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).
Trematode infections such as schistosomiasis have emerged as important tropical infections. An estimated 200 million people in the tropical belts of the world may have schistosomal infection. This makes Schistosoma infection the second most prevalent tropical infectious disease in areas such as sub-Saharan Africa after malaria.[1]
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 (monoecious). 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.[2]
Classification of trematodes according to their habitat
Blood flukes include Schistosoma haematobium, Schistosoma mansoni, Schistosoma japonicum, Schistosoma mekongi, and Schistosoma intercalatum (clade B – mammalian freshwater schistosomes).
Liver flukes include F hepatica, Fasciola gigantica, C sinensis, Opisthorchis felineus, O viverrini, Dicrocoelium dendriticum,Dicrocoelium hospes, and Metorchis conjunctus.
Pancreatic flukes include Eurytrema pancreaticum, Eurytrema coelomaticum, and Eurytrema ovis.
Lung flukes include Pwestermani, Paragonimusheterotremus, Paragonimus kellicoti, Paragonimusmexicana, Paragonimus skrjabin, Paragonimus miyazakii, Paragonimus compactus, and Paragonimushueit’ungensis.
Intestinal flukes include F buski, M yokogawai, Echinostoma ilocanum, Watsonius watsoni, H heterophyes, and Gastrodiscoides hominis.
Eye flukes include Philophthalmus lacrimosus, Philophthalmus palpebrarum, and Philophthalmus gralli (Philophthalmus lucipetus).
Other flukes include Alaria americana and Clinostomum complanatum.
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). See the figures below.
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Adult worms in humans reside in the veins in various locations: Schistosoma mansoni in the inferior mesenteric veins, Schistosoma japonicum in the sup....
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Eggs are excreted unembryonated in the sputum, or, alternately, they are swallowed and passed with stool (1). In the external environment, the eggs be....
Snails that act as intermediate hosts for trematodes of medical importance are listed in Table 1. The list of these hosts for different trematodes and the source of infections are summarized in Table 2.
Table 1. Vectors and Geographical Areas Associated With Certain Trematode Types
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Table 2. List of Definitive and Intermediate Hosts and Sources of Infection of Major Trematodes
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Blood flukes (Schistosoma species)
Schistosomiasis, or bilharzia, is a tropical parasitic disease caused by blood-dwelling fluke worms of the genus Schistosoma, from the Greek for skhistos (split) and soma (body).Originally thought to be a single organism with a split body, the parasite was eventually recognized as having male and female forms. 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. Shaematobium eggs have a terminal spine, whereas Smansoni and Sjaponicum 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 (Shaematobium) or the mesentery (Sjaponicum, Smansoni). 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 (Shaematobium) or stool (Sjaponicum, Smansoni). 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.
Table 3. Comparative Features of Major Human Schistosoma Species
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Lung flukes
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 Pwestermani is the most important. Pwestermani, also known as the Oriental lung fluke, is the most widespread species in Africa, South America, and parts of Asia. Among other species of Paragonimus that have been reported to cause human disease from around the world is Paragonimus heterotremus, which has been reported from northeastern parts of the Indian subcontinent.[3]
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. Note the image below.
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The average egg size is 85 μm by 53 μm (range, 68-118 μm X 39-67 μm). They are yellow-brown, ovoidal or elongate, have a thick shell, and are often as....
Liver flukes (CsinensisandFhepatica)
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 generations of rediae to fork-tailed cercariae.
See the image below.
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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 "shoulde....
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 Fhepatica (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.
Fhepatica, 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.
D dendriticum, D hospes
Dicrocoeliasis is a parasitic disease caused by the small liver flukes D dendriticum and Dhospes. 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 by ingestion of infected ants, human dicrocoeliasis remains a relatively rare occurrence in humans. Cases of human dicrocoeliasis have been reported throughout Eastern Europe, Western Europe, Africa, Australia, India, and Saudi Arabia.
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, Epancreaticum, Ecoelomaticum, 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 (Fbuski, Hheterophyes, Myokogawai, G hominis)
F buski is the most common intestinal nematode that causes infections in humans. The trematodes Hheterophyes, Myokogawai, and G hominis 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.
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. The liver fluke M conjunctus is found to be endemic in North America (Canadian population).
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 Csinensis 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 (Csinensis,Fasciola species, Opisthorchis species), lung flukes (Paragonimus species), and intestinal flukes (Echinostoma species, Fbuski, heterophyids), is an emerging public health problem in Southeast Asia and the West Pacific region.
