eMedicine Specialties > Infectious Diseases > Parasitic Infections

Trematode Infection

Author: Subhash Chandra Parija, MBBS, MD, PhD, FRCPath, Director-Professor of Microbiology, Head of Department of Microbiology, Jawaharlal Institute, Postgraduate Medical Education and Research, India
Coauthor(s): Thomas J Marrie, MD, Chair, Professor, Department of Medicine, Division of Infectious Diseases, University of Alberta College of Medicine; Shekhar Koirala, MD, Vice Chancellor, Department of Medicine, BP Koirala Institute of Health, Dharan, Nepal
Contributor Information and Disclosures

Updated: Nov 13, 2009

Introduction

Background

Trematode infections occur worldwide. Trematodes, also called flukes, cause various clinical infections in humans. The parasites are so named because of their conspicuous suckers, the organs of attachment (trematos means "pierced with holes"). All the flukes that cause infections in humans belong to the group of digenetic trematodes. Important features exhibited by adult digenetic trematodes are summarized below (see Features of digenic trematodes).

Depending on the habitat in the infected host, flukes can be classified as blood flukes, liver flukes, lung flukes, or intestinal flukes (see Classification of trematodes according to their habitat). The flukes that cause most human infections are Schistosoma species (blood fluke), Paragonimus westermani (lung fluke), and Clonorchis sinensis (liver fluke). Other less important flukes include the liver flukes Fasciola hepatica and Opisthorchis viverrini and the intestinal flukes Fasciolopsis buski, Heterophyes heterophyes, and Metagonimus yokogawai.

Features of digenic trematodes

• Digenic trematodes are unsegmented leaf-shaped worms that are flattened dorsoventrally.
• They bear 2 suckers, one surrounding the mouth (oral sucker) and another on the ventral surface of the body (ventral sucker). These serve as the organs of attachment.
• The sexes of the parasites are not separate (monecious). An exception is schistosomes, which are diecious (unisexual).
• The alimentary canal is incomplete, and no anus is present.
• The excretory system is bilaterally symmetrical. It consists of flame cells and collecting tubes. These flame cells provide the basis for the identification of the species.
• The reproductive system consists of male and female reproductive organs and is complete in each fluke.
• The flukes are oviparous. They lay operculated eggs. An exception is schistosome eggs, which are not operculated.
• All have complicated life cycles, with alternating asexual and sexual developments in different hosts.

Classification of trematodes according to their habitat

• Blood flukes -Schistosoma haematobium, Schistosoma mansoni, Schistosoma japonicum, Schistosoma mekongi, and Schistosoma intercalatum
• Liver flukes -F hepatica, Fasciola gigantica, C sinensis, Opisthorchis felineus, O viverrini, Dicrocoelium dendriticum, and Dicrocoelium hospes
• Pancreatic flukes -Eurytrema pacreaticum, Eurytrema coelomaticum, and Eurytrema ovis
• Lung flukes -P westermani, Paragonimus mexicana, and Paragonimus skrjabini
• Intestinal flukes -F buski, M yokogawai, Echinostoma ilocanum, Watsonius watsoni, H heterophyes, and Gastrodiscoides hominis

Pathophysiology

The life cycle of trematodes is completed in 2 different classes of hosts: definitive (ie, humans, domestic animals, wild animals) and intermediate (ie, freshwater snails). Snails that act as intermediate hosts for trematodes of medical importance are listed in Table 2. The list of these hosts for different trematodes and the source of infections are summarized in Table 3.

Blood flukes (Schistosoma species)

Schistosomiasis, or bilharzia, is a tropical parasitic disease caused by blood-dwelling fluke worms of the genus Schistosoma. The main schistosomes that infect human beings include S haematobium (transmitted by Bulinus snails and causing urinary schistosomiasis in Africa and the Arabian peninsula), S mansoni (transmitted by Biomphalaria snails and causing intestinal and hepatic schistosomiasis in Africa, the Arabian peninsula, and South America), and S japonicum (transmitted by the amphibious snail Oncomelania and causing intestinal and hepatosplenic schistosomiasis in China, the Philippines, and Indonesia).

