History
Humans become infected with intestinal flukes by consuming contaminated food and water that consists of or contains the second intermediate hosts (eg, vegetation, snails, fish).
Most infected persons are asymptomatic and exhibit no physical signs.
Individuals with moderate infection present with occasional loose stools, some weight loss, malaise, and, occasionally, generalized abdominal pain.
In severe infection, diarrhea may alternate with constipation. Hunger pangs may be one of the first symptoms to appear, usually occurring near the end of the incubation period. Minimal bleeding at the site of attachment may manifest as bloody diarrhea. As the infection progresses and the worm burden increases, intoxication results from the absorption of worm metabolites by the host. [38] The patient then experiences toxic and allergic symptoms such as edema of the face, abdominal wall, and lower limbs. Patients may report increased abdominal girth and abdominal pain. Anorexia, nausea, and vomiting are also common. The diarrhea persists, becoming greenish-yellow and exceptionally malodorous.
Heavy F buski infection may cause intestinal obstruction. On the other hand, eggs of H heterophyes may embolize, causing myocarditis, heart failure, and intracerebral hemorrhage. Eggs lodged in the spinal cord may manifest as motor and sensory deficits at the level of the lesion. [38]
Mild to moderate infections with the liver flukes C sinensis and O viverrini may also manifest as symptoms similar to those of intestinal flukes. However, distinction between these parasites is important, as these liver flukes are known to be associated with the development of cholangiocarcinoma. [39]
Physical
In mild infections, patients are asymptomatic on physical examination.
In severe infections, patients are asthenic, with gray and harsh skin and edema of the face and lower extremities.
Causes
Intestinal flukes are endemic in areas that contain abundant snail hosts (eg, China, Vietnam, India, other parts of Asia).
Table 2. Commonly Associated Exposures and Clinical Features of Certain Intestinal Trematodes* (Open Table in a new window)
Infection |
Source |
Clinical Features |
Alaria americana |
Undercooked frog legs |
Disseminated fatal thoracic, gastrointestinal, retroperitoneal, and CNS manifestations; intraocular infections |
Echinostomiasis (16 species) |
Freshwater fish, aquatic plants, clams, snails, mollusks, contact with aquatic birds |
May be asymptomatic; mild abdominal pain, bloating, dyspepsia, diarrhea, eosinophilia |
Fibricola species |
Tadpoles |
Abdominal pain, diarrhea, fever, eosinophilia |
Fasciolopsis species |
Water chestnut, water calthrop, water bamboo, water morning glory lotus and water hyacinth |
May be symptomatic; may be subclinical; gastritis, nausea, diarrhea, eosinophilia; generalized edema in persons with heavy infection burden |
Gastrodiscoides species |
Vegetables, aquatic plants |
Often asymptomatic; may manifest as abdominal pain and diarrhea in severe cases |
Watsonius watsoni |
Water bamboo |
Severe diarrhea |
Fischoederius elongates |
Aquatic plants |
Epigastric pain and vomiting |
Heterophyes species |
Mullets, fish; brackish water |
May be asymptomatic; intestinal mucosal disease, ulcer-related abdominal pain, dyspepsia, nausea, vomiting, diarrhea, weight loss |
Gymnophalloides seoi |
Oysters |
Fever, abdominal pain, anorexia, weight loss, diarrhea, pancreatitis |
Carneophallus brevicaeca |
Shrimp |
Fatal when infection involves CNS and heart |
Brachylaima ruminae |
Poultry, rats |
Abdominal pain, diarrhea |
Metagonimiasis species |
Fish (ayu, golden carp) |
May be asymptomatic; intestinal mucosal disease, ulcer-related abdominal pain, dyspepsia, nausea, vomiting, diarrhea, weight loss |
Nanophyetus salmincola |
Undercooked fish (eg, salmon, trout, steelhead) |
May be symptomatic; mild diarrhea, abdominal pain |
*Adapted from Berger SA, Marr JS. Human Parasitic Diseases Sourcebook. 1st ed. Sudbury, MA: Jones and Bartlett; 2006. |
||
-
Life cycle of Fasciolopsis buski. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
-
The life cycle of Fasciolopsis. Immature eggs are discharged into the intestine and stool and become embryonated in water. The eggs then release miracidia, which invade a suitable snail intermediate host, in which the parasites undergo several developmental stages (sporocysts, rediae, cercariae). The cercariae are released from the snail and encyst as metacercariae on aquatic plants, which are eaten by mammalian hosts (humans and pigs), who become infected. After ingestion, the metacercariae excyst in the duodenum and attach to the intestinal wall, where they develop into adult flukes (20-75 mm X 8-20 mm) in approximately 3 months and attach to the intestinal wall of the mammalian hosts. The adults have a life span of about one year. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
-
Egg of Fasciolopsis buski. Images reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
-
Adult fluke of Fasciolopsis buski. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
-
The life cycle of Heterophyes. The adult parasites release embryonated eggs (each with a fully developed miracidium), which are then passed in the host's feces. After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia, which penetrate the snail's intestine. Snails of the genera Cerithidea and Pirenella are important hosts in Asia and the Middle East, respectively. The miracidia undergo several developmental stages in the snail (sporocysts, rediae, cercariae). Many cercariae are produced from each redia. The cercariae are released from the snail and encyst as metacercariae in the tissues of a suitable freshwater or brackish-water fish (second intermediate host). The definitive host becomes infected by ingesting undercooked or salted fish that contains metacercariae. After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine, and mature into adults (measuring 1-1.7 mm X 0.3-0.4 mm). Heterophyes heterophyes infects humans, various fish-eating mammals (eg, cats, dogs), and birds. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