The liver fluke M conjunctus has been reported in Russia. The life cycle of this parasite resembles that of Opisthorchis species. Aquatic snails serve as the first intermediate host, white sucker (Catostomus commersoni) serve as the second intermediate host, and humans are incidental hosts. Unlike Clonorchis and Opisthorchis, M conjunctus is not linked to the causation of cholangiocarcinoma.[4] Clonorchiasis is believed to be the third most prevalent worm parasite disease in the world. It is endemic to Japan, China, Taiwan, and South Asia and is currently infecting an estimated 35 million humans, of which 15 million are from China.[5]
The different species of Schistosoma have different geographic distributions. Urinary schistosomiasis caused by Shaematobium is found in 54 countries in Africa and the eastern Mediterranean. Intestinal schistosomiasis is caused by Sjaponicum and is limited to 4 countries in the Far East (ie, China, Thailand, Indonesia, Philippines). Smansoni is found in 52 countries in Africa and Latin America. Smekongi is found along the banks of the Mekong River area in Southeast Asia. It has been estimated that after malaria, schistosomiasis is the most common tropical infectious disease, with 207 million estimated cases.[6, 7] Schistosoma gimvicum and Schistosoma incognitum are rare species of schistosomes reported from India that are known to cause urinary and intestinal schistosomiasis, respectively.[8]
Schistosomal coinfection has been found to affect the malaria transmission in areas in Africa, where both these infections are common. Higher incidence of parasitemia and density of gametocytes has been seen in patients with coinfections.[1]
Cercarial dermatitis is commonly caused by various mammalian schistosomes (belonging to clade B) and rarely by avian schistosomes belonging to clade A (marine avian schistosomes - Austrobilharzia and Ornithobilharzia) and clade D (freshwater avian schistosomes - Trichobilharzia, Gigantobilharzia, Dendritobilharzia, among others). In India and Nepal, clade B mammalian freshwater schistosomes such as like Schistosoma turkestanicum, Schistosoma nasale, Schistosoma indicum, and Schistosoma spindale have been implicated in outbreaks of cercarial dermatitis. The snail Indoplanorbis exustus, seen abundantly in Asia (in contrast to Bulinus and Biomphalaria in Africa), acts as the intermediate host, and domesticated farm cattle act as definitive hosts.[9]
Liver fluke infection is endemic in China, Japan, Korea, Taiwan, and Vietnam (Csinensis); Thailand and Laos (Oviverrini); and the Russian Federation and Eastern Europe (Ofelineus). People who habitually eat raw or partially cooked fish or dried, salted, or pickled fish are more susceptible to infection by Clonorchis species. C sinensis the most common liver fluke in East Asia. Its importance lies in the fact that it has been shown to be responsible for cholangiocarcinoma in humans. In 2009, it was given the status of a group 1 biological carcinogen.[10]
Human fascioliasis occurs worldwide in temperate regions. F hepatica is found on every continent except Antarctica, with the estimated number of infected people over 2 million. The prevalence is highest in areas of extensive sheep and cattle farming 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 Fgigantica infection have been reported from tropical areas of Southeast Asia, Africa, and Hawaii. Human fascioliasis has been reported with increasing frequency from countries such as Turkey in the past few years.[11]
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.
Metagonimiasis is common in East Asia (mainly Japan, China, Taiwan, and the Republic of Korea). Echinostoma is endemic in Southeast Asia and the Far East (eg, China, Indonesia, Vietnam, Taiwan, Thailand). The life cycle of all the intestinal flukes is similar.[12]
Human lung fluke infection, most commonly with P westermani, is most common in China, Korea, Thailand, Philippines, and Laos. P skrjabini is more prevalent in China and P miyazakii is common in Japan. Isolated endemic foci have also been reported from the states of Manipur, Nagaland, and Arunachal Pradesh in India, where P heterotremus is the most common agent, followed by P westermani. Infection with a new subspecies of P miyazakii named P miyazakii manipurinus has been recently reported from India. P kellicotti is the agent responsible for pulmonary paragonimiasis in North America.[13]
A low prevalence has been reported from the African countries of Cameroon and Nigeria, where infections with Paragonimus africanus and Paragonimus uterobilateralis are reported. Rare reports of infections with other species of Paragonimus such as P compacta and P hueit’ungensis have been reported from a few countries, including India.[14]
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.
Systemic and intraocular infections in humans have been reported to be caused by A Americana, a 3-host trematode affecting the definitive host, dog. Humans acquire infection by consuming undercooked frogs (intermediate host) that contain the infective mesocercaria stage of the parasite. Humans and snakes act as paratenic hosts. Two intraocular (diffuse subacute neuroretinitis) and one disseminated infection by this parasite has been reported worldwide.[15]
The trematode Philophthalmus affects birds’ eyes (definitive host), and snails act as intermediate host. New definitive hosts (birds and, accidently, humans) acquire infections when encysted metacercaria are ingested or when they come into direct contact with the eye. Cases have been reported from Yugoslavia, Thailand, Sri Lanka, Japan, Mexico, and the United States.[16]
Clinostomum complanatum primarily affects birds (definitive host). Freshwater fish act as a second intermediate host, containing metacercaria, which are infective to birds. Humans acquire infection by ingesting undercooked freshwater fish, releasing the metacercaria into the stomach. The fluke migrates through the esophagus to the upper respiratory tract, causing pharyngitis or laryngitis. Most human Clinostomum infections have been reported in Japan, Korea, and Thailand.[17]
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.
