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PARASITOLOGY - CHAPTER FIVE

 

CESTODES (TAPE WORMS)

 

 

 

 

 

TEACHING OBJECTIVES

Epidemiology, morbidity and mortality

Morphology of the organism

Life cycle, hosts and vectors

Disease, symptoms, pathogenesis and site

Diagnosis

Prevention and control

 

 

Clinically important cestodes pathogenic to man are Tenia solium (pork

tapeworm), T. saginata (beef tapeworm), Diphyllobothrium lattum (fish

or broad tapeworm), Hymenolepis nana (dwarf tapeworm) and Echinococcus

granulosus and E. multilocularis (hydatid).

 

 

Tenia solium T. saginata (Teniasis)

 

Epidemiology

These cestodes have a worldwide distribution but incidence is higher

in developing countries. Infection rate is as low as 1 per 1000 in

most of North America and as high as 10% in the third world. Pork

tapeworm shows a higher incidence but this is dependent on dietary habits.

 

Morphology

 

T. saginata can be up to 4 to 6 meters long and 12 mm broad; it

has a pear-shaped head (scolex) with four suckers but no hooks or

neck. It has a long flat body with several hundred segments

(proglottids). Each segment is about 18 x 6 mm with a branched uterus

(15-30 branches). The egg is 35 x 45 micrometers, roundish and

yellow-brown. It has peripheral radial striations and contains an

embryo with 3 hooklets (figure 2).

 

T. solium is slightly smaller than T. saginata. It has a globular

scolex with four suckers and a circular row of hooks (rostellum) that

gives it a solar appearance. There is a neck and it has a long flat

body (0.1 meter in length). The proglottids are 5 x 10 mm with a 7-12

branch uterus. The eggs of T. solium and T. saginata are

indistinguishable (figure 2).

 

Life cycle

A tapeworm larval cyst (cysticercus) is ingested with poorly cooked

infected meat; the larva escapes the cyst and passes to the small

intestine where it attaches to the mucosa by the scolex suckers. The

proglottids develop as the worm matures in 3 to 4 months. The adult

may live in the small intestine as long as 25 years and pass gravid

proglottids with the feces. Eggs extruded from the proglottid

contaminate and persist on vegetation for several days and are

consumed by cattle or pigs in which they hatch and form cysticerci

(Figure 1).

 

Symptoms

Light infections remain asymptomatic, but heavier infections may

produce abdominal discomfort, epigastric pain, vomiting and diarrhea.

 

Cysticercosis

T. solium eggs can also infect humans and cause cysticercosis (larval

cysts in lung, liver, eye and brain) resulting in blindness and

neurological disorders. The incidence of cerebral cysticercosis can be

as high 1 per 1000 population and may account for up to 20% of

neurological case in some countries (e.g., Mexico); cysticercosis

ocular involvement occurs in about 2.5% of patients and muscular

involvement is as high as 10% (India).

 

Pathology and Immunology

Gastrointestinal symptoms are due to the presence of the tape worm.

Cysticercosis symptoms are a result of inflammatory/immune responses.

Antibodies are produced in cysticercosis and are useful

epidemiological tools.

 

Diagnosis

Diagnosis is based on the recovery of eggs or proglottids in stool or

from the perianal area. Cysticercosis is confirmed by the presence of

antibodies.

 

Treatment and control

Praziquantel is the drug of choice. Expulsion of scolex must be

assured to assume a satisfactory treatment. A thorough inspection of

beef and pork, adequate cooking or freezing of meat are effective

precautions, since cysticerci do not survive temperatures below -10o C

and above 50o C.

 

 

 

Figure 1

Taenia-lc.gif (21938 bytes) Figure 1

 

Life cycle of Taenia saginata and Taenia solium

Humans are the only definitive hosts for Taenia saginata and Taenia

solium. Eggs or gravid proglottids are passed with feces ; the eggs

can survive for days to months in the environment. Cattle (T.

saginata) and pigs (T. solium) become infected by ingesting vegetation

contaminated with eggs or gravid proglottids . In the animal's

intestine, the oncospheres hatch , invade the intestinal wall, and

migrate to the striated muscles, where they develop into cysticerci.

A cysticercus can survive for several years in the animal. Humans

become infected by ingesting raw or undercooked infected meat . In

the human intestine, the cysticercus develops over 2 months into an

adult tapeworm, which can survive for years. The adult tapeworms

attach to the small intestine by their scolex and reside in the small

intestine . Length of adult worms is usually 5 m or less for T.

saginata (however it may reach up to 25 m) and 2 to 7 m for T. solium.