-
Life cycle of Metagonimus. The adult parasites release fully embryonated eggs (each with a fully developed miracidium), which are then passed in the host's feces. After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia, which penetrate the snail's intestine. Snails of the genus Semisulcospira are the most common intermediate host for Metagonimus yokogawai. The miracidia undergo several developmental stages in the snail (sporocysts, rediae, cercariae). Many cercariae are produced from each redia. The cercariae are released from the snail and encyst as metacercariae in the tissues of a suitable freshwater or brackish-water fish (second intermediate host). The definitive host becomes infected by ingesting undercooked or salted fish that contains metacercariae. After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine, and mature into adults (measuring 1-2.5 mm X 0.4-0.75 mm). M yokogawai infects humans, fish-eating mammals (eg, cats, dogs), and birds. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
-
Various animals may be definitive hosts for different Echinostoma species, such as aquatic birds, carnivores, rodents, and humans. Unembryonated eggs are passed in stool (1), and development occurs in the water (2). The miracidium takes an average of 10 days to mature and then hatches (3), penetrating the first intermediate host, a snail (4). Snails, in general, serve as the first intermediate host. The intramolluscan stages are as follows: sporocyst (4a); rediae (4b); and cercariae (4c). Cercariae may then encyst as metacercariae in the same first intermediate host or leave to penetrate a new second intermediate host (5). Several animals may become the second intermediate host, such as other snails, bivalves, fish, and tadpoles. The definitive host gets infected after eating infected second intermediate hosts (6). The metacercariae excyst in the duodenum (7). Adults then live in the small intestine (8). Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.
-
Magnified image of Echinostoma spp. egg. Echinostoma eggs resemble F busk and Fasciola eggs, with the latter two only bigger in size. Specimen slides courtesy of the University of the Philippines - College of Public Health, Department of Parasitology.
-
Low power magnification (arrow) of a Heterophyid egg. Specimen slides courtesy of the University of the Philippines - College of Public Health, Department of Parasitology.
-
Magnified view of a Heterophyid egg. Specimen slides courtesy of the University of the Philippines - College of Public Health, Department of Parasitology.
Tables
Infection |
Source* |
Geographic Distribution** |
|---|---|---|
Fasciolopsiasis |
Freshwater plants (water caltrop, water chestnut) |
Asia and Indian subcontinent, especially in areas where humans raise pigs and consume freshwater plants (as of December 8, 2017) |
Echinostomiasis |
Tadpoles, freshwater snails, fish, frogs |
Worldwide, but human cases seen most frequently in Southeast Asia and in areas where undercooked or raw freshwater snails, clams, and fish are eaten (as of December 27, 2017) |
Heterophyiasis |
Fish |
Egypt, Middle East, Far East (as of January 3, 2018) |
Metagonimiasis |
Fish (cyprinid) |
Far East, Siberia, Manchuria, the Balkan states, Israel, Spain (as of December 29, 2017) |
* Adapted with permission from Tribble D, Wagner KF. Trematode infections. Infectious Disease Practice. 1996; 20:69-73. ** Updated data from the Centers for Disease Control and Prevention (DPDx Laboratory Identification of Parasites of Public Health Concern. Available: http://www.cdc.gov/dpdx/) |
||
Infection |
Source |
Clinical Features |
Alaria americana |
Undercooked frog legs |
Disseminated fatal thoracic, gastrointestinal, retroperitoneal, and CNS manifestations; intraocular infections |
Echinostomiasis (16 species) |
Freshwater fish, aquatic plants, clams, snails, mollusks, contact with aquatic birds |
May be asymptomatic; mild abdominal pain, bloating, dyspepsia, diarrhea, eosinophilia |
Fibricola species |
Tadpoles |
Abdominal pain, diarrhea, fever, eosinophilia |
Fasciolopsis species |
Water chestnut, water calthrop, water bamboo, water morning glory lotus and water hyacinth |
May be symptomatic; may be subclinical; gastritis, nausea, diarrhea, eosinophilia; generalized edema in persons with heavy infection burden |
Gastrodiscoides species |
Vegetables, aquatic plants |
Often asymptomatic; may manifest as abdominal pain and diarrhea in severe cases |
Watsonius watsoni |
Water bamboo |
Severe diarrhea |
Fischoederius elongates |
Aquatic plants |
Epigastric pain and vomiting |
Heterophyes species |
Mullets, fish; brackish water |
May be asymptomatic; intestinal mucosal disease, ulcer-related abdominal pain, dyspepsia, nausea, vomiting, diarrhea, weight loss |
Gymnophalloides seoi |
Oysters |
Fever, abdominal pain, anorexia, weight loss, diarrhea, pancreatitis |
Carneophallus brevicaeca |
Shrimp |
Fatal when infection involves CNS and heart |
Brachylaima ruminae |
Poultry, rats |
Abdominal pain, diarrhea |
Metagonimiasis species |
Fish (ayu, golden carp) |
May be asymptomatic; intestinal mucosal disease, ulcer-related abdominal pain, dyspepsia, nausea, vomiting, diarrhea, weight loss |
Nanophyetus salmincola |
Undercooked fish (eg, salmon, trout, steelhead) |
May be symptomatic; mild diarrhea, abdominal pain |
*Adapted from Berger SA, Marr JS. Human Parasitic Diseases Sourcebook. 1st ed. Sudbury, MA: Jones and Bartlett; 2006. |
||
