Prevention

Prevention is attained primarily by controlling the mice and rat populations along with good hygiene and sanitation.

Figure 51 D. caninum scolex demonstrating the armed rostellum.

466

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Taenia solium

General characteristics

 T. solium, the pork tapeworm, is the intestinal cestode capable of causing serious pathologic damage to the human host.

Humans serve as the definitive host, whereas pigs serve as the intermediate host. Humans can alsoserve as the intermediate

host. T. solium may result in an intestinal infection in which the larvae mature and reside in the small intestine for up to 25

years. The organisms can grow to be 1.5 to 8 m long and produce more than 1000 proglottids, each containing about

50,000 eggs. Cysticercosis is the extraintestinal form of the disease and can be much more severe. The disease is life

threatening if the organism invades the central nervous system.

Pathogenesis and spectrum of disease

T. solium infection can result in the presence of both adult and larval stages in the human host Infection begins

when the intermediate host ingests embryonated eggs in feces. Once the egg is ingested, the hexacanth embryo

is released into the intestine where the embryo penetrates the mucosa. The embryo then matures into a cyst

(cysticercus) in the tissue. Humans may become infected when they eat raw or undercooked pork containing

embedded cysts. Pork tapeworm infection is usually caused through the ingestion of multiple worms. During

ingestion and subsequent digestion of the infected meat, the cysticercus is released and attaches to the mucosa

within the small intestine of the human host. The cysticercus matures into an adult worm within approximately

5 to 12 weeks. The eggs are then released in the host’s feces. Accidental ingestion of the eggs by the human

host may also result in migration of the embryo through the intestine to other areas of the body.including the

eyes, brain, muscle, or bone. In addition, the proglottids are motile and may migrate out of the anus. Infection

of the adult tapeworm causes few clinical symptoms, although abdominal pain, diarrhea, indigestion, and loss

of appetite may be present as a result of irritation to the mucosa of the intestinal wall. The major complication

with T. solium is cysticercosis (larval forms throughout the body), in which the human host becomes the

intermediate host and harbors the larvae in tissues as previously described.

Laboratory diagnosis

Serologic diagnosis is unreliable for infections with T. solium. Diagnosis of Taenia tapeworm infection is

through the examination of stool samples. Individuals suspected of infection with T. solium should be asked if

they have passed any notable tapeworm segments in their stool.

Additional laboratory findings may include a low-grade eosinophilia, increased serum IgE level, and the

presence of atypical lymphocytes in the cerebrospinal fluid.

Figure 52 T. saginata scolex with suckers.

467

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Therapy

Adult worms can be eradicated with praziquantel or niclosamide. Expulsion of the scolex must be assured to

assume satisfactory treatment.

Prevention

Good hygiene and immediate treatment are essential for the prevention of autoinfection.

Taenia saginata

General characteristics:

T. saginata or beef tapeworm has a worldwide distribution and is more common than T. solium. The worm can

grow 4 to 12 m and contain 1000 to 2000 segments. T. saginata may produce 100,000 eggs and live up to 25

years in the human intestine.

Epidemiology

T. saginata has a similar life cycle to that of T. solium. Cattle are the intermediate hosts and humans are

infected through the ingestion of cysticerci (larval form) in raw or undercooked beef.

Pathogenesis and spectrum of disease

The life cycle of T. saginata begins with human ingestion of undercooked or raw meet infected with larvae.

The larvae are ingested in the meat and, following digestion, released into the small intestine where the worm

attaches to the mucosa and matures. In about 3 months, the worm may grow up to 4 to 5 m in length and gravid

segments begin to break off and pass in stool. Following deposition of gravid segments in the soil, an

intermediate bovine host may ingest the segments. The segments are digested and the eggs hatch, releasing an

oncosphere that penetrates the muscle tissue. Following penetration of the mucosa the organisms are carried via

the lymphatic vessels and bloodstream throughout the intermediate host. Humans then ingest the infected meat

of the intermediate host, as previously indicated. Humans typically are asymptomatic, or have very mild

indigestion, loss of appetite, vomiting, and abdominal discomfort. A rare case of severe infection may result in

intestinal obstruction and appendicitis. Patients are often unaware of their infection until gravid motile

segments are passed in the feces and cause psychological distress.

Laboratory diagnosis

The stool should be examined for proglottids and eggs; eggs may also be present on anal swabs. The eggs of T.

saginata are indistinguishable from those of T. solium. The uterus of T. saginata is longer than wide and

typically contains 15 to 18 lateral branches on each side . The scolex has four suckers and is unarmed or does

not contain any hooklets (Figure 52). Stool specimens should be handled with care since the eggs cannot be

distinguished from those of T. solium. Slight eosinophilia may develop.