The adults produce proglottids which mature, become gravid, detach

from the tapeworm, and migrate to the anus or are passed in the stool

(approximately 6 per day). T. saginata adults usually have 1,000 to

2,000 proglottids, while T. solium adults have an average of 1,000

proglottids. The eggs contained in the gravid proglottids are

released after the proglottids are passed with the feces. T. saginata

may produce up to 100,000 and T. solium may produce 50,000 eggs per

proglottid respectively. CDC DPDx Parasite Image Library

 

 

Diphyllobothrium latum (fish or broad tapeworm)

 

Epidemiology

Fish tapeworm infection is distributed worldwide, in the subarctic and

temperate regions; it is associated with eating of raw or improperly

cooked fresh water fish.

 

Morphology

This is the longest tapeworm found in man, ranging from 3-10 meters

with more than 3000 proglottids. The scolex resembles two

almond-shaped leaves and the proglottids are broader than they are

long, a morphology reflected in the organism's name. Eggs are 30 x 50

micrometers in size and contain an embryo with 3 pairs of hooklets

(figure 4).

 

Life cycle

Man and other animals are infected by eating uncooked fish that

contains plerocercoid larvae (15 x 2 mm) which attach to the small

intestinal wall and mature into adult worms in 3 to 5 weeks. Eggs

discharged from gravid proglottids in the small intestine are passed

in the feces. The egg hatches in fresh water to produce a ciliated

coracidium which needs to be ingested by a water flea (Cyclops) where

it develops into a procercoid larva. When infected Cyclops are

ingested by the freshwater fish, the procercoid larva penetrates the

intestinal wall and develops into a plerocercoid larva, infectious to

man (figure 3).

 

Symptoms

Clinical symptoms may be mild, depending on the number of worms. They

include abdominal discomfort, loss of weight, loss of appetite and

some malnutrition. Anemia and neurological problems associated with

vitamin B12 deficiency are seen in heavily infected individuals.

 

Diagnosis

Diagnosis is based on finding many typical eggs and empty proglottids

in feces (Figure 3). A history of raw fish consumption and residence

in an endemic locality is helpful.

 

Treatment and control

Praziquantel is the drug of choice. Freezing for 24 hours, thorough

cooking or pickling of fish kills the larvae. Fish reservoirs should

be kept free of raw sewage.

 

Hymenolepis nana (dwarf tapeworm)

 

This is a small tapeworm (20 x 0.7 mm) which infects children. Rodents

are the reservoir. Infection is by the oro-fecal mode and, hence,

cross infection and auto infection by eggs in feces in normal (figure

6). The worm develops from ingested eggs into an adult in the small

intestine and resides there for several weeks (figure 5). Light

infections produce vague abdominal disturbances but heavier infections

may cause enteritis. Diagnosis is based on finding eggs in the feces.

Nicolsamide is the drug of choice. Hygiene is the best control.

 

 

Echinococcosis (hydatid)

 

Echinococcus granulosus and E. multilocularis are causative agents of

hydatid cysts.

 

 

Echinococcus granulosus

 

Epidemiology

The organism is common in Asia, Australia, Eastern Africa, southern

Spain, southern parts of South America and northern parts of North

America. The incidence of human infection about 1 to 2 per 1000

population and may be higher in rural areas of affected regions.

 

Morphology

This is the smallest of all tapeworms (3 to 9 mm long) with only 3

proglottids.

 

Life cycle

The adult worm lives in domestic and wild carnivorous animals. Eggs,

passed by infected animals, are ingested by the grazing farm animals

or man, localize in different organs and develop into hydatid cysts

containing many larvae (proto-scolices or hydatid sand) (Figure 8).

When other animals consume infected organs of these animals,

proto-scolices escape the cyst, enter the small intestine and develop

into adult worms (Figure 7). Echinococcus eggs, when swallowed by man,

produce embryos that penetrate the small intestine, enter the

circulation and form cysts in liver, lung, bones, and sometimes,

brain. The cyst is round and measures 1 to 7 cm in diameter, although

it may grow to be 30 cm. The cyst consists of an outer anuclear

hyaline cuticula and an inner nucleated germinal layer containing

clear yellow fluid. Daughter cysts attach to the germinal layer,

although some cysts, known as brood cysts, may have only larvae

(hydatid sand). Man is a dead end host.

