses

 

Because of its limited contact with the epithelial lining, the

gut-dwelling adult tapeworm induces little host inflammatory,

allergic, cell-mediated, or humoral response. The sucking action of

the scolex appears to have relatively limited immunogenic effect. The

long life span of the worm suggests the absence of an effective

inhibitory mechanism.

Epidemiology

 

Taenia saginata, the commonest large tapeworm of humans, is

transmitted as cysticerci in beef ( " measly beef " ). Partially cooked,

smoked, or pickled beef can be infective, although raw beef (steak

tartare) is the commonest bearer of infection, as witnessed by the

frequency of taeniasis in countries such as Ethiopia and Argentina

where raw or undercooked beef is often eaten.

 

Large worms may grow by 15 to 30 cm a day in the human gut, passing 10

segments daily, which may convey up to a million eggs a day into the

environment throughout the long life span of the worm. Eggs may also

be found in pastures flooded by human sewage or on which human sewage

is used as fertilizer.

Diagnosis

 

Adult infections can be diagnosed by identifying segments in the

feces. The species of Taenia can be identified only by the segments,

because their eggs are identical. The uterus of T saginata usually

forms 12 to 20 branches on each side of the main uterine stem, whereas

there are 7 to 10 branches in the smaller and relatively wider T

solium segment (see Fig. 89-2).

Control

 

Inspection of beef for cysticerci is the best preventive measure. Beef

must be thoroughly cooked in endemic areasto at least 56°C throughout

the meat, which may be difficult to accomplish with large cuts of

fatty meat, particularly pork. Freezing at 10°C for 10 days usually is

lethal to Taenia cysticerci, but they can withstand 70 days at 0°C.

 

Treatment is readily available for the intestinal adult worms.

Niclosamide, is a nonabsorbed oxidative phosphorylation inhibitor that

kills the scolex and anterior segments on contact, after which the

worm is expelled. Praziquantel, a synthetic isoquinoline-pyrazine

derivative, is an equally effective and relatively nontoxic cesticidal

compound. Since the scolex is usually but not always destroyed, and a

new worm can regenerate if the scolex and a minute portion of the neck

survive, the patient should be observed for several months, as

egg-bearing segments can reappear in 10-12 weeks.

Taenia solium, The Pork Tapeworm

Clinical Manifestations

 

The clinical effects of adult T solium infection are similar to those

caused by T saginata.

Structure

 

The scolex of T solium differs from that of T saginata in possessing

an anterior circle of sharply spined hooks arranged in a double row.

These are under muscular control and work with the four suckers to

adhere to the gut wall. As described above, gravid segments of T

solium can be distinguished from those of T saginata by the number of

outpocketing branches of the uterus. The adult worm is usually 3 to 5

m long. Gravid segments tend to be less muscular and more square than

those of T saginata.

Multiplication and Life Cycle

 

As shown in Figure 89-2, the life cycle of T solium is similar to that

of T saginata except that the pig is the principal intermediate host.

Because the gravid segments are less motile than those of T saginata,

they are usually eliminated in the fecal matter and remain in the

fecal bolus (which increases the chance of infecting pigs, which are

coprophagous).

 

Of great clinical importance is the fact that humans who ingest eggs

from human feces, as distinct from cysticerci in undercooked pork, may

develop the larval infection just as pigs do, resulting in the serious

disease cysticercosis.

Pathogenesis

 

The hooked scolex of T solium may cause greater intestinal

disturbance, pain, and inflammatory response than that caused by T

saginata, but symptoms are still generally mild and the pathology

minor. However, T solium larval infection (cysticercosis) is a

potentially dangerous systemic infection, the degree of damage

depending on the site and number of cysticerci that develop. Infection

most commonly occurs in the central nervous system (CNS). It is also

frequently found in muscles and subcutaneous tissues. The globe of the

eye is also a common site. In the CNS, the larvae most often occupy

the brain hemispheres. They may also be found in the cisternae and

ventricles. Hydrocephalus may result from obstruction of cerebrospinal

fluid flow. Infection in specific sites can induce epilepsy, mental

disturbances, or a meningeal syndrome. Nevertheless, up to one-half of

CNS infections are thought to be asymptomatic. After death of the

scolex within the cystoften years after infectionthe capsule becomes

fibrosed or calcified.