S intercalatum and S mekongi are only of local importance. S japonicum is a zoonotic parasite that infects a wide range of animals, including cattle, dogs, pigs, and rodents. S mansoni also infects rodents and primates, but human beings are the main host. A dozen other schistosome species are animal parasites, some of which occasionally infect humans.

Unlike other trematodes, schistosomes have separate sexes, but males and females are found together. The male is short and stout and holds the relatively long female worm in its gynecophoric canal, a groovelike structure. With S haematobium, both male and female live together in the veins that drain the urinary bladder, pelvis, and ureter, whereas S japonicum and S mansoni live in the inferior and superior mesenteric veins, respectively. Hence, these flukes are known as blood flukes. These species are distinguished from the other schistosomal species based on the morphology of their eggs and their adult and cercarial forms. S haematobium eggs have a terminal spine, whereas S mansoni and S japonicum eggs have lateral spines and central spines, respectively.

Humans are infected by free-swimming, fork-tailed cercaria in fresh water by penetration of the skin. The cercaria loses its tail and outer layer of glycocalyces, transforms into a schistosomula (a larval form), and travels through venous circulation to the heart, lungs, and portal circulation. Larvae mature and develop into adult worms in approximately 3 weeks and reach the vessels that drain the urinary bladder (S haematobium) or the mesentery (S japonicum, S mansoni). At these venous sites, they live and lay eggs for the duration of the host’s life.

The eggs penetrate the vascular endothelium, enter the bladder or gut lumen, and are excreted in urine (S haematobium) or stool (S japonicum, S mansoni). If these excreted eggs gain access to fresh water, the miracidium emerges from the egg and swims freely until it finds an appropriate snail. In the snail host, after 2 generations of asexual multiplication (sporocysts), the forked-tailed cercariae emerge in water to infect other susceptible human hosts. A single miracidium can multiply in the snail to produce nearly 100,000 cercariae.

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Lung flukes (Paragonimus species)

The genus Paragonimus contains more than 30 species that have been reported to cause infections in animals and humans. Among these, approximately 10 species have been reported to cause infection in humans, of which P westermani is the most important. P westermani, also known as the Oriental lung fluke, is the most widespread species in Africa, South America, and parts of Asia.

P westermani is a thick, fleshy, reddish brown, egg-shaped worm (7.5-12 mm in length, 4-6 mm in breadth, and 3.5-5 mm in thickness). It inhabits parenchyma of the lung close to bronchioles in humans, foxes, wolves, and various feline hosts (eg, lions, leopards, tigers, cats).

The infection is typically transmitted via ingestion of metacercariae contained in raw freshwater crabs or crayfish. Additionally, consumption of the raw meat of paratenic hosts (eg, omnivorous mammals) may also contribute to human infection. Freshwater snails and crabs are first and second intermediate hosts of Paragonimus species, respectively. In the duodenum, the cyst wall is dissolved, and the metacercariae are released. The metacercariae migrate by penetrating through the intestinal wall, peritoneal cavity, and, finally, through the abdominal wall and diaphragm into the lungs. There, the immature worms finally settle close to the bronchi, grow, and develop to become sexually mature hermaphrodite worms.

Adult worms begin to lay the eggs, which are unembryonated and are passed out in the sputum. However, if they are swallowed, they are excreted in the feces. The eggs develop further in the water. In each egg, a ciliated miracidium develops during a period of 2-3 weeks. The miracidium escapes from the egg and penetrates a suitable species of snail (first intermediate host), in which it goes through a generation of sporocysts and 2 generations of rediae to form the cercariae. The cercariae come out of the snail, invade a freshwater crustacean (crayfish or crab), and encyst to form metacercariae. When ingested, these cause the infection, and the cycle is repeated.