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 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 longstanding 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.
Neuroschistosomiasis is a severe manifestation of schistosomal infection. The neurological symptoms result from the inflammatory response of the host to the deposition of eggs in the brain and spinal cord. Myelopathy is the most common neurological complication of Smansoni infection.[18]
Paragonimiasis
Acute manifestations include acute pulmonary infection is characterized by low-grade fever, cough, night sweats, chest pain, and blood-stained rusty-brown sputum.
Chronic manifestations can include lung abscess or pleural effusion.[19] 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.
Pulmonary paragonimiasis has been found to mimic metastatic pulmonary tumors on evaluation with imaging methods such as computed tomography (CT) and positron emission tomography (PET) scanning.[20]
Paragonimiasis can affect all parts of the human body, and reports have described cerebral paragonimiasis in the last few years. The rate of cerebral paragonimiasis has been found to be about 0.8% of all active cases of paragonimiasis.[21]
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. A 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
These infections are frequently asymptomatic. Diarrhea and abdominal pain are common symptoms in individuals with acute infection.
Generalized abdominal pain; ascites; and edema of the face, abdomen wall, and lower limbs are the main symptoms.
Acute infections manifest hepatosplenomegaly, lymphadenopathy, and rashes.
Chronic schistosomiasis manifests 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.
Diagnosis is made after microscopic demonstration of eggs in the stool (intestinal schistosomiasis; intestinal, liver, and lung fluke infections), sputum (pulmonary paragonimiasis), or urine (genitourinary schistosomiasis).
For improving the sensitivity of sputum examination for pulmonary paragonimiasis, serial samples (up to 6) should be examined.
A study by Slesak et al has reported the utility of performing Ziehl-Neelsen staining for demonstration of Paragonimus eggs. Ziehl-Neelsen staining was found to be much better than the conventional method of just performing a wet film direct examination of sputum in endemic areas.[22] Paragonimus eggs measure around 100 µm long by 50 µm wide, are ovoid and thick-shelled, and can be detected in sputum and stool samples.
Less frequently, nonoperculate, terminal-spined eggs of S haematobium can be demonstrated in the rectal biopsy and aspiration findings obtained with proctoscopy or cystoscopy.
The flask-shaped eggs of C sinensis can also be demonstrated in the duodenal contents. Examination of fluid obtained from duodenal intubation is diagnostically more sensitive than examination of 2 stool specimens.
Formalin ether and/or ethyl acetate concentration is the most sensitive method for processing stool specimens for egg examination.
The Kato-Katz technique is a simple and sensitive quantitation technique used successfully in the field.[22] It is a commonly used semiquantitative method (measured in a defined quantity of stool - 5 mg) for counting eggs in persons with intestinal schistosomiasis and allows the degree of infection and treatment response to be assessed.
Schistosomal species can be differentiated based on the morphology of the eggs. Rectal or intestinal biopsy demonstrates a granulomatous reaction around the eggs. These biopsy examinations are more much sensitive than multiple stool examinations for detection of intestinal schistosomiasis.[23]
Urine, the specimen of choice for diagnosing urinary schistosomiasis, is collected between noon and 2 pm, the period when an increased number of eggs are excreted. The eggs in the urine are concentrated by centrifugation or membrane filtration.
The eggs of Fasciola and Fasciolopsis species are morphologically similar and indistinguishable. The eggs are oval and yellowish-brown, measuring around 130-150 µm long by 60-90 µm wide. Stool concentration techniques facilitate detection of these trematode eggs. Since the eggs are operculated, they do not float during a saturated salt flotation test. F buski is the largest intestinal trematode, measuring 2-8 cm long by 1-2 cm wide, while the smallest intestinal trematodes are H heterophyes and M yokogawai, measuring less than 0.5 mm.
Similarly, the eggs of Clonorchis, Heterophyes, Metagonimus, and Opisthorchis species are also morphologically similar and indistinguishable. Clonorchis eggs measure around 20 μm long by 15 μm wide. In rare cases, adult worms seen during upper GI endoscopy or when patients are started on antihelminthic therapy. Adult worms can be differentiated based on size; Clonorchis measures 10-25 mm long by 3-5 mm wide, O felineus measures 8-12 mm long by 2-3 mm wide, and O viverrini is narrower (1-2 mm wide).[24]
In Fasciola and Paragonimus species infections, the eggs cannot be demonstrated during the migratory phase of infection or in ectopic infections because no eggs are passed in the stool.
Coproantigen detection
Detection of antigen in the stool (coproantigen) is a nonmicroscopic method of diagnosis for intestinal trematodes. An enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody to an 89-kd antigen of O viverrini has been used to detect coproantigen in the stool of individuals with O pisthorchis infection. This test has been found to be highly sensitive and specific. Coproantigen detection tests are also available for the diagnosis of fascioliasis and capillariasis.[25, 26, 27]
Soluble egg antigen (SEA) detection
ELISA-based tests are available to assess urine or stool samples for SEA; this method provides an effective diagnosis of schistosomiasis and correlates well with quantitation egg count. Although these antigen detection tests have better efficacy than microscopy, they must be used in conjunction with microscopy to increase the probability of detection.[28, 29]
Serology
Several serologic tests, which can be used to detect either specific antibodies or antigens in the serum, are used in diagnosing trematode infections.