Host Defenses

 

Owing to the systemic migration and tissue localization of the

cysticerci, cysticercosis elicits considerable host sensitization.

This response is usually insufficient to block the initial infection

but probably renders the normal host immune to a subsequent one. Much

of the damage from cysticercosis is caused by the severe inflammatory

host response that occurs after the death and disruption of the parasite .

Epidemiology

 

A remarkable and tragic aspect of T solium infection is the ability of

this worm to develop both adult and larval stages in humans. If T

solium eggs are ingested (from fecally contaminated water or by

anus-to-mouth transfer of infective eggs), they may hatch in the gut

and spread systemically, causing human cysticercosis. It appears

likely (although it is unproven) that human cysticercosis may also be

caused when reverse peristalsis, induced by adult T solium in the gut,

returns gravid segments into the duodenum, where the eggs hatch and

release invasive oncospheres. Cysticerci develop to potency in about 3

months and may live many years. Cysticerci that die may become

calcified, rendering them demonstrable by radiography.

 

Human cysticercosis is a serious and widespread disease, being

especially common in Latin America. The disease is frequently found

among Mexican agricultural workers in California and other Western

states. Human cysticercosis apparently cannot develop from eggs of T

saginata.

Diagnosis

 

Diagnosis of adult worm infection is similar to that for T saginata.

Cysticercosis is difficult to diagnose and usually requires

radiologic, serologic, and clinical assessment. Subcutaneous nodules

can usually be felt or observed, and can be sampled by biopsy. The

enzyme-linked immunosorbent assay (ELISA) is useful, especially with

the purified antigens that are now available. Plain radiographs of

soft tissues may demonstrate the oval or elongated cysts (4-10 mm X

2-5 mm) if they are wholly or partially calcified. Cysts in muscles

are usually aligned with the fibers. Soft tissue or brain

calcifications are strongly indicative of cysticercosis. Plain skull

films may show cerebral calcifications or indicate intracranial

hypertension. Computed tomography is the most useful procedure, as it

detects calcified and noncalcified cysts as well as edema or

intracranial hypertension.

Control

 

The control of infection of humans as definitive hosts is the same as

that for T saginata, except that the control measures apply to pork

not beef. In addition, human sewage from infected individuals may

contaminate the source of drinking water. The eggs are highly

resistant and can withstand many months of environmental exposure over

a broad temperature range. Treatment for adult T solium is the same as

for T saginata.

 

Cysticercosis may require surgery for ophthalmic or brain involvement,

but chemotherapy should precede surgery when possible. Tissue

infection can be treated with albendazol (taken with a fatty meal to

increase absorption or praziquantel (combined with corticosteroids to

reduce the inflammatory response to the dead cysticerci). Praziquantel

should not be used for ocular or spinal cord infections.

Taenia multiceps, The Coenurus Tapeworm

 

The adult worm of Taenia multiceps is found in dogs or wild canids.

The larva is a bladderworm with multiple scolecesfrom a few to 100 or

morein an encysted vesicle. This vesicle, usually 2 - 5 cm in

diameter, is called a coenurus (Fig. 89-1). The usual intermediate

host is the sheep. Human infection can occur from accidental ingestion

of dog feces containing the eggs.

 

Infection in humans usually occurs in the brain in temperate areas,

and in the eye or subcutaneous tissues in tropical areas. Diagnosis

and treatment are similar to those for Echinococcus infection, which

may be difficult to distinguish from coenurus infection. Treatment is

chiefly surgical, although the drugs used for cysticercosis may also

be effective against coenurus infection.

Echinococcus granulosus, The Hydatid Tapeworm

Clinical Manifestations

 

Echinococcosis (hydatid disease) results from the presence of one or

more massive cysts, or hydatids, which can develop in any tissue site,

including the liver, lungs, heart, brain, kidneys, and long bones. The

clinical manifestations of this infection therefore vary greatly,

depending on the site and size of the cyst, but resemble those of a

slow-growing tumor that causes gradually increasing pressure.