Therapy

 Recommended treatment includes praziquantel or niclosamide. Treatment of T. saginata can be considered

468

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

successful when no proglottids are passed for 4 consecutive months.

Prevention

Beef should be inspected for cysticerci and thoroughly cooked before ingesting

Tissue Cestodes:

Cestodes (Tapeworms) Tissue (Larval Forms):

Taenia solium

Echinococcus granulosus

Echinococcus multilocularis

Taenia multiceps

Spirometra mansonoides

 Tissue cestodes do not reach the adult stage in the human host. The organism infect the human in their

intermediate or cyst stage. The infections are much more serious than those caused by the adult tapeworm. The

parasites can cause serious disease, or even death. Larval cestodes cause infection by accidental ingestion of

eggs excreted from the intermediate hos and they lodge in various organs and tissues in the human body.

Diagnosis of larval infections can be problematic.

TAENIA SOLIUM

General characteristics

 Taenia solium, also known as the pork tapeworm, causes an intestinal infection from eating contaminated

pork . The adult worm usually causes no clinical disease. Humans may accidentally become the intermediate

host and ingest eggs from human feces. This typically occurs when an individual is already infected with adult

T. solium. Autoinfection occurs when the individual swallows eggs from improper hand washing. Humans may

develop the larval infection, which could result in cysticercosis. Cysticercosis is usually asymptomatic unless

larvae invade the central nervous system (CNS), the globe of the eye, or other muscle and tissues.

Pathogenesis and spectrum of disease

Clinical signs and symptoms depend on the location, viability, and number of the cysticerci present. Cysticerci

can develop in any organ or tissue of the body. The severity of the symptoms depends on the body site involved

and may not appear for years after the initial infection. The most severe cases are found in the central nervous

system and the eye. Once cysticerci localize in the brain, the organism causes a condition referred to as

neurocysticercosis. Infection can cause epileptic-type seizures, headaches, mental disturbances, meningitis, or

sudden death. Cysticerci can also be found in the eye and must be removed to avoid permanent eye damage,

including blindness. Much of the damage from cysticercosis is caused by the severe inflammatory host

response that occurs after the cysticerci have died. Antibodies are produced and offer the patient secondary

immunity.

469

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Laboratory diagnosis

 Cysticercosis can be difficult to diagnose. T. solium eggs are found in stools in fewer than half the patients with

cysticercosis. Demonstration of eggs or proglottids in the feces is an indication of Taenia infection but does not provide a

diagnosis for cysticercosis . Definitive diagnosis usually requires the identification of cysticercus in the tissue. The

organism is surgically removed and microscopically examined for the presence of suckers and hooks on the scolex. The

cysticercus is round to oval, translucent, and about 5 mm or more in diameter. The organism has a scolex with four suckers

and a rostellum with a circle of hooks. Fine needle aspiration cytology may be helpful in the diagnosis and eliminates the

need for surgical biopsy. Diagnosis may also be made using computed tomography (CT) scans and magnetic resonance

imaging (MRI). Radiographs may also be useful in detecting calcifying cysticerci within tissue. Ocular cysticercosis may be

diagnosed by visual identification of the larval worm. Serologic procedures (such as enzymelinked immunosorbent assay

[ELISA]) may also be used as a useful tool to aid in diagnosis, but may not be sensitive enough in light infections. The

Centers for Disease Nucleic acid-based methods and species-specific polymerase chain reaction (PCR) have been described

to differentiate Taenia species.

Therapy

Cysticercosis should be treated with corticosteroids, anticonvulsants, and surgery if deemed appropriate.

Surgery may be required for ocular, spinal, or brain involvement.

Prevention

Education, meat inspection, and improvement of sanitation measures are the key preventive measures.

ECHINOCOCCUS GRANULOSUS

General characteristics

Echinococcus is the smallest of all tapeworms (3 to 9 mm long) with three to five proglottids. It contains a

scolex with four suckers and a rostellum with hooks to attach to the intestinal wall. E. granulosus is a tapeworm

found in the small intestine of the definitive host, the canine. Eggs are ingested by the intermediate hosts and

include a variety of mammals including sheep, cattle, moose, and humans. There are several strains of

Echinocococcus granulosus that have been identified, with the dog-sheep strain being the most common.