Various antibody-based serologic tests are used in the diagnosis of most trematode infections. These tests are used for diagnosis and for seroepidemiologic studies. Commonly used tests include indirect hemagglutination, indirect immunofluorescence, and ELISA. ELISA is most sensitive and practical.
These serologic tests are especially useful in the following situations:
Prepatent period and in chronic and ectopic cases of schistosomiasis, in which the eggs are difficult to demonstrate in the stool
Acute fascioliasis, because the eggs are not passed in the stool for as many as 4 months of infection
Cerebral and abdominal paragonimiasis, because the eggs are not passed in the sputum or stool
A major disadvantage of antibody-based serologic tests is the inability to differentiate between recent and past infections because antibodies remain in the serum even after parasitologic cure of the disease. Low sensitivity and cross-reactions between trematodes are other noted disadvantages.
Detection of specific antigen in serum and urine is particularly useful during acute and end-stage disease, when excretion of eggs is minimal. Knowing whether infection is recent or old is also useful because, in active or recent infection, the circulating antigen is present in the serum or urine but is absent in patients with older or treated infection.
Falcon assay screening test (FAST) ELISA is sensitive (95%) and specific (99%) for the diagnosis of urinary schistosomiasis. This test uses S hematobium adult worm microsomal antigen (HAMA) to reveal serum antibodies.
In schistosomiasis, antigen titers in serum and urine correlate well with the degree of infection, as demonstrated by the egg counts. ELISA is used for detection of proteoglycan gut-associated antigens such as circulating anodic antigen (CAA) and circulating cathodic antigen (CCA) in the urine and serum. The sensitivities of the urine CCA and serum CAA ELISA are substantially higher than those of a single egg count. The sensitivity of these assays increases with egg output. Both CAA and CCA can also be detected in sera and urine of egg-negative individuals. Recently, a novel dipstick test detecting the CCA of intestinal schistosomes in urine samples has been devised. The dipstick test is found to have a better sensitivity than the conventional stool microscopy for the diagnosis of intestinal schistosomiasis.[30]
For its convenience, ELISA has replaced the complement fixation test in the diagnosis of paragonimiasis. For serologic diagnosis, the criterion standard is a Western blot assay, which yields a sensitivity and specificity of nearly 99%. Newer techniques such as the dot immunogold filtration assay (DIGFA) are of supplementary value. Serological assays are particularly useful in the early phase of the disease (prior to egg excretion) and in extrapulmonary paragonimiasis, in which eggs are not expected in sputum or stool.
Immunoblot is a specific and sensitive test to detect schistosomiasis.
The circulating antigen has been detected in the sera of patients with C sinensis infection with the ELISA double-sandwich method. A dipstick ELISA can be used to assess urine samples for SEA; this method provides an effective diagnosis of schistosomiasis and correlates well with quantitation egg count. Diagnosis of fascioliasis by detection of circulating 28.5-kd tegumental antigen is also evaluated.[31] Fas2-ELISA is based on the detection of circulating immunoglobulin G (IgG) antibodies. Results show that Fas2-ELISA is a highly sensitive (92.4%) immunodiagnostic test for the detection of F hepatica infection in children living in human fascioliasis–endemic areas.[32] The CDC recommends the use of ELISA utilizing excretory secretory (ES) antigens followed by confirmation of positive results with immunoblot techniques.[33] Protein-banding patterns after isoelectric focusing have been used to differentiate F hepatica from F gigantica. This is useful for monitoring therapeutic studies. No cross-reaction with heterophyid flukes has been reported.
Skintest
Intradermal skin testing has been used for epidemiologic studies but cannot be used to differentiate past from current infection.
Skin testing using extracts of adult C sinensis or P westermani antigens has been used in Korea and China as an epidemiologic tool.
Molecular methods
Most molecular methods are still in the experimental stage. A polymerase chain reaction (PCR) using the primer named OV-6F/OV-6R has been developed for the detection of Oviverrini in experimentally infected hamsters. The method has been found to be 100% sensitive in hamsters.[34] Duplex PCR has been designed for the simultaneous detection and differentiation of the liver flukes F hepatica and F gigantica from stool samples.[35]
Parvathi et al have evaluated a nested PCR for the specific detection of Csinensis. The PCR assay was found not to show any amplification with closely related trematode, O viverrini.[36] Detection of Csinensis in stool samples has been attempted using a real-time PCR assay. The sensitivity of the assay was found to be 100%, and the PCR cycle threshold values showed significant correlation with egg counts.[37] Real-time PCR (targeting the internal-transcribed-spacer-2 sequence of the parasite) to detect Csinensis –specific DNA in fecal samples was found to correlate with the egg counts in the stool, thus also being useful for quantification.