Liver flukes (C sinensis, F hepatica)

• C sinensis
  • C sinensis is a widespread parasite found in Southeast Asia that infects the biliary passage in humans. The fluke is oblong, flat, transparent, and relatively small (10-25 mm long by 3-5 mm wide). It has a pointed anterior and rounded posterior end. Humans are infected by eating raw or partially cooked freshwater fish or dried, salted, or pickled fish infected with the metacercariae. In the duodenum, the cyst is digested and an immature larva released. The larva enters the biliary duct, where it develops and matures into an adult worm. The adult worm feeds on the mucosal secretions and begins to lay fully embryonated operculated eggs, which are excreted in the feces. Upon reaching fresh water and upon ingestion by a suitable species of operculate snails (first intermediate host), the eggs hatch to produce a miracidium. Inside the snail, the miracidia multiply asexually through a single generation of sporocysts and 2 generations of rediae to fork-tailed cercariae.
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    Trematode infection. Clonorchis sinensis egg. These are small operculated eggs. Size is 27-35 μm X 11-20 μm. The operculum, at the smaller end of the egg, is convex and rests on a visible "shoulder." At the opposite (larger, abopercular) end, a small knob or hooklike protrusion is often visible (as here). The miracidium is visible inside the egg. Image courtesy of the US Centers for Disease Control and Prevention.

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    Trematode infection. Clonorchis sinensis egg. These are small operculated eggs. Size is 27-35 μm X 11-20 μm. The operculum, at the smaller end of the egg, is convex and rests on a visible "shoulder." At the opposite (larger, abopercular) end, a small knob or hooklike protrusion is often visible (as here). The miracidium is visible inside the egg. Image courtesy of the US Centers for Disease Control and Prevention.

  • The cercariae escape from the snail to the water and penetrate under scales of freshwater cyprinid fish (second intermediate host). In the fish, the cercariae lose their tails and encyst in the scale or muscle of the fish to the metacercariae, which are infectious to humans. When ingested, the infected fish cause infection in humans.
• F hepatica
  • Fascioliasis, a zoonotic disease caused by infection with F hepatica (a digenetic trematode), is a major disease of livestock that is associated with important economic losses due to mortality; liver condemnation; reduced production of meat, milk, and wool; and expenditures for anthelmintics. The disease has a cosmopolitan distribution, with cases reported from Scandinavia to New Zealand and southern Argentina to Mexico.
  • F hepatica, also known as the sheep liver fluke, is a large liver fluke. This fluke primarily causes zoonotic disease in sheep and other domestic animals. Humans are infected by eating watercress and other aquatic plants contaminated by the metacercariae, which enter the duodenum and excyst. They then penetrate the intestinal wall, peritoneal cavity, and liver capsule (Glisson capsule) to reach the bile duct of the liver, where they develop and mature into adult worms.
  • The adult worms begin to lay the unembryonated eggs, which are excreted in the stool. They develop further in the fresh water. A miracidium hatches out of the egg and invades the appropriate snail host. Inside the snail host, the larva multiplies asexually through a single generation of sporocysts and 2 generations of rediae to finally develop into cercariae. Upon exiting the snail, the cercariae encyst on aquatic plants to form metacercariae. When humans and sheep eat these plants, they become infected, repeating the life cycle.
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    Trematode infection. Fasciola hepatica eggs. Wet mounts with iodine. The eggs are ellipsoidal. They have a small, barely distinct operculum (upper end of the eggs in panel A). The operculum can be opened (egg in panel B), for example, when slight pressure is applied to the coverslip. The eggs have a thin shell that is slightly thicker at the abopercular end. They are passed unembryonated. Size range is 120-150 μm X 63-90 μm. Image courtesy of the US Centers for Disease Control and Prevention.

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    Trematode infection. Fasciola hepatica eggs. Wet mounts with iodine. The eggs are ellipsoidal. They have a small, barely distinct operculum (upper end of the eggs in panel A). The operculum can be opened (egg in panel B), for example, when slight pressure is applied to the coverslip. The eggs have a thin shell that is slightly thicker at the abopercular end. They are passed unembryonated. Size range is 120-150 μm X 63-90 μm. Image courtesy of the US Centers for Disease Control and Prevention.