Humans are typically accidental hosts and are considered a dead-end since the life cycle of the organism is

unable to continue in a human host. Oncospheres hatch in the intestine of the intermediate host and invade the

circulatory system, where they develop into hydatid cysts. Disease symptoms vary with the site and size of the

cyst. Echinococcosis (hydatid disease) results from the presence of one or more cysts (hydatids), which can

develop in any tissue.

Pathogenesis and spectrum of disease

 Hydatid disease in humans is potentially dangerous depending on the size and location of the cyst. Some cysts

may remain undetected for many years until they grow large enough to affect other organs. Many humans live

470

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

day-to-day without ever knowing they are infected. The cyst is very slow growing in humans. It is usually

fluid-filled and has a germinal layer from which many thousands of scolices are budded. These are known as

daughter cysts (brood capsules), which attach to the germinal layer or free-float in the cyst. The scolices in the

hydatid fluid resemble grains of sand and are called hydatid sand (Figures 53). The result is a unilocular cyst

containing future adult worms. The cyst may resemble a slow-growing tumor. Infections in the liver or lungs

may be asymptomatic for many years, but the pressure eventually causes noticeable symptoms. The majority of

the hydatid cysts occur within the liver. Cysts within the liver cause chronic abdominal pain and allergic

reactions and may result in cholangitis (infection of the common bile duct) and cholestasis (interference with

flow of bile from the liver). Cysts that develop in the lungs may cause infections and abscesses and result in

chronic cough, shortness of breath, and chest pain. During the life cycle of the cyst, there may be occasional

seepage of fluid into the host tissue and circulation causing sensitization or activation of the immune response

from the presence of the parasite. The rupture and release of the fluid of a hydatid cyst may cause anaphylactic

shock as a result of the primary sensitization in a previously asymptomatic individual. If a cyst bursts within the

human body, many new cysts may be released that are typically eliminated via the host’s cellular immune

response. Leaking fluid from a cyst may cause notable eosinophilia.

Laboratory diagnosis

Clinical symptoms of a slow-growing abdominal tumor with or without eosinophilia are suggestive of infection.

Human infection ranges from asymptomatic to severe, including death. Diagnosis is made through the

identification of cysts in the infected organ, accompanied with positive serologic tests. A variety of serologic

tests are available including ELISA, indirect hemagglutination, and latex agglutination. Both false positives and

false negatives may occur; therefore clinical history is extremely important for diagnosis. Ultrasound, magnetic

resonance imaging (MRI), and computed tomography (CT) have

Figure 53:, Echinococcus granules.A B, Ovum. C, Scolex.

471

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

improved the diagnosis and may provide visualization of the fluid-filled cysts. Calcified cysts can be visualized

using conventional x-ray. Microscopic examination of the cyst fluid for the identification of the scolices can be

useful in diagnosis. A 1% eosin stain may be added to the fluid to assist in the visualization and determination

as to whether or not the cyst is viable. Nonviable scolices will stain with the eosin whereas viable scolices will

not.

Therapy

Surgery is the most common form of treatment. The procedure involves surgical removal of cysts or

inactivation of hydatid sand by injecting the cyst with 10% formalin and then removing it. Extreme care must

be taken to avoid spillage. Albendazole is the drug of choice to kill the scolices within the cyst, reduce the size

of the cyst, and prevent recurrence. Mebendazole and praziquantel have also been shown to be effective.

Prevention

Preventive measures include avoiding contact with infected dogs and deworming animals regularly. Effective

control includes educating the population concerning the danger and means of transmission of hydatid disease

as well as maintaining good hygiene and practicing safe disposal of dog feces. Slaughtered animals must be

disposed of properly, to prevent dogs from exposure to contaminated materials and interrupt the Echinococcus

life cycle.

ECHINOCOCCUS MULTILOCULARIS

General characteristics

Although rarely found in the brain of humans, E. multilocularis causes alveolar hydatid disease, which is a fatal

form of echinococcosis. It is the most lethal of all helmintic diseases. The cyst is extremely dangerous because

it lacks a laminated membrane and develops a series of connected chambers. The chambers contain little or no

fluid and rarely contain a scolex. The morphology of the cyst is very similar to that of E. granulosus, but the

adult organisms are much smaller (1.2 to 3.7 mm).The cysts are very resistant to cold temperatures.