Infections in the liver, lungs, or subcutaneous tissue sites may be

asymptomatic for many years, but pressure effects eventually develop.

In sensitive or vital areas, hydatids produce a panoply of symptoms,

chiefly owing to mechanical compression or blocking effects but also

include collapse of infected long bones, blindness, and epileptiform

seizures. The rupture of a hydatid cyst may induce sudden anaphylactic

shock in a previously asymptomatic individual.

Structure

 

Adult E granulosus tapeworms are relatively minute, consisting of 3 to

5 segments, and usually are less than 1 cm long. Dogs and wild canids

are the only final hosts in which the adults are found, often adhering

in great numbers to the small intestinal mucosa. The scolex has four

suckers and is crowned with a circle of spines as in T solium. It is

followed by a germinative neck region, one developing segment, and

usually one gravid segment containing several hundred eggs.

 

The hydatid larva is found in sheep, in many other herbivoresand in

humans. In humans the cyst is slow-growing, but in a period of years

may reach a diameter of 30 cm with a 1 mm thick, laminated sheath

surrounded by fibrous reactive host tissue. The cyst is usually

fluid-filled and, if viable, has a germinative inner lining from which

many thousands of scoleces are budded off into the lumen or remain

attached to the germinative wall (Fig. 89-1). The floating scoleces

often enlarge, become vesicular, and develop into daughter floating

colonies within the parent cyst. These in turn may bud off a third

generation of cysts within themselves. The result is an enclosed sac

containing myriads of future adult worms ready to infect a dog or

other susceptible carnivore that feeds on the hydatid-infected animal

and the scolex-filled cyst fluid.

Multiplication and Life Cycle

 

Echinococcus granulosus causes a zoonosis; the adult is a parasite of

canids. The dog-sheep cycle (Fig. 89-3) is the one most germane to

humans Other animal cycles, such as dog-camel, dog-horse, wolf-moose,

wolf- or coyote-deer, also occur, each with a degree of specific

host-parasite adaptation, as well as a distinct geographic range. Eggs

passed by the dog can be ingested by sheep or other herbivores, or by

humans who have close contact with feces-contaminated dog fur. Within

the sheep or human intermediate host, the eggs hatch, the oncospheres

penetrate the gut, migrate, and ultimately one or several may form the

enormous hydatid cysts. Because the scoleces in the hydatid fluid

resemble sand grains, they are called hydatid sand. If a cyst bursts

within the human body, it can give rise to dozens of new cystslimited,

in most cases, by a strong cellular immune response of the host.

 

FIGURE 89-3 Life cycles of Echinococcus granulosus and E multiocularis.

 

A. Final host: dog (and other canids)

 

1.a. Echinococcus granulosus (beside it, the worm is reproduced

approximately half its natural size)

 

b. Embryophore, the so-called egg with its six-hooked larva

(oncosphere)

 

B. Intermediate host: sheep (for E granulosus); field mouse (for E

granulosus): mouse (for E multilocularis)

 

2a. Liver with Echinococcus cyst (hydatid stage)

 

b. Diagram of an Echinococcus hydatid with daughter cysts and

scoleces (compare with III)

 

c. Humans as the accidental intermediate host (echinococcosis);

the organs most often infected are the liver and the brain

 

3-4. Isolated scoleces: invaginated (3), and evaginated (4) on

which the crown of hooklets and the suckers can be seen

 

I. Echinococcus multiocularis, sexually mature worm

 

II. Human liver infected by the hydatid of E multilocularis

 

III. Hydatid cyst of E granulosus opened up; daughter cysts

visible.

 

Pathogenesis

 

Jaundice and portal hypertension can result from pressure effects of a

cyst in the liver; hemoptysis and dyspnea from a lung cyst; and acute

inflammatory effects may follow brain or spinal cord infections.