• D dendriticum, D hospes
  • Dicrocoeliasis is a parasitic disease caused by the small liver flukes D dendriticum and D hospes. The disease represents a worldwide and widespread problem in grazing livestock, and it is sometimes (although rarely) found in humans. However, because of its unusual method of transmission, human dicrocoeliasis remains a relatively rare occurrence in individuals who do not engage in risky activities such as eating raw or undercooked liver.
  • Cases of human dicrocoeliasis have been reported throughout Eastern Europe, Western Europe, Africa, Australia, India, and Saudi Arabia.

Pancreatic flukes (Eurytrema pancreaticum, E coelomaticum, E ovis)

• These flukes have a thick body and are 8-16 mm long and 6 mm wide. They parasitize the pancreatic ducts and occasionally the bile ducts of sheep, pigs, and cattle in Brazil and Asia. Three species, E pancreaticum, E coelomaticum, and E ovis are recognized.
• The first intermediate hosts are terrestrial snails (Bradybaena species), and the cercariae encyst in grasshoppers (Conocephalus species), which are the second intermediate host. After a suitable animal hosts ingests a grasshopper with cercariae, the immature flukes are released and migrate to the pancreatic duct, where they mature and produce eggs within approximately 11-14 weeks.
• There are no obvious clinical signs of infection with these parasites. Dicrocoelium -like eggs can be demonstrated in feces. Light infections cause proliferative inflammation of the pancreatic duct, which may become enlarged and occluded. In heavy infections, fibrotic, necrotic, and degenerative lesions develop. Losses are reported due to condemned pancreas, but the pathogenesis suggests an additional loss of production.

Intestinal flukes (F buski, H heterophyes, M yokogawai)

F buski is the most common intestinal nematode that causes infections in humans. The trematodes H heterophyes and M yokogawai are less-common causes of human infection.

F buski, known as the giant intestinal fluke, is found in the duodenum and jejunum of pigs and humans and is the largest intestinal fluke to parasitize humans. Humans are infected by eating freshwater aquatic plants such as water caltrops, water chestnuts, and water bamboo, which can harbor the metacercariae. In the intestine, the metacercariae excyst, attach to the duodenum or jejunum, develop, and grow into adult worms. They lay unembryonated eggs, which are excreted in the feces.

In water, inside the egg, a ciliated miracidium develops, comes out, and penetrates a suitable snail host. Inside the snail, after several stages of asexual multiplication, large numbers of cercariae are produced. The latter emerge from the snail and encyst on the surface of aquatic plants to metacercariae. Ingestion of these plants causes infection in humans, and the cycle is repeated.

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Frequency

United States

Infection with blood flukes, lung flukes, liver flukes, and intestinal flukes in the United States is extremely rare. The condition is observed in travelers and emigrants from endemic areas.

International

Trematode infections in general are becoming more prevalent. Schistosomiasis affects about 200 million people worldwide, and more than 650 million people live in endemic areas. Worldwide, more than 250 million people in 74 countries are infected. Currently, 601 million individuals are at risk for C sinensis infection, 293.8 million for infection with Paragonimus species, 91.1 million for infection with Fasciola species, and 79.8 million for infection with Opisthorchis species.

The geographic distribution of schistosomiasis depends on the presence of the freshwater snails that act as the intermediate hosts. Human infection is caused by skin penetration by the schistosomal cercariae upon contact with the contaminated water sources. Persons susceptible to infection include farmers working in irrigated fields, anglers working in culture ponds and rivers, and persons who wash utensils or clothes along banks of canals or rivers.

Residents who live near freshwater bodies have a risk of infection that is 2.15 times that of persons who live farther from water. Exponential growth of aqua culture may be the most important risk factor for the emergence of foodborne trematodiasis.