Pathogenesis and spectrum of disease

Figure 54:Echinococcus granulosus, hydatid sand (300×). (Inset)

Two individual hooklets (1000×)

472

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Alveolar hydatid disease is a highly lethal, destructive disease. The cyst of E. multilocularis grows slowly and

may take years to produce clinical symptoms. Many cysts are asymptomatic during the life of the infected

individual and are sometimes found during autopsy, surgery, or imaging scans related to other clinical

conditions. The severity of symptoms depends on the location of the cyst and the size, as seen with E.

granulosus. Cysts form primarily in the liver and metastasize to the lung or brain. Cysts in the liver are not

restricted with a laminated cyst wall and are capable of expansion into a multicystic structure. This

multilocular (many chambers) hydatid cyst is often mistaken for a hepatic sarcoma, making diagnosis difficult.

This disease is often fatal.

Laboratory diagnosis

Ultrasound, CT scans, and MRI are used to visualize the cyst and can be supported with serologic testing.

Serologic tests, such as ELISA, are sensitive and highly specific.

Therapy

The most common treatment is to remove the parasite Surgically. Treatment with mebendazole and albendazole

has been used successfully

Prevention

Controlling rodents is an important means of prevention along with educating the public at risk to avoid

exposure to infective feces. Practicing good hygiene and periodically deworming household pets are also

helpful.

Taenia multiceps

General characteristics

T. multiceps is a tapeworm that causes coenurosis in humans. The coenurus (larval form) may cause

destructive damage or death, but is an extremely rare disease in

humans. The coenurus is a unilocular cyst similar to cysticercus, although the worm has multiple scolices.

Daughter cysts may also be seen. The body of T. multiceps is 5 to 6 cm long and consists of 200 to 250

segments. The scolex has 4 suckers and a proboscis (tubular appendage) with 22 to 32 hooks arranged in 2

rows.

Pathogenesis and spectrum of disease

The oncosphere hatches and penetrates the intestinal wall of the intermediate host. The embryo is carried via

the bloodstream to various parts of the body including the brain, eyes, and central nervous system, where the

organism lodges and the coenurus develops. The coenurus develops into multiple daughter cysts. Symptoms

include headache, vomiting, paralysis, and blindness. The coenurus causes a serious disease called coenurosis

in sheep and in dogs that have eaten the brains of infected sheep. This clinical condition is known as gid,

sturdy, or staggers.

473

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Laboratory diagnosis

Diagnosis is similar to that for Echinococcus infection. CT and MRI may be useful for detecting the cysts.

Microscopic identification can be used if the cyst has been removed surgically. Currently, there are no serologic

tests available.

Therapy

Treatment is similar to that for Echinococcus. The most common treatment is surgery if possible, although the

drugs used for cysticercosis may also be effective against coenurus infection.

Prevention

dogs associated with sheep and other livestock should not be fed the brain or spinal cord from infected animals

and should be dewormed regularly. good hygiene should be practiced and care taken not to eat or drink

anything contaminated with dog feces.

Spirometra mansonoides

General characteristics

Sparganosis is an infection caused by the plerocercoid larvae of Spirometra. The larvae (spargana) are white,

wrinkled, and ribbon-shaped. They may be 3 mm wide and up to 30 cm long. The sparganum has bothria

(longitudinal grooves) instead of suckers. No scolex is present, which can help differentiate Spirometra from

Taenia solium.

Pathogenesis and spectrum of disease

Spargana migrate and lodge anywhere in the human body. Clinical symptoms depend on which organs or

tissues are involved. Spargana can live for several years before symptoms develop. Sparganosis is usually

asymptomatic until the larvae grow and cause an inflammatory reaction. Painful nodules can develop in the

tissues. A variety of symptoms may occur, including seizure, weakness, headache, and eye pain that can lead to

blindness if left untreated.

Laboratory diagnosis

Definitive diagnosis is usually made by removal and identification of the sparganum from infected tissue.

Clinical history, ELISA, MRI, and CT can all be used together to presumptively diagnose sparganosis.

Eosinophilia may also be present.

Therapy

Praziquantel has been used with limited success. Surgical removal of the complete sparganum is the treatment

of choice.

474

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Prevention

Prevention strategies should include safe drinking water practices, and awareness of the dangers of consuming

raw fish and amphibians. Water in contaminated areas should be boiled before consumption.

Intestinal Trematodes:

Helminths Trematodes (Flukes) Intestinal:

Fasciolopsis buski

Heterophyes heterophyes

Metagonimus yokogawai

 The intestinal trematodes (flukes) are members of the phylum Platehelminthe (flatworms), are

dorsoventrically flattened, and require at least one intermediate host (a freshwater snail).