Various PCR protocols have been evaluated for the detection of intestinal schistosomiasis among which the most widely used molecular targets are the 121-bp rDNA sequence and mitochondrial NADH1 gene for S mansoni and 230-bp sequence from retrotransposon SjR2 for S japonicum.[38] Recently, PCR targeting the 121-bp Dra1 gene fragments in urine is being used for the diagnosis of urinary schistosomiasis caused by S haematobium.[39]
PCR-based techniques have the advantage in that they can detect the presence of trematodes irrespective of the stage of their life cycle. A species-specific PCR assay using internal transcribed spacer (ITS2) sequences that can distinguish between common food-borne trematodes such as Paragonimus, Fasciolopsis, and Fasciola species has been evaluated in a study from Shillong, India, and has been found to be unaffected by the life-cycle stages of the trematode parasites.[40]
Other parameters
A complete blood cell count may reveal eosinophilia in patients with fasciolopsiasis, schistosomiasis, heterophyiasis, metagonimiasis, early stages of paragonimiasis, and acute Clonorchis species infection (disappears in chronic Clonorchis species infection). Anemia may be found in patients with schistosomiasis, fascioliasis, and paragonimiasis. Gross and microscopic hematuria may be found in individuals with schistosomiasis. Neutropenia may be found in patients with fasciolopsiasis. Elevation of cerebrospinal fluid (CSF) pressure and pleocytosis and eosinophilia in the CSF may occur in individuals with cerebral paragonimiasis.
Chest radiographs in patients with schistosomiasis may reveal cor pulmonale and pulmonary hypertension, if present.
Radiographs of the liver exhibit tractlike small abscesses and subcapsular lesions in patients with fascioliasis.
Patchy foci of fibrotic change with a characteristic "ring shadow" (ie, circular or oval thin-walled cyst with a crescent-shaped opacity along one side) is the characteristic finding on chest radiographs in patients with paragonimiasis.
Ultrasonography
Ultrasonography is useful in evaluating the gall bladder and biliary tract in individuals with fascioliasis. Adult worms may be visible on sonograms or may appear as curvilinear lucent areas in the contrast medium on cholangiograms.
This is a sensitive procedure used to demonstrate urinary obstruction and hepatosplenic disease in persons with schistosomiasis.
Portable ultrasonography can be used for determining the extent of pathological changes, particularly in the liver and bladder, and can be used to screen populations at the community level. In addition, it can be used to assess the effects of chemotherapy.
CT scanning and MRI
CT scanning is useful in the study of CNS manifestations of trematode infections.
In persons with cerebral paragonimiasis, longstanding cerebral infection forms and cystlike structures may calcify and may be seen as clusters similar in appearance to soap bubbles.
In recent years, CT scanning and MRI have also been found to be useful in spinal paragonimiasis in addition to cerebral paragonimiasis, with imaging features specific for spinal involvement.[21]
CT scanning helps detect parenchymal lesions in individuals with fascioliasis
MRI may be useful in the study of CNS manifestations of trematode infections. MRI can also reveal granuloma of the liver parenchyma in cases of fascioliasis.
Biliary and pancreatic imaging
Cholangiography in individuals with fascioliasis reveals the multiple cystic dilatations of the ducts. Large cystic dilatation, small cystic ectasias, and mulberrylike dilatation are considered diagnostic of fascioliasis.
Endoscopic retrograde cholangiopancreatography (ECRP) has been found to be helpful in the diagnosis and treatment of biliary fascioliasis.[41]
Colonic biopsy is a sensitive and specific procedure to aid in identifying parasite eggs in biopsy specimens for the diagnosis of intestinal schistosomiasis and intestinal trematode infections.
Biopsy of neural tissue can be performed for diagnosis of neuroschistosomiasis.[18]
Cystoscopy is useful to help identify schistosome eggs in mucosal biopsy specimens from the urinary bladder and to exclude other causes of hematuria.
Egg granuloma is the typical pathologic lesion in urinary schistosomiasis. These are found in the ureter and urinary bladder. The granuloma consists mainly of eosinophils, macrophages, and lymphocytes surrounding the egg at the center. In chronic infection, fibroblast proliferation and fibrosis are characteristic.
Finger-sized fibrosis in the portal areas is characteristic of S mansoni infection.
Periportal fibrosis, Symmers fibrosis, and perisinusoidal blockage are the typical findings in S japonicum infection.
Adult Paragonimus flukes elicit an acute inflammatory reaction with formation of eosinophilic granulomas and small multiple fibrous cysts in the liver. The eggs also elicit an acute inflammatory reaction consisting of eosinophils, formation of a fibrous capsule, rupture of cysts in bronchioles, eosinophilic empyema, and, finally, calcification. The cystic encapsulation of the eggs in the lung and, less frequently in the brain and in other abdominal organs, is the key pathologic feature in paragonimiasis.