Foodborne trematodiasis, which is caused by liver flukes (C sinensis, Fasciola species, Opisthorchis species), lung flukes (Paragonimus species), and intestinal flukes (Echinostoma species, F buski, heterophyids), is an emerging public health problem in Southeast Asia and the West Pacific region. In China, the number of clonorchiasis cases have more than tripled over the past decade; approximately 15 million people were infected with C sinensis in 2004.¹

The different species of Schistosoma have different geographic distributions. Urinary schistosomiasis caused by S haematobium is found in 54 countries in Africa and the eastern Mediterranean; intestinal schistosomiasis caused by S japonicum is limited to 4 countries in the Far East (ie, China, Thailand, Indonesia, Philippines). S mansoni is found in 52 countries in Africa and Latin America. S mekongi is found along the banks of the Mekong River area in Southeast Asia.

Approximately 30 million people are infected by liver flukes, of whom 19 million are infected by C sinensis, 9 million by O viverrini, and 1.2 million by O felineus. Of these, approximately 15 million are in China. Liver fluke infection is endemic in China, Japan, Korea, Taiwan, and Vietnam (C sinensis); Thailand and Laos (O viverrini); and the Russian Federation and Eastern Europe (O felineus). People who habitually eat raw or partially cooked fish or dried, salted, or pickled fish are more susceptible to infection by Clonorchis species. Human fascioliasis occurs worldwide in temperate regions.

F hepatica is found on every continent. The prevalence is highest in areas of extensive sheep and cattle raising and where dietary practices include the consumption of raw aquatic vegetables. In many locations (eg, Portugal, the Nile delta, northern Iran, parts of China, the Andean highlands of Ecuador, Bolivia, and Peru), high infection rates have made fascioliasis a serious public health concern. Outbreaks of F gigantica infection have been reported from tropical areas of Southeast Asia, Africa, and Hawaii.

Nearly 100 million people worldwide are infected with F buski. The infection is found most commonly in China, Taiwan, Thailand, Indonesia, Bangladesh, and India. Human infection occurs after ingestion of various parts (eg, fruits, pods, roots, stems) of infected water chestnut, lotus, and other aquatic plants when they are bitten or peeled off with the teeth. Human infection with H heterophyes has been reported in Egypt's Nile delta.

Human lung fluke infection, most commonly with P westermani, is most common in China, Korea, Thailand, Philippines, and Laos. Isolated endemic foci have also been reported from the states of Manipur, Nagaland, and Arunachal Pradesh in India. A low prevalence has been reported from African countries of Cameroon and Nigeria, where infections with Paragonimus africanus and Paragonimus uterobilateralis were reported. Humans are infected by eating raw or partially cooked crab or crayfish or crabs soaked in wine as a food delicacy or by drinking juice from raw crabs or crayfish as a part of a food habit.

Mortality/Morbidity

Because of the large numbers of people infected worldwide, trematode infections can cause considerable morbidity. Many of the trematode infections, such as schistosomiasis, clonorchiasis, and pulmonary paragonimiasis, can be fatal if left untreated. Infection with intestinal trematodes is rarely fatal.

Race

No racial predisposition to trematode infections is apparent.

Sex

Most trematode infections have no sexual predisposition.

Age

Most trematode infections affect people of all ages equally. However, with intestinal trematode infections, children are affected more severely, as are children and adolescents with schistosomiasis.