 Human infection occurs by ingestion of metacercariae encysted on freshwater vegetation or fish. Most

trematodes are hermaphroditic (both ovaries and testes are contained within each adult worm).

The parasites are typically identified from eggs shed in the feces. The adult worms are located in the small

intestine, where they lay eggs that may be embryonated or remain unembryonated until they are shed from the

body via feces. The egg continues developing after reaching the water, and a ciliated, free-swimming

miracidium larva is released. The miracidium enters a snail host and develops into a redia (cylindrical larvae),

followed by development into tailed cercariae.

 The cercariae emerge from the snail and encyst as a metacercariae (encrusted larvae) on water plants or fish.

A human host ingests raw or undercooked plants (Fasciolopsis buski) or fish (Heterophyes heterophyes,

Metagonimus yokogawai) containing the metacercariae, which exycyst in the intestinal tract, attach, and mature

into adults .

Fasciolopsis buski

General characteristics

The adults of F. buski have an elongated shape and range from 20 to 75 mm long to approximately 8 to 20 mm

wide , They have an oral sucker at the anterior end and a ventral sucker located about midway to the posterior

end. The eggs, which are indistinguishable from those of Fasciola hepatica (Figure 55), are oval and elongated,

transparent, and yellow-brown with an operculum (lid) at one end, and they range in size from 130 to 140 μm

long to 80 to 85 μm wide and may be unembryonated.

Pathogenesis and spectrum of disease

 the intestinal attachment site of the adult worms often becomes locally inflamed and ulcerated, and may

hemorrhage. moderate to heavy infections may cause abdominal pain, diarrhea, intestinal obstruction, and

edema of the abdomen and lower extremities, and may result in inadequate absorption of vitamin b12.

eosinophilia is commonly observed.

475

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Prevention

Infection can be prevented by making sure that water plants and fish are properly cooked before eating. In

addition, changes are needed in agricultural practices and health education in the endemic areas.

Heterophyes heterophyes: and metagonimus yokogawai

General characteristics

Adult H. heterophyes worms range in size from 1.0 to 1.7 mm in length by 0.3 to 0.4 mm in width, and have a

broadly rounded posterior. M. yokogawai adults range in size from 1.0 to 2.5 mm long to approximately 0.4 to

0.8 mm wide.

 The adult H. heterophyes also has an additional sucker, the genital sucker, which surrounds the genital pore.

The eggs are small, yellow-brown, embryonated, and operculated and may have minimal opercular shoulders.

Eggs range in size from 26 to 30 long μm to 15 to 17 μm wide, and may be indistinguishable between the two

species.

Pathogenicity and spectrum of disease

 Infections with a small number of worms may be asymptomatic. Symptoms in heavy infections may include

abdominal pain, diarrhea with a large amount of mucus, and ulceration of the intestinal wall. Eggs may gain

entry into intestinal capillaries and lymphatics, where they can be carried to the heart, brain, spinal cord, or

other tissues, causing emboli or granuloma formation.

Prevention

Avoid ingestion of raw, inadequately cooked, and pickled or salted fish. The risk of infection could be greatly

reduced by improved sanitary conditions and health education programs.

Laboratory diagnosis

 Identification of the intestinal trematodes is made by recovery of eggs, or in rare cases adults, from stool

specimens using a sedimentation method such as formalinethyl acetate. The sediment may be examined in a

wet mount with or without iodine. Because the eggs of Fasciolopsis buski are identical to those of Fasciola

Figure 55 Fasciola egg. The eggs of F. buski and F. hepatica are indistinguishable

morphologically.

476

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

hepatica, and those of Heterophyes heterophyes and Metagonimus yokogawai are very similar, diagnosis may

also require assessment of symptoms, obtaining a travel history, and/or recovery of adult worms.

Treatment

The drug of choice for treatment of intestinal trematode infection is praziquantel (Biltricide), an isoquinoline

derivative administered orally in three doses for 1 day.

Liver and Lung Trematodes:

Trematodes (Flukes) Liver/Lung:

Clonorchis (Opisthorchis) sinensis

Opisthorchis viverrini

Fasciola hepatica

Paragonimus westermani

Paragonimus mexicanus

 Clonorchis sp., Opisthorchis sp. and Fasciola sp. Live in the biliary ducts of humans. Paragonimus spp. are

found in the lungs and in other body sites.