During the acute stage of fascioliasis, the liver is enlarged and exhibits hemorrhagic necrotic tracts in the subcapsular areas infiltrated by eosinophils and other inflammatory cells. In chronic infection, the bile duct exhibits epithelial hyperplasia with minimal pericholangitis and proliferation of tissues.
The infection of the biliary tract by C sinensis, O viverrini, and Ofelineus demonstrates adenomatous hyperplasia, periductal inflammation, periductal fibrosis, and diffuse or localized dilatation of ducts and may be associated with cholangiocarcinoma in C sinensis.
Ulceration of gut epithelium and localized inflammation are the features of infection caused by F buski and other intestinal flukes.
Causes of mortality include recurrent pyogenic cholangitis in persons with schistosomiasis; hemiplegia, cephalgia, and paresis in those with cerebral paragonimiasis; cholangiocarcinoma in those with clonorchiasis; and intercurrent bacterial infections in those with fascioliasis and/or intestinal fluke infections.
Praziquantel remains the drug of choice for all trematode infections except fascioliasis, for which bithionol is the drug of choice. Praziquantel is recommended when bithionol is not available.
Bithionol is the drug of choice for Fasciola infections. The drug of choice in the treatment of fascioliasis is triclabendazole, a member of the benzimidazole family of anthelmintics. The drug works by preventing the polymerization of the molecule tubulin into the cytoskeletal structures and microtubules. However, resistance of F hepatica to triclabendazole has already been recorded in Australia and Ireland. Artemether has been shown to be effective in a rat model of fascioliasis.
Emetine, dehydroemetine, chloroquine, albendazole, and mebendazole were once used in many trematode infections; however, this practice is now discontinued because these drugs are associated with toxicity and their efficacy is in doubt.
When trematode infections are complicated by intercurrent bacterial infections, institute antibiotic therapy.
Surgical management may be needed for complications of trematode infection, which include bladder carcinoma in patients with urinary schistosomiasis, fibrosis and thickening of the intestinal wall in those with intestinal schistosomiasis, ascending cholangitis in those with fascioliasis, and cholangiocarcinoma in those with clonorchiasis.
Schistosomicidal drugs, steroids, and surgery are the currently available treatments for neuroschistosomiasis.[18] A multidisciplinary approach is warranted for effective treatment of schistosomiasis.[42]
Ectopic fluke infections (eg, abdominal and cerebral paragonimiasis; ectopic schistosomiasis in intestine, lung, brain, or spinal cord; ectopic clonorchiasis) - Consultations with appropriate specialists as required per particular manifestations
To prevent paragonimiasis and clonorchiasis, avoid eating raw or undercooked fish.
To prevent infection with intestinal flukes and fascioliasis, properly clean and thoroughly wash raw vegetables, watercress, and other water-grown vegetables before eating.
Cook water-grown vegetables thoroughly before eating.
Chemotherapy objectives in trematode infections are to cure the disease, to reduce morbidity, and to prevent transmission of parasitic infection in endemic areas.
Bithionol (Lorothidol, Bitin) is the drug of choice for Fasciola infections; however, it is an investigational drug with distribution limited to physicians with patients who are unable to take praziquantel. Doses of 30-50 mg/kg/d PO for 5-15 days have been used to treat Fasciola infections. Repeat doses may be administered to some patients. Pediatric patients have been administered the same weight-based dosing used in adults. Adverse effects include nausea, vomiting, diarrhea, and abdominal pain.
Further outpatient care includes health education. Thorough cooking of fish and aquatic vegetables, fruits, and plants is necessary to prevent ingestion of infective forms of the parasite.
Note the following practices for deterrence and prevention of trematode infection:
Elimination of molluscan hosts using molluscicides, such as metal salts, and organophosphates or carbamates to control infection with schistosomes and intestinal, liver, and lung flukes
Proper sanitary disposal of human and pig excreta to avoid water contamination
Chemotherapy of infected persons
Avoidance of consumption of contaminated water, water plants, fruits, fish, crab, and raw liver
Thorough cleaning and washing of raw vegetables and aquatic fruits to prevent infection with intestinal flukes
Thorough cooking of water-grown vegetables before eating
Cooking of crab and crayfish before eating to prevent infection with lung flukes
Schistosomiasis complications can include the following:
Cor pulmonale
Portal hypertension
Urinary bladder carcinoma
Neurological complications: Cerebral and cerebellar tumour–like neuroschistosomiasis can present with increased intracranial pressure, headache, nausea and vomiting, and seizures. Myelopathy (acute transverse myelitis and subacute myeloradiculopathy) is the most common neurological complication of S mansoni infection. Schistosomal myelopathy tends to occur early after infection and is more likely to be symptomatic than cerebral schistosomiasis. The conus medullaris and cauda equina are the most common sites of involvement. Severe schistosomal myelopathy can provoke a complete flaccid paraplegia with areflexia, sphincter dysfunction, and sensory disturbances.[18]
Lung fluke complications can include the following:
Lung abscess
Pleural effusion
Ectopic lesions in the brain
Liver fluke complications can include the following:
Intercurrent bacterial infections
Less commonly, pancreatitis in fascioliasis[43]
Anemia
Recurrent pyogenic cholangitis and cholangiocarcinoma in clonorchiasis
Intestinal fluke complications can include asthenia with ascites in fasciolopsiasis.