Clinical

History

• Schistosomiasis
  • Acute manifestations
    • Cercarial dermatitis, also known as swimmer's itch, is an allergic reaction caused by the penetration of cercariae in persons who have been exposed to cercariae in salt water or fresh water. Cercarial dermatitis manifests as petechial hemorrhages with edema and pruritus, followed by maculopapular rash, which may become vesicular. The process is usually related to avian schistosomal species of the genera Trichobilharzia, Gigantobilharzia, and Orientobilharzia, which do not develop further in humans.
    • Katayama syndrome corresponds to maturation of the fluke and the beginning of oviposition. This syndrome is caused by high worm load and egg antigen stimuli that result from immune complex formation and leads to a serum sickness –like illness. This is the most severe form and is most common in persons with S mansoni and S japonicum infections. Symptoms include high fever, chills, headache, hepatosplenomegaly, lymphadenopathy, eosinophilia, and dysentery. A history of travel in an endemic area provides a clue to the diagnosis.
  • Chronic manifestations
    • Symptoms depend on the Schistosoma species that causes the infection, the duration and severity of the infection, and the immune response of the host to the egg antigens.
    • Terminal hematuria, dysuria, and frequent urination are the main clinical symptoms of urinary schistosomiasis.
    • The earliest bladder sign is pseudotubercle, but, in long-standing infection, radiography reveals nests of calcified ova (sandy patches) surrounded by fibrous tissue in the submucosa.
    • Dysentery, diarrhea, weakness, and abdominal pain are the major symptoms of intestinal schistosomiasis.
    • A reaction to schistosomal eggs in the liver causes a periportal fibrotic reaction termed Symmers clay pipestem fibrosis.
    • Hemoptysis, palpitation, and dyspnea upon exertion are the symptoms of schistosomal cor pulmonale that develops as a complication of hepatic schistosomiasis.
    • Headache, seizures (both generalized and focal), myeloradiculopathy with lower limb and back pain, paresthesia, and urinary bladder dysfunction are the noted symptoms of CNS schistosomiasis due to S japonicum infection.
• Paragonimiasis
  • Acute manifestations: Acute pulmonary infection is characterized by low-grade fever, cough, night sweats, chest pain, and blood-stained rusty-brown sputum.
  • Chronic manifestations: Lung abscess or pleural effusion² develops in individuals with chronic infections. Fever, hemoptysis, pleurisy pain, dyspnea, and recurrent attacks of bacterial pneumonia are the common symptoms. The condition mimics pulmonary tuberculosis.
  • Fever, headache, nausea, vomiting, visual disturbances, motor weakness, and localized or generalized paralysis are the symptoms of cerebral paragonimiasis.
• Liver fluke infections
  • Acute manifestations
    • Fascioliasis is mostly subclinical. Acute manifestations are due to migration of larva through lung parenchyma. Malaise, intermittent fever, night sweats, and pain in the right costal area are early symptoms of acute infection.
    • Clonorchiasis is frequently asymptomatic. Serum sickness–like illness with symptoms of high fever, eosinophilia, and rash occurs in individuals with acute infection.
  • Chronic manifestations
    • Chronic fascioliasis is frequently asymptomatic. In symptomatic patients, irregular fever, anemia, hepatobiliary manifestations (colicky pain, jaundice), and secondary bacterial infections are present.
    • In its end stage, chronic clonorchiasis may be complicated by recurrent pyogenic cholangitis and jaundice associated with cholangiocarcinoma.
• Intestinal fluke infections
  • Acute manifestations: These infections are frequently asymptomatic. Diarrhea and abdominal pain are common symptoms in individuals with acute infection.
  • Chronic manifestations: Generalized abdominal pain; ascites; and edema of the face, abdomen wall, and lower limbs are the main symptoms.

Physical

• Schistosomiasis
  • Acute infections: Patients may have hepatosplenomegaly, lymphadenopathy, and rashes.
  • Chronic schistosomiasis: Patients may have anemia, pedal edema, ascites, and abdominal distension with distended abdominal veins. Patients may also have intestinal polyposis and signs of malnutrition.
• Paragonimiasis: Abdominal mass, pain in the abdomen, and mucosanguineous diarrhea characterize abdominal paragonimiasis.
• Liver fluke infections: Patients with chronic clonorchiasis may have tender hepatomegaly, progressive ascites, catarrhal cholecystitis, progressive edema, and jaundice.
• Intestinal fluke infections: Patients with mild infection are usually asymptomatic. Patients with severe infections may have ascites and edema of the face, abdomen wall, and lower limbs.

Causes

See Pathophysiology.

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