Host Defenses

 

The migrating and growing larvae, and antigens that leak from the

cyst, induce a strong immune responsebut rarely one capable of

penetrating and destroying the cyst. Ruptured cysts may cause

anaphylaxis and the appearance of new cysts in other sites, suggesting

an active but ineffectual immune response.

Epidemiology

 

Echinococcus granulosus is most common in temperate sheep-raising

areas: southern South America, the southern and central Russia, East

Africa, and the western United States. The source of most human

infections is sheepdog feces containing E granulosus eggs (which often

adhere to the fur of dogs petted by humans). Killed sheep fed to dogs

maintain the infection; sheep ingest eggs with dog feces in their

grazing. Other species of Echinococcus are also found in Africa, South

America, and elsewhere, sustained by similar canid-prey relationships.

Diagnosis

 

Symptoms of a tumorlike, slowly growing mass with eosinophilia are

strongly suggestive, especially in an endemic sheep-raising area.

Isolated hooks in the sputum suggest rupture of a lung cyst. The

serologic tests that are currently most useful are indirect

hemagglutination, latex agglutination, and ELISA. However, both false

positives and false negatives are frequent and may have dangerous

repercussions (see Morris and Richard, 1992; Schantz et al., 1990).

Ultrasound imaging (US), magnetic resonance (MR), and computed axial

tomography (CT) have greatly improved the diagnosis of deep-seated

lesions and also can demonstrate avascular fluid-filled cysts.

Radiographs are less precise or useful, although they demonstrate the

hollow cyst areas.

Control

 

Effective control is chiefly epidemiologic: denying sheep dogs access

to carcasses of infected sheep, obligatory testing and treatment of

all sheep dogs, prevention of contact of children with possibly

infected sheep dogs, and widespread education on the danger and method

of spread of hydatid disease.

 

Treatment is chiefly by surgical resection (with extreme care to avoid

or decontaminate spillage). Recent work suggests that a long course of

albendazole may kill the scoleces within the cyst and even reduce the

size of the cyst. Long, continued use of mebendazole has also proved

effective, although the results are variable. A recent approach

involves percutaneous puncture under sonographic guidance, aspiration

of cyst fluid, instillation of a protoscolecidal agent such as 95%

methanol or cetrimide, and respiration (PAIR), along with albendazole

treatment to reduce the danger of subsequently renewed disease from

spillage (see Giorgio et al., 1992).

Echinococcus multiocularis, The Multioculate or Alveolar Hydatid Tapeworm

 

Echinococcus multilocularis, which normally follows a fox-rodent cycle

in northern Siberia and North America, is occasionally conveyed to

human fur trappers via fox pelts. In humans it causes a frequently

fatal form of echinococcosis. The appearance and life cycle of this

cestode closely resemble those of E granulosus, except for the

restricted range and small number of hosts. The cyst, however, is

extremely dangerous as it lacks the laminated membrane that confines

the cyst of E granulosus, and develops an invasive, uncontrolled

series of connected chambers (hence the designation " multiloculate "

and the alternative name alveolar hydatid). It therefore resembles a

malignant growth, capable of budding off to cause metastatic spread.

The primary cyst usually forms in the liver. The disease is usually

diagnosed late, when it is inoperable, and ends fatally. Early

radiologic imaging by US, CT, or MR is essential. Serological tests,

particularly with purified E multilocularis antigens, are sensitive

and highly specific. Treatment with mebendazole, albendazole, or

praziquantel, and surgery should follow.

Hymenolepis nana, The Dwarf Tapeworm

Clinical Manifestations

 

Hymenolepis nana infections are often asymptomatic, especially in

light cases. Heavy infections can induce enteritis with nausea and

vomiting, diarrhea, abdominal pain, and dizziness. Massive infection

with several thousand worms may follow autoreinfection.

Structure

 

These small worms, 15 to 50 mm long, have minute segments that are

wider than long, a foursucker scolex with a retractable spined

anterior rostellum, and terminal gravid segments that break up and

release their egg load after they are caught up in the fecal bolus.