The liver flukes:

General characteristics

 The adults of these trematodes live in the biliary ducts and in heavy infections may be also found in the

gallbladder. Two of these, Clonorchis sinensis (the Chinese liver fluke) and Opisthorchis viverrini (the

Southeast Asian liver fluke), are elongated and narrow and much smaller than Fasciola (the sheep liver fluke).

These flukes also all require a freshwater snail as an intermediate host.

Life cycle

 The life cycle of the liver flukes is very similar to that of the intestinal flukes. The adult worms produce eggs

in the biliary ducts that are then excreted from the body in the feces. The free-swimming miracidium is released

from the egg in freshwater and enters the snail host where it develops into a redia and then a cercariae, which

leaves the snail and enters the water (Figure 56).

 The cercariae of Clonorchis and Opisthorchis are ingested by a second intermediate host, a freshwater fish.

The cercariae then encyst and develop into the metacercariae within the intermediate host. The metacercaria is

the infective stage for humans. When infected freshwater fish are eaten raw or undercooked, the metacercariae

will excyst in the duodenum and then travel to the bile duct where they mature. The cercariae of Fasciola

encyst on freshwater vegetation, such as watercress and water chestnuts, and develop into metacercariae. When

the infected vegetation is eaten raw, the metacercariae will excyst in the duodenum and then travel to the bile

duct and mature.

477

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Pathogenesis and spectrum of disease:

 Light infections with C. sinensis or O. viverrini are most common, and may be asymptomatic. Heavier

infections with these flukes may present with fever, abdominal pain, and jaundice. Eosinophilia and increased

serum levels of immunoglobulin E (IgE) may be observed. Severe infections may cause obstruction of the

biliary ducts, resulting in enlargement and tenderness of the liver, cirrhosis, cholecystitis (inflammation of the

gallbladder), and cholangiocarcinoma (cancerous growth in bile duct epithelium).

Even light infections with Fasciola may cause fever, abdominal pain, nausea, diarrhea, enlargement and

tenderness of the liver, jaundice, nonproductive cough, eosinophilia, and elevated serum IgE levels. More

severe infections may result in obstruction of the biliary ducts, cirrhosis, cholecystitis, and cholangiocarcinoma.

During migration in the human body, the larvae may penetrate the peritoneal cavity, and adult flukes may then

be found in the intestinal walls, lungs, heart, or brain.

Laboratory diagnosis

 Identification of the liver flukes is primarily made by recovery of the eggs in feces using a sedimentation

method and a wet mount with or without iodine staining. There is also serologic testing available in the United

States for diagnosis of Fasciola. Enzyme immunoassay (EIA) and enzyme-linked immunosorbent assay

(ELISA) serum IgG antibody testing is performed at private references laboratories.

Therapy :

 The drug of choice for treatment of infections with Clonorchis and Opisthorchis is praziquantel given orally

three times for 1 day. An alternative drug is albendazole, a benzimidazole group drug, given once daily for 7

days. The drug of choice for Fasciola is bithionol (praziquantel is not effective) given orally every other day for

10 to 15 doses.

Prevention :

 Human infection can be prevented by ensuring that fish and aquatic vegetation are properly cooked before

consumption, as well as by the improvement of sanitary conditions along with the education of good personal

hygiene.

Figure 56 Cercaria of a liver fluke. Figure 57Clonorchis sinensis egg.

478

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

THE LUNG FLUKES:

General characteristics

 The genus Paragonimus contains several species known to infect humans. Paragonimus westermani is the

most common and widely distributed lung fluke. The adult worms live in the lungs and produce eggs that may

be present in sputum, or if expectorated and swallowed, may be present in feces. Like other trematodes, a

freshwater snail is required as an intermediate host.

Life cycle:

 Reservoir hosts for P. westermani include dogs and cats, and those for P. mexicanus include domestic and

wild pigs, and dogs. Species of Paragonimus may also be found in other freshwater crab- or crayfish-eating

mammals. The adult worms, encapsulated in the lungs, produce eggs that leave the lung via the bronchioles,

stimulating a cough response. The eggs are then swallowed and eventually excreted in the feces. The freeswimmingmiracidium is released from the egg in freshwater and enters the snail host where it develops into a

redia and then a cercariae, which leaves the snail and enters the water. The cercariae then enter a second

intermediate host, a crab or crayfish, where they encyst and develop into metacercariae. The metacercaria is the

infective stage for humans. When infected freshwater crabs and crayfish are eaten raw or undercooked, the

metacercariae will excyst in the duodenum and then migrate through the intestinal wall, and eventually through

the diaphragm and into the lungs where they encapsulate (usually in pairs) and mature .

