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Fascioliasis

[Fasciola gigantica] [Fasciola hepatica]

Causal Agent

The trematodes Fasciola hepatica (the sheep liver fluke) and Fasciola gigantica, parasites of herbivores that can infect humans accidentally.

Life Cycle

lifecycle

Immature eggs are discharged in the biliary ducts and in the stool The number 1. Eggs become embryonated in water The number 2, eggs release miracidia The number 3, which invade a suitable snail intermediate host The number 4, including the genera Galba, Fossaria and Pseudosuccinea. In the snail the parasites undergo several developmental stages (sporocysts The number 4a, rediae The number 4b, and cercariae The number 4c). The cercariae are released from the snail The number 5 and encyst as metacercariae on aquatic vegetation or other surfaces. Mammals acquire the infection by eating vegetation containing metacercariae. Humans can become infected by ingesting metacercariae-containing freshwater plants, especially watercress The number 6. After ingestion, the metacercariae excyst in the duodenum The number 7 and migrate through the intestinal wall, the peritoneal cavity, and the liver parenchyma into the biliary ducts, where they develop into adults The number 8. In humans, maturation from metacercariae into adult flukes takes approximately 3 to 4 months. The adult flukes (Fasciola hepatica: up to 30 mm by 13 mm; F. gigantica: up to 75 mm) reside in the large biliary ducts of the mammalian host. Fasciola hepatica infect various animal species, mostly herbivores.

Geographic Distribution

Fascioliasis occurs worldwide. Human infections with F. hepatica are found in areas where sheep and cattle are raised, and where humans consume raw watercress, including Europe, the Middle East, and Asia. Infections with F. gigantica have been reported, more rarely, in Asia, Africa, and Hawaii.

Clinical Presentation

During the acute phase (caused by the migration of the immature fluke through the hepatic parenchyma), manifestations include abdominal pain, hepatomegaly, fever, vomiting, diarrhea, urticaria and eosinophilia, and can last for months. In the chronic phase (caused by the adult fluke within the bile ducts), the symptoms are more discrete and reflect intermittent biliary obstruction and inflammation. Occasionally, ectopic locations of infection (such as intestinal wall, lungs, subcutaneous tissue, and pharyngeal mucosa) can occur.

Fasciola hepatica eggs.

 

Eggs of Fasciola hepatica are broadly ellipsoidal, operculated and measure 130-150 µm long by 60-90 µm wide. The eggs are unembryonated when passed in feces. The eggs of F. hepatica can be difficult to distinguish from Fasciolopsis spp., although the abopercular end of the former often has a roughened or irregular area.

Figure A: Egg of F. hepatica in an unstained wet mount, taken at 400x magnification.

Figure B: Egg of F. hepatica in an unstained wet mount, taken at 400x magnification.

Figure C: Egg of F. hepatica in an unstained wet mount.

F. hepatica adults.

 

Adults of Fasciola hepatica are large and broadly-flattened, measuring up to 30 mm long and 15 mm wide. The anterior end is cone-shaped, unlike the rounded anterior end of Fasciolopsis buski. Adults reside in the bile ducts of the liver in the definitive host.

Figure A: Unstained adult of F. hepatica fixed in formalin.

Figure B: Adult of F. hepatica stained with carmine.

F. hepatica adults observed in endoscopic retrograde cholangiopancreatography (ERCP).

 

Adults of Fasciola hepatica observed with endoscopic retrograde cholangiopancreatography (ERCP) imaging.

Figure A: Adult of F. hepatica observed with ERCP imaging in the common bile duct of a human patient. Image courtesy of Dr. Subhash Agal, Kokilaben Dhirubhai Ambani Hospital, Mumbai, India.

Figure B: Adult of F. hepatica observed with ERCP imaging in the common bile duct of a human patient. Image courtesy of Dr. Subhash Agal, Kokilaben Dhirubhai Ambani Hospital, Mumbai, India.

Intermediate hosts of Fasciola spp.

 

Members of the genus Fasciola require a snail in the family Lymnaeidae to complete their life cycle. The species of snail can vary, in terms of location, habitat and elevation. In places where both F. hepatica and F. gigantica occur, each species of fluke has its own species or more of intermediate hosts.

Figure A: Galba truncatula, the main intermediate host of F. hepatica throughout most of the fluke's natural range in Europe and western Asia. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Figure B: Galba humilis, a host of F. hepatica in Canada and parts of the United States. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Figure C: Fossaria bulamoides, a host for F. hepatica in the western United States. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Figure D: Pseudosuccinea columella, a lymnaeid snail that has been introduced into South America and serves as an intermediate host for F. hepatica in Venezuela and Colombia. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Diagnostic Findings

Microscopic identification of eggs is useful in the chronic (adult) stage. Eggs can be recovered in the stools or in material obtained by duodenal or biliary drainage. They are morphologically indistinguishable from those of Fasciolopsis buski. False fascioliasis (pseudofascioliasis) refers to the presence of eggs in the stool resulting not from an actual infection but from recent ingestion of infected livers containing eggs. This situation (with its potential for misdiagnosis) can be avoided by having the patient follow a liver-free diet several days before a repeat stool examination. Antibody detection tests are useful especially in the early invasive stages, when the eggs are not yet apparent in the stools, or in ectopic fascioliasis.

Antibody Detection

The acute manifestations of human fascioliasis may precede the appearance of eggs in the stool by several weeks; immunodiagnostic tests may be useful for early indication of Fasciola infection as well as for confirmation of chronic fascioliasis when egg production is low or sporadic and for ruling out “pseudofascioliasis” associated with ingestion of parasite eggs in sheep or calves’ liver. The current tests of choice for immunodiagnosis of human Fasciola hepatica infection are enzyme immunoassays (EIA) with excretory-secretory (ES) antigens combined with confirmation of positives by immunoblot. Specific antibodies to Fasciola may be detectable within 2 to 4 weeks after infection, which is 5 to 7 weeks before eggs appear in stool. Sensitivity for the FAST-ELISA format of EIA was reported to be 95%, while sensitivity for the immunoblot using 12-, 17-, and 63-kDa antigens appeared to be 100%. However, some cross-reactivity occurs in the FAST-ELISA with serum specimens of patients with schistosomiasis. Antibody levels decrease to normal 6 to 12 months after chemotherapeutic cure and can be used to predict the success of therapy. CDC has developed an immunoblot assay for fascioliasis based on a recombinant F. hepatica antigen (FhSAP2). A positive reaction is defined if a band at ~ 38 kDa is present. The sensitivity of the assay is ≥ 94% (16/17) and specificity is ≥ 98% (113/115) for human with chronic fascioliasis. Currently we don’t have any information on the performance of this assay on acute fascioliasis cases.

Please note: Serologic testing for fascioliasis is available at the CDC. Pre-approval is necessary before submitting a specimen for fascioliasis testing. To obtain approval please contact the Parasitic Diseases Public Inquiries at (404) 718 4745 or at parasites@cdc.gov.

References:

Shin SH et al. Development of Two FhSAP2 Recombinant-Based Assays for Immunodiagnosis of Human Chronic Fascioliasis. Am. J. Trop. Med. Hyg., 2016: 95(4), pp. 852-855.

Hillyer GV. Serological diagnosis of Fasciola hepatica. Parasitol al Dia 1993;17:130-6.

Morphologic comparison with other intestinal parasites

Treatment Information

Treatment information for fascioliasis can be found at: https://www.cdc.gov/parasites/fasciola/health_professionals/index.html

DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.

  • Page last reviewed: December 8, 2017
  • Page last updated: December 8, 2017
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