 

The larval form is a cysticercoid, a tailed structure that has a

withdrawn scolex and lacks a fluid filled bladder. Typically,

Hymenolepis larvae are found in insect or crustacean intermediate

hostswith the sole and remarkable exception of H nana, whose

cysticercoid larvae can develop either in an insect or in the small

intestinal villi of its human (or rodent) final host.

Multiplication and Life Cycle

 

The life cycle of this parasite is shown in Figure 89-4. Infection is

acquired most commonly from eggs in the feces of another infected

individual, which are transferred in food, by contaminated fingers, or

in sewage-contaminated drinking water.

 

FIGURE 89-4 Life cycle of Hymenolepis nana.

 

A. Final host: humans (also dog, rodents)

 

1. Hymenolepis nana, sexually mature worm, about 1/2 natural size

 

1a-1b. Development without an intermediate host

 

1a. Egg with six-hooked larva (oncosphere)

 

1b. Cysticercoid from the intestinal mucosa (compare with I below)

 

B. Intermediate host: for example, a rodent flea (including larva and

pupa)

 

2a. Development within intermediate host, in which the flea larva

takes up the egg of the tapeworm, to a

 

2b. Cysticercoid from the body cavity of the flea (tail appendage

with the hooks left in it)longitudinal section

 

I. Villus of the small intestine of a mouse with cysticercoid

(transverse section)

 

II. Scolex of Hymenolepis nana

 

III. Immature segments of the tapeworm

 

IV. Mature segments of the tapeworm

 

(From Piekarski G: Medical Parasitology. Springer-Verlag, Heidelberg,

1989, with permission.)

 

The ingested eggs hatch in the duodenum, and the oncospheres penetrate

only into the villi (Fig. 89-5). There, each oncosphere forms a

cysticercoid larva that emerges, 4 to 5 days later, into the gut lumen

as a young scolex and neck; the scolex attaches to the mucosa, the

neck proceeds to strobilate, and the worm reaches full size in 5 to 10

days. The adult worm sheds gravid terminal segments, which

disintegrate in the intestine, releasing eggs that are passed in the

feces. When these eggs are ingested by another (nonimmune) human, this

direct or one-host life cycle begins again. Worms live only a short

time, perhaps 4 to 6 weeks. Rodents also can harbor these worms and

may serve as reservoir hosts, infecting humans via their pellets.

 

FIGURE 89-5 Diagrammatic section of an adult and larval H nana in the

gut of a mouse. In heavy infections in this host, much of the mucosal

lining is abraded. (From Muller R: Worms and Disease: A Manual of

Medical Helminthology. William Heinemann Medical Books, 1975, with

permission.)

 

Remarkably, an indirect, two-host cycle may also occur, involving

grain beetles, fleas, or other insects that feed on contaminated

rodent droppings. Insects that ingest the H nana eggs can serve as

hosts for the cysticercoid larvae. Humans who accidentally ingest

infected grain beetles (some of which, such as Tribolium, are only 2

to 3 mm long) digest the cysticercoid free; digestive enzymes then act

on the cysticercoid to release the scolex, which attaches and develops

by this indirect cycle into an adult worm identical to that acquired

by the direct life cycle.

 

A third mode of infection is by internal infection or autoreinfection.

Eggs from worms acquired in an initial infectionprobably via the

indirect, insect cycle, which is nonimmunizing (see Host Defenses,

below)can hatch, invade the villi, and produce a second generation of

worms. Since many eggs can be involved, this pathway can lead to

massive infection with several thousand worms.

Pathogenesis

 

Little or no pathology occurs from development of cysticercoids in the

villi, and only after a heavy infection (perhaps produced by

autoreinfection) do symptoms develop from the adult worms. Children

may be particularly subject to massive worm loads and show the most

severe intestinal symptoms.

Host Defenses

 

The tissue phase of the direct cycle of H nana infection (Figs. 89-4

and 89-5) initiates a profound cellular and humoral response,

rendering most hosts immune to subsequent infection (as demonstrated

experimentally in rodents). In contrast, the indirect cycle through

infected insects does not involve mucosal embryogenesis in humans and

induces little or no immunity, even permitting occasional massive

internal reinfection to occur. The immune response is seldom effective

against the initial infection because the tissues have already been

invaded and a protective cyst formed by the time the response develops.