Avoid high-risk food habits. Inform patients about the danger of eating raw or undercooked vegetables and fish and the importance of cleaning, washing, and adequately cooking vegetables and fish or raw liver.[2]
Subhash Chandra Parija, MD, MBBS, PhD, DSc, FRCPath, Director, Jawaharlal Institute of Postgraduate Medical Education and Research, India
Disclosure: Nothing to disclose.
Coauthor(s)
Shekhar Koirala, MBBS, Vice Chancellor, Department of Medicine, BP Koirala Institute of Health, Nepal
Disclosure: Nothing to disclose.
Thomas J Marrie, MD, Dean of Faculty of Medicine, Dalhousie University Faculty of Medicine, Canada
Disclosure: Nothing to disclose.
Specialty Editors
Francisco Talavera, PharmD, PhD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference
Disclosure: Received salary from Medscape for employment. for: Medscape.
Chief Editor
Mark R Wallace, MD, FACP, FIDSA, Infectious Disease Physician, Skagit Valley Hospital, Skagit Regional Health
Disclosure: Nothing to disclose.
Additional Contributors
Larry I Lutwick, MD, FACP, Editor-in-Chief, ID Cases; Moderator, Program for Monitoring Emerging Diseases; Adjunct Professor of Medicine, State University of New York Downstate College of Medicine
Allam G, Bauomy IR, Hemyeda ZM, Sakran TF. Evaluation of a 14.5 kDa-Fasciola gigantica fatty acid binding protein as a diagnostic antigen for human fascioliasis. Parasitol Res. 2012. 110:1863-71.
Grenfell RF, Martins W, Enk M, Almeida A, Siqueira L, Silva-Moraes V, et al. Schistosoma mansoni in a low-prevalence area in Brazil: the importance of additional methods for the diagnosis of hard-to-detect individual carriers by low-cost immunological assays. Mem Inst Oswaldo Cruz. 2013. 108:
Centers for Disease Control and Prevention. DPDx: Fascioliasis. Centers for Disease Control and Prevention. Available at http://www.cdc.gov/dpdx/fascioliasis/index.html
Le TH, Nguyen KT, Nguyen NT, Doan HT, Le XT, Hoang CT, et al. Development and evaluation of a single-step duplex PCR for simultaneous detection of Fasciola hepatica and Fasciola gigantica (family Fasciolidae, class Trematoda, phylum Platyhelminthes). J Clin Microbiol. 2012. 50:2720-6.
Guo JJ, Zheng HJ, Xu J, Zhu XQ, Wang SY, Xia CM. Sensitive and specific target sequences selected from retrotransposons of Schistosoma japonicum for the diagnosis of schistosomiasis. PLoS Negl Trop Dis. 2012. 6:e1579.
Adult worms in humans reside in the veins in various locations: Schistosoma mansoni in the inferior mesenteric veins, Schistosoma japonicum in the superior mesenteric veins, and Schistosoma haematobium in the vesical veins (these locations are not absolute). The females (size 7-20 mm; males slightly smaller) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S mansoni and S japonicum) and of the bladder and ureters (S haematobium), and they are eliminated with feces or urine, respectively. Under optimal conditions, the eggs hatch and release miracidia, which swim and penetrate specific snail intermediate hosts. The stages in the snail include 2 generations of sporocysts and the production of cercariae. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host, and migrate through several tissues and stages to their residence in the veins. Human contact with water is thus necessary for infection by schistosomes. Various animals serve as reservoirs for S japonicum and Schistosoma mekongi. Image courtesy of the US Centers for Disease Control and Prevention.
Eggs are excreted unembryonated in the sputum, or, alternately, they are swallowed and passed with stool (1). In the external environment, the eggs become embryonated (2), and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues (3). Miracidia go through several developmental stages inside the snail (4): sporocysts (4a), rediae (4b), with the latter giving rise to many cercariae (4c), which emerge from the snail. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, in which they encyst and become metacercariae. This is the infective stage for the mammalian host (5). Human infection with Paragonimus westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite (6). The metacercariae excyst in the duodenum (7), penetrate through the intestinal wall into the peritoneal cavity, and then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults (8) (7.5-12 mm X 4-6 mm). The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this occurs, completion of the life cycle is not achieved because the eggs laid cannot exit these sites. Time from infection to oviposition is 65-90 days. Infections may persist for 20 years in humans. Animals such as pigs, dogs, and a variety of feline species can also harbor P westermani. Image courtesy of the US Centers for Disease Control and Prevention.