Pathogenesis and spectrum of disease

 Light infections may be asymptomatic. The migration of the metacercariae through muscle and tissue may

cause local pain and immune response to tissue damage.

 In the lungs, the immune response causes infiltration of eosinophils and neutrophils. Serum IgE levels are

usually elevated. Eventually the adult worms are encapsulated in a granuloma. Presence of the worms in the

lungs usually results in a chronic cough, with possible production of blood-tinged sputum. The cough provides

a mechanism to transport eggs up into the throat where they are swallowed and then may be excreted in the

feces. The larvae of P. mexicanus may migrate to other areas of the body, frequently causing the formation of

subcutaneous or lower abdominal nodules. The larvae of Paragonimus may even enter the brain (rarely), where

they can cause severe damage.

Laboratory diagnosis

 The eggs may be recovered from sputum, and occasionally in feces using a sedimentation concentration

method. The eggs may be observed in a wet mount (with/without iodine stain) (Figure 58).

Charcot-Leyden crystals may also be observed in sputum or lung tissue specimens. Charcot-Leyden crystals are

slender and pointed at both ends. The crystals normally appear colorless and stain purplish to red with

trichrome. Elevated levels of eosinophils in whole blood and elevated IgE levels in serum may be present.

Lesions in the lungs may be observed in x-ray. Serum IgG EIA and immunoblot testing, and EIA serum and

479

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

cerebrospinal fluid (CSF) IgG antibody testing is performed at private reference laboratories; cross reactivity

with other species and trematodes may occur.

Therapy

The drug of choice for treatment of Paragonimus infections is praziquantel given three times a day for 2 days.

Prevention

 Human infection can be prevented by not eating pickled, raw, or undercooked crabs and crayfish. Care

should also be taken to properly clean utensils used in the preparation of these foods. Improvement of sanitary

conditions and practices may also help to reduce the prevalence of these infections.

BloodTrematodes:

Schistosoma mansoni

Schistosoma haematobium

Schistosoma japonicum

Schistosoma intercalatum

Schistosoma Mekongi

 There are four species of blood flukes that are primarily associated with disease in humans (known as

schistosomiasis, bilharziasis, or snail fever), all belonging to the genu Schistosoma. These four species are

Schistosoma haematobium, S. japonicum (Oriental blood fluke), S. mekongi, and S. mansoni. A fifth species, S.

intercalatum, is a pathogen primarily in animals but has been associated with human disease. The blood flukes

differ

in morphology and life cycle characteristics from the other trematodes, but because they all belong to the same

genus, they are very similar, and may be difficult to distinguish from each other. They do, however, require a

freshwater snail as the only intermediate host.

Figure 58: Paragonimus westermani egg.

480

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

General characteristics

 Unlike the other trematodes, adult schistosomes are not flattened, but are rather long, thin, and rounded in

shape. There is an oral sucker surrounding the mouth and a ventral sucker located just slightly below the oral

sucker. The adult male averages 1.5 cm in length and is wider than the female, having a ventral fold that wraps

around the female when they mate (Figure 59). The adult female averages 2 cm in length and is very thin.

The eggs of each species are distinct, and can be distinguished by size, spine morphology, and sometimes

specimen type (Figure 60). The size range for eggs of S. haematobium is 110 to 170 μm long by 40 to 70 μm

wide and they have a sharply pointed terminal spine. They are fully embryonated without an operculum. The size

range for the eggs of S. japonicum is 70 to 100 μm long by 50 to 65 μm wide, and they have a small lateral spine

that is sometimes difficult to detect .

 S. mekongi eggs are smaller than those of S. japonicum, ranging in size from 50 to 65 μm long by 30 to 55 μm

wide. They are fully embryonated without an operculum and have a small lateral spine. The size range for eggs of

S. mansoni is 115 to 180 μm long by 40 to 75 μm wide, and they have a large lateral spine. S. mansoni eggs are

unoperculate, immature when released, and take up to 8 to 10 days to develop a miracidium. S. intercalatum eggs

are fully embryonated without an operculum, have a terminal spine, and range in size from 140 to 240 μm long by

50 to 85 μm wide. S. intercalatum eggs resemble those of S. haematobium and can be differentiated by ZiehlNeelsen acid-fast positivity. In addition, S. intercalatum eggs are only found in feces, not in urine specimens

One of the main differences in the schistosomes from other trematodes is that instead of being hermaphroditic,

there are separate male and female adult worms. In human infection, the adult worms live in either the veins that

supply the intestine (S. japonicum and S. mansoni) or the veins that supply the urinary bladder (S. haematobium).