Epidemiology

 

Hymenolepis nana is probably the commonest human cestode, owing to its

wide distribution, particularly in crowded areas, such as India and

China. The direct infectiousness of the eggs frees the parasite from

its former dependence upon an insect intermediate host, making rapid

infection and person-to-person spread possible. The short life span

and rapid course of development also facilitate the spread and ready

availability of this worm. Congested areas, day-care centers, and

crowded institutions such as prisons frequently have high levels of

infection with H nana, despite its strong immunizing capacity and

short life span.

Diagnosis

 

H nana infections can be diagnosed accurately and rapidly by

inspecting the stool for eggs.

Control

 

Preventing fecal contamination of food and water in institutions and

crowded areas is of primary importance. General sanitation and rodent

and insect control (especially control of fleas and grain insects) are

also essential for prevention of H nana infection. Treatment with

praziquantel or niclosamide is usually effective, and can be repeated

if necessary.

Hymenolepis diminuta, The Rat Tapeworm

 

The rat tapeworm, which is larger than H nana (up to 40 cm long), has

a life cycle involving grain insects, similar to the indirect cycle of

H nana. H diminuta rarely infects humans, but may do so if a human

eats an insect carrying cysticercoids of this worm. The infection is

most common in children, causes a mild diarrhea, is diagnosed by

finding the characteristic eggs in the stool, and is readily treated

with praziquantel.

Dipylidium caninum, The Double-Pored Tapeworm

 

Dipylidium caninum causes a cosmopolitan infection of dogs and cats.

Fleas are the intermediate hosts in which the infective cysticercoids

develop. Children in close and continuous contact with pets are

occasionally infected as a result of the accidental ingestion of an

infected flea. The infection is usually asymptomatic and is

self-limited, although praziquantel would probably be an effective

treatment. Flea control of pets would largely eliminate the infection

from household pets and children.

Diphyllobothrium latum, The Broad Fish Tapeworm

Clinical Manifestations

 

Infection with Diphyllobothrium latum is usually asymptomatic,

although occasional diarrhea, abdominal pain, fatigue, vomiting,

dizziness, or numbness of fingers and toes may be present.

Eosinophilia develops during the early stages of worm growth.

Structure

 

Diphyllobothrium latum is the largest parasite of humans, reaching

lengths up to 10 m and consisting of a chain of 3,000 to 4,000

segments, each up to 2 cm wide. The adult worm, a member of the order

Pseudophyllidea, is characterized by a scolex with a pair of linear

sucking grooves instead of suckers and hooks, and by having a

rosette-shaped uterus connected to the outside by a uterine pore

through which the eggs are passed. Hence, mature segments produce eggs

until they die and are shed, rather than by breaking off as intact

egg-filled segments, as in Taenia. Up to a million eggs can be

produced daily. The developmental stages are (1) the ciliated,

swimming coracidium that hatches from the egg, (2) the procercoid that

develops in the copepod primary host, and (3) the plerocercoid (or

sparganum), a nonencysted, nonsegmented larval worm, 20 mm or more in

length, found in the fish secondary hosts. The plerocercoid develops

into the adult tapeworm in the small intestine of a fish-eating final

host, such as human, cat, dog, or bear.

Multiplication and Life Cycle

 

Diphyllobothrium latum is the only adult cestode of humans that has an

aquatic life cycle (Fig. 89-6). Eggs are passed in feces of an

infected human (or bear, dog, cat, wolf, raccoon, or other freshwater

fish-eating reservoir host). If passed into lake or pond water, the

eggs develop in 2 or more weeks (varying with the temperature) and

hatch, releasing the spherical ciliated coracidium that contains the

oncosphere. When ingested by an appropriate water flea (copepods such

as Cyclops or Diaptomus), the coracidium sheds the ciliated coat,

penetrates into the hemocoel, and changes in 2 to 3 weeks into the 0.5

mm, tailed second-stage embryo, the procercoid. If the infected

copepod is then ingested by a minnow or other fish, the procercoid

penetrates the fish gut in a few hours and later develops into a

third-stage larva, the plerocercoid or sparganum. Usually, these small

infected fish are eaten by larger ones; in each new fish host, the

plerocercoid penetrates into the fascia or muscles. Eventually, a

large game fish, such as a perch or pike, is infected; after being

eaten by a human, the fish releases its tapeworm passenger, which

attaches and begins adult life. In a few months, the worm is 5 to 10 m

long.