The average egg size is 85 μm by 53 μm (range, 68-118 μm X 39-67 μm). They are yellow-brown, ovoidal or elongate, have a thick shell, and are often asymmetrical with one end slightly flattened. At the large end, the operculum is clearly visible. The opposite (abopercular) end is thickened. The eggs of P westermani are excreted unembryonated. Image courtesy of the US Centers for Disease Control and Prevention.
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.
Adult worms in humans reside in the veins in various locations: Schistosoma mansoni in the inferior mesenteric veins, Schistosoma japonicum in the superior mesenteric veins, and Schistosoma haematobium in the vesical veins (these locations are not absolute). The females (size 7-20 mm; males slightly smaller) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S mansoni and S japonicum) and of the bladder and ureters (S haematobium), and they are eliminated with feces or urine, respectively. Under optimal conditions, the eggs hatch and release miracidia, which swim and penetrate specific snail intermediate hosts. The stages in the snail include 2 generations of sporocysts and the production of cercariae. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host, and migrate through several tissues and stages to their residence in the veins. Human contact with water is thus necessary for infection by schistosomes. Various animals serve as reservoirs for S japonicum and Schistosoma mekongi. Image courtesy of the US Centers for Disease Control and Prevention.
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.
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.
Adult flukes size range is 20-75 mm by 8-20 mm. Image courtesy of the US Centers for Disease Control and Prevention.
Eggs are excreted unembryonated in the sputum, or, alternately, they are swallowed and passed with stool (1). In the external environment, the eggs become embryonated (2), and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues (3). Miracidia go through several developmental stages inside the snail (4): sporocysts (4a), rediae (4b), with the latter giving rise to many cercariae (4c), which emerge from the snail. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, in which they encyst and become metacercariae. This is the infective stage for the mammalian host (5). Human infection with Paragonimus westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite (6). The metacercariae excyst in the duodenum (7), penetrate through the intestinal wall into the peritoneal cavity, and then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults (8) (7.5-12 mm X 4-6 mm). The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this occurs, completion of the life cycle is not achieved because the eggs laid cannot exit these sites. Time from infection to oviposition is 65-90 days. Infections may persist for 20 years in humans. Animals such as pigs, dogs, and a variety of feline species can also harbor P westermani. Image courtesy of the US Centers for Disease Control and Prevention.
The average egg size is 85 μm by 53 μm (range, 68-118 μm X 39-67 μm). They are yellow-brown, ovoidal or elongate, have a thick shell, and are often asymmetrical with one end slightly flattened. At the large end, the operculum is clearly visible. The opposite (abopercular) end is thickened. The eggs of P westermani are excreted unembryonated. Image courtesy of the US Centers for Disease Control and Prevention.
Vector
Geographical Area
Type of Trematode
Biomphalaria glabrata
Brazil
S mansoni
Bulinus globosa
Nigeria
S haematobium
Bulinus truncate
Iran
S haematobium
Oncomelania hupensis nosophora
Japan
S japonicum
Thiara granifera
China
P westermani; M yokogawai
Semisulcospira libertine
China
P westermani; M yokogawai
Polypylis hemisphaerula
China
F buski
Parafossarulus manchouricus
China
C sinensis
Bithynia leachi
Germany
O felineus
Pirenella conica
Egypt
H heterophyes
Lymnaea truncatula
England
F hepatica
Trematode
Definitive Host
Intermediate Host
1st 2nd
Source of Infection
S haematobium
Humans
Freshwater snails (genus Bulinus)
Absent
Contact with water contaminated by cercariae
S mansoni
Humans, occasionally baboons and rodents
Freshwater snails (genus Biomphalaria)
Absent
Penetration of skin by cercariae
S japonicum
Humans, dogs, pigs, cattle, mice, mustelids, and monkeys
Amphibian snails (Oncomelania species)
Absent
Penetration of skin by cercariae
S mekongi
Humans and dogs
Aquatic snails (Tricula aperta)
Absent
Penetration of skin by cercariae
F hepatica
Sheep, goats, cattle, and other herbivorous animals
Amphibian snails (family Lymnaeidae)
Aquatic vegetations and watercress
Ingestion of aquatic plants and watercress infected with metacercariae
C sinensis
Humans, dogs, pigs, cats, rats, and several species of wild animals
Freshwater snails (family Bulinidae)
Freshwater fish (family Cyprinidae)
Eating raw or partially cooked freshwater fish or dried, salted, or pickled fish infected with encysted metacercariae
O felineus
Humans and other fish-eating mammals
Aquatic snails
Freshwater fish
Eating fish infected with metacercariae
P westermani
Humans, wolves, foxes, tigers, leopards, lions, cats, dogs, and monkeys
Freshwater snails (family Pleuroceridae and Thiaridae)
Freshwater crab or crayfish
Ingestion of freshwater crabs or crayfish infected with metacercariae
F buski
Pigs and humans
Planorbid snails of the genera Segmentina, Hippeutis, and Polypylis
Freshwater plants such as water caltrops, water chestnut, water bamboo, water hyacinth, and lotus
Ingestion of freshwater aquatic plants that harbor metacercariae
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