Medical Care
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 triclabendazole is the drug of choice. [45]
Triclabendazole was approved in the United States in 2019 for fascioliasis in patients aged 6 years or older after being available from the CDC for many years. [46] 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.
Based on limited data from the CDC, nitazoxanide might be effective therapy in some patients with fascioliasis. Bithionol, a halogenated phenol previously used as a first-line agent for the treatment of fascioliasis in the United States, is no longer available. Praziquantel, which is active against most trematodes (flukes), is typically not active against Fasciola parasites. Therefore, the CDC does not recommend praziquantel therapy for fascioliasis. [47]
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 Care
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. [48]
Consultations
The following consultations may be necessary:
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Intestinal and liver trematode infections - Infectious diseases specialist
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Urinary schistosomiasis - Infectious diseases specialist, gastroenterologist, and urologist
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Pulmonary paragonimiasis - Chest disease specialist
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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
Diet
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.
Activity
The patient should be given adequate bed rest supplemented with an adequate protein-rich diet.
Prevention
Note the following practices for deterrence and prevention of trematode infection:
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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
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Proper sanitary disposal of human and pig excreta to avoid water contamination
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Chemotherapy of infected persons
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Avoidance of consumption of contaminated water, water plants, fruits, fish, crab, and raw liver
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Thorough cleaning and washing of raw vegetables and aquatic fruits to prevent infection with intestinal flukes
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Thorough cooking of water-grown vegetables before eating
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Cooking of crab and crayfish before eating to prevent infection with lung flukes
Further Outpatient Care
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.
Further Inpatient Care
Anemia may be treated with iron supplements and vitamins, which may be administered orally to facilitate iron absorption.
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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 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.
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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 "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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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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Adult flukes size range is 20-75 mm by 8-20 mm. Image courtesy of the US Centers for Disease Control and Prevention.
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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 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.
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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 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.
Tables
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 |
|
S haematobium |
S mansoni |
S japonicum |
Adult |
|
|
|
Body surface of male |
Finely tuberculate |
Grossly tuberculate |
Nontuberculate (smooth) |
Testes |
4-6, in a cluster |
6-9, in a cluster |
7, in a linear series |
Position of ovary |
Posterior to middle of body |
Anterior to middle of body |
Posterior to middle of body |
Number of eggs in uterus |
20-30 |
1-4 |
50-300 |
Egg |
|
|
|
Size and shape |
110-170 μm long 40-70 μm wide Terminal spine |
114-175 μm long 45-68 μm wide Lateral spine |
70-100 μm long 50-65 μm wide Central spine |
Cercaria |
|
|
|
Cephalic glands |
2 pairs, oxyphilic |
2 pairs, basophilic |
4 pairs, oxyphilic |