 

FIGURE 89-6 Life cycle of Diphyllobothrium latum.

 

A. Final host: Humans, dogs, cats (and other fish-eating domesticated

and wild animals). Site of the tapeworm: small intestine

 

1. Egg after it is passed in feces

 

2. Ciliated larva, the coracidium, containing the embryo with six

hooklets (oncosphere)

 

B. First intermediate host: small crustacean (copepod such as Cyclops)

 

3a. Six-hooked larva emerged from the coracidium in gut of a Cyclops

 

3b. Procercoid in body cavity of Cyclops

 

C. Second intermediate host: predatory fish or fish such as carp.

 

4. Trout with a plerocercoid (sparganum); final larval stage

 

4a. Isolated plerocercoid

 

I. Sexually mature D latum; pieces of tapeworm com posed of

proglottides indifferent stages of maturity

 

Ia. Mature proglottid with rosette-shaped uterus (See III)

 

II.a. Scolex, spatula-shaped

 

b. Transverse section of a scolex; the suctorial grooves on the

two sides are seen clearly

 

III. Mature tapeworm segment (proglottid)

 

e. Mehlis' gland

 

h. Testes

 

o. Ovary

 

g. Sexual opening

 

u. Uterus

 

IV. A single egg from the stool

 

(From Piekarski G: Medical Parasitology. Springer-Verlag, Heidelberg,

1989, with permission.)

Pathogenesis

 

Infection with this tapeworm usually produces no pathology, although

the minor symptoms noted above are occasionally present. Megaloblastic

anemia ( " tapeworm anemia " ), which is exacerbated by the worm's uptake

of vitamin B12, is now seldom seen, as a result of improved diet,

prenatal care, and ready treatment. This condition was formerly most

common in Finland.

Host Defenses

 

Little or no protective immunity develops, owing to the lack of an

intimate tissue phase in the human host. Reinfection is common.

Epidemiology

 

Infection with the broad fish tapeworm is common in temperate and

subarctic regions, wherever freshwater fish are eaten rawas in

Scandinavia, Siberia, the Great Lakes, Japan, central Europe, and Chile.

Diagnosis

 

The ovoid, operculated eggs passed in abundance in the human stool are

diagnostic. Occasionally, strands of exhausted segments with the

characteristic rosette-shaped uteri are also passed.

Control

 

Plerocercoids in fish are quickly killed by thorough cooking, freezing

at -10°C for 15 minutes, or thorough pickling. Treatment of sewage

before it enters lakes greatly reduces the prevalence of infection, as

has been demonstrated in Finland. Treatment with praziquantel or

niclosamide is effective and nontoxic.

Spirometra

 

Sparganosis is a tissue infection with the sparganum (or plerocercoid)

of Spirometra, a genus related to Diphyllobothrium. These two genera

have similar life cycles, but Spirometra usually utilizes frogs,

reptiles, or various small mammals as intermediate hosts, whereas

Diphyllobothrium uses fish. In the Southeast Asia, frog flesh (rather

than beef steak) is used as a poultice over a wound or black eye,

which allows the sparganum to crawl into the wound or orbit,

initiating a severe inflammatory response. Humans can also acquire the

infection as a result of drinking water containing infected Cyclops

and possibly from undercooked snake or other infected meat. The

procercoids from Cyclops invade the gut wall of the human or animal

intermediate host and usually migrate to subcutaneous tissues to form

a sparganum, which induces in humans formation of a fibrous 2-cm

nodule that encloses and destroys the worm. The nodule can usually be

removed surgically or can be treated with praziquantel if the cyst is

inaccessible to surgery.

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