 

Symptoms

The symptoms, comparable to those of a slowly growing tumor, depend

upon the location of the cyst. Large abdominal cysts produce

increasing discomfort. Liver cysts cause obstructive jaundice.

Peribronchial cysts may produce pulmonary abscesses. Brain cysts

produce intracranial pressure and Jacksonian epilepsy. Kidney cysts

cause renal dysfunction. The contents of a cyst may produce

anaphylactic responses.

 

Diagnosis

Clinical symptoms of a slow-growing tumor accompanied by eosinophilia

are suggestive. Intradermal (Casoni) test with hydatid fluid is

useful. Pulmonary cysts and calcified cysts can be visualized using

x-rays. Antibodies against hydatid fluid antigens have been detected

in a sizable population of infected individuals by ELISA or indirect

hemagglutination test.

 

Treatment and control

Treatment involves surgical removal of cyst or inactivation of hydatid

sand by injecting the cyst with 10% formalin and its removal within

five minutes. It has been claimed that a high dose of Mebendazole

results in some success. Preventive measures involve avoiding contact

with infected dogs and cats and elimination of their infection.

 

he adult Echinococcus granulosus (3 to 6 mm long) (1) resides in the

small bowel of the definitive hosts, dogs or other canids. Gravid

proglottids release eggs (2) that are passed in the feces. After

ingestion by a suitable intermediate host (under natural conditions:

sheep, goat, swine, cattle, horses, camel), the egg hatches in the

small bowel and releases an oncosphere (3) that penetrates the

intestinal wall and migrates through the circulatory system into

various organs, especially the liver and lungs. Inthese organs, the

oncosphere develops into a cyst (4) that enlarges gradually, producing

protoscolices and daughter cysts that fill the cyst interior. The

definitive host becomes infected by ingesting the cyst-containing

organs of the infected intermediate host. After ingestion, the

protoscolices (5) evaginate, attach to the intestinal mucosa (6), and

develop into adult stages (1) in 32 to 80 days. The same life cycle

occurs with E. multilocularis (1.2 to 3.7 mm), with the following

differences: the definitive hosts are foxes, and to a lesser extent

dogs, cats, coyotes and wolves; the intermediate host are small

rodents; and larval growth (in the liver) remains indefinitely in the

proliferative stage, resulting in invasion of the surrounding tissues.

With E. vogeli (up to 5.6 mm long), the definitive hosts are bush dogs

and dogs; the intermediate hosts are rodents; and the larval stage (in

the liver, lungs and other organs) develops both externally and

internally, resulting in multiple vesicles. E. oligarthrus (up to 2.9

mm long) has a life cycle that involves wild felids as definitive

hosts and rodents as intermediate hosts. Humans become infected by

ingesting eggs (2), with resulting release of oncospheres (3) in the

intestine and the development of cysts (4) in various organs CDC

DPDx Parasite Image Library

 

E. multilocularis

 

This is a tapeworm, similar to E. granulosus, that also causes hydatid

in northern parts of Asia and North America. It has a very similar

morphology and life cycle except that rodents are its intermediate

host. Humans, when infected with this worm, also develop hydatid cysts

which produce symptoms similar to those caused by E. granulosus.

However, the cysts are multilocular (many chambers). The organism is

resistant to praziquantel; high doses of Albendazole has some

anti-parasitic effect. Surgery is the means of removing the cyst.

Rodent control is the means of prevention.

 

 

Organism

 

 

Transmission

 

 

Symptoms

 

 

Diagnosis

 

 

Treatment

Tenia saginata Cyst in beef Epigastric pain, vomiting, diarrhea

 

Proglottids or eggs in stool or perianal area

Praziquantel

Tenia solium Cyst in pork Epigastric pain, vomiting, diarrhea

 

Proglottids or eggs in stool or perianal area

Praziquantel

T. solium Cysticercosis Oro-fecal Muscle pain and weakness, ocular

and neurologic problems

 

Roentgenography, anti-cysticercal antibody (EIA)

Praziquantel

D. latum Cyst in fish Abdominal pain, loss of weight, anorexia,

malnutrition and B12 deficiency problems Proglottids or eggs in stool

or perianal area Praziquantel

E. granulosus Oro-fecal Large cysts produce various symptoms

depending on the location of the organism. Roentgenography,

anti-hydatid fluid antibody (EIA), Casoni skin test Surgery, formalin

injection and drainage, Praziquantel

E. multiloculoris Oro-fecal As above As above Surgery, Albendazole

 

 

 

 

 

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