 The eggs are passed from the body in either the feces or the urine. To reach the inside of the intestine or bladder,

the eggs must penetrate the tissue from the veins. This is accomplished via a spine that is distinctive among the

major species. The embryonated egg will release the miracidium (Figure 62) once it reaches freshwater, and will

enter the snail host, where it will develop into the infectious cercaria. The freeswimming cercariae are capable of

penetrating through the human skin directly and do not encyst on aquatic vegetation or other aquatic wildlife

(Figure 63). The cercariae penetrate the host tissue until they reach a vein; then they travel to capillaries near the

lungs and then to the portal vein of the liver, where they mature. When they are mature, the adult males will pair

with the females and then travel to the veins of either the intestine or thebladder, where the eggs are produced.

Pathology and spectrum of disease

 Infection with only a small number of worms may be asymptomatic. Quite often, penetration of the skin by

the cercariae causes localized swelling and itching. The migration of the larvae through the body may cause

transient symptoms of fever, malaise, cough (when they migrate in the lungs), or hepatitis (when in the liver).

The adults are able to acquire some host antigens on their outer surface, and so may not elicit an immune

response, although the eosinophil count may be high.

Severe tissue damage, with associated pain, fever, and chills, may occur when the eggs travel through the

tissue to reach the intestine or bladder. There may also be bloody diarrhea or blood in the urine (hematuria).

Necrosis, lesions, and granulomas may develop, as well as obstruction of the bowel or ureters. Penetration of

481

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

human skin by the cercariae of blood flukes that commonly infect other mammals or aquatic birds may cause a

schistosomal dermatitis known as “swimmer’s itch.” Erythema, edema, and intense itching may develop that

usually disappear within 1 week. The cercariae of these species are not able to complete the life cycle by

entering the human bloodstream, and are destroyed by the host immune system.

Figure 59: Mating of Schistosoma mansoni male and femaleWorms.

Figure 60, Schistosoma mansoni egg. B, Schistosoma japonicum egg. C, Schistosoma haematobium egg.

Figure 61 :S. japonicum egg. Figure 62 :S. mansoni miracidium.

482

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

Laboratory diagnosis

 The standard method of diagnosis is by the detection of characteristic eggs in feces or rectal biopsy, for S.

japonicum, S. mekongi, S. mansoni, and S. intercalatum (and perhaps S. haematobium if these worms have

migrated to a bladder vein that is close to the intestine); and in urine (usually concentrated before examination)

or bladder tissue biopsy for S. haematobium.

A wet mount with/without iodine from a sedimentation or concentration method can be examined for eggs.

Figure provides images of three different schistosome eggs. To optimize recovery of S. haematobium in urine,

the specimen should be collected between noon and 2 pm. There are some antibody-based assays that are

available for diagnosis of schistosomal IgG antibody (enzyme immunoassay [EIA], enzyme-linked

immunosorbent assay [ELISA], and immunoblot), but these methods cannot distinguish between current and

previous infections. This type of assay may, however, be useful for travelers who have returned from endemic

areas.

Several nucleic acid-based testing methods have been developed that demonstrate high sensitivity and

specificity using genomic or mitochondrial sequences. In addition, schistosome DNA has been identified in

patients’ plasma using real-time polymerase chain reaction (PCR).

Therapy

 The drug of choice for treatment of schistosome infections is praziquantel, given in two or three doses in a

day. Infection with S. mansoni may require a larger dose than that for the other species.

Prevention

 Because human infection is by direct penetration of the cercariae, prevention of schistosome infection is more

difficult to achieve. Educational programs are required to help people in endemic areas understand how to help

prevent infection.

 Sanitary conditions need to be improved with proper disposal not only of human wastes but also that of

domestic animals (in areas with S. japonicum and S. mekongi). A safe water supply for bathing and washing

Figure 63: S. mansoni cercaria.

483

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

clothes is also necessary. Various snail control methods have been tried, but these methods are very costly and

would need to be repeated on a regular basis to have the desired effect.

Table Body Sites and Parasite Recovery (Trophozoites, Cysts, Oocysts, Spores, Adults, Larvae, Eggs,

Amastigotes,Trypomastigotes)

Site Parasites

Blood

Red cells

Plasmodium spp.

Babesia spp.

White cells Leishmania spp.