Fascioliasis

Causal Agent

The trematodes Fasciola hepatica (also known as the common liver fluke or the sheep liver fluke) and Fasciola gigantica are large liver flukes (F. hepatica: up to 30 mm by 15 mm; F. gigantica: up to 75 mm by 15 mm), which are primarily found in domestic and wild ruminants (their main definitive hosts) but also are causal agents of fascioliasis in humans.

Although F. hepatica and F. gigantica are distinct species, “intermediate forms” that are thought to represent hybrids of the two species have been found in parts of Asia and Africa where both species are endemic. These forms usually have intermediate morphologic characteristics (e.g. overall size, proportions), possess genetic elements from both species, exhibit unusual ploidy levels (often triploid), and do not produce sperm. Further research into the nature and origin of these forms is ongoing.

Life Cycle

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Immature eggs are discharged in the biliary ducts and passed in the stool . Eggs become embryonated in freshwater over ~2 weeks ; embryonated eggs release miracidia , which invade a suitable snail intermediate host . In the snail, the parasites undergo several developmental stages (sporocysts , rediae , and cercariae ). The cercariae are released from the snail and encyst as metacercariae on aquatic vegetation or other substrates. Humans and other mammals become infected by ingesting metacercariae-contaminated vegetation (e.g., watercress) . After ingestion, the metacercariae excyst in the duodenum and penetrate through the intestinal wall into the peritoneal cavity. The immature flukes then migrate through the liver parenchyma into biliary ducts, where they mature into adult flukes and produce eggs . In humans, maturation from metacercariae into adult flukes usually takes about 3–4 months; development of F. gigantica may take somewhat longer than F. hepatica.

Hosts

Fasciola hepatica and F. gigantica are primarily parasites of domestic and wild ruminants (most commonly, sheep, cattle, and goats; also, camelids, cervids, and buffalo). Infections occasionally occur in aberrant, non-ruminant herbivore hosts, including equids, lagomorphs, macropods, and rodents. Detection of Fasciola spp. eggs in the feces of carnivores probably represents spurious passage following consumption of contaminated liver.

The snail intermediate hosts for Fasciola spp. are in the family Lymnaeidae, particularly species in the genera Lymnaea, Galba, Fossaria, and Pseudosuccinea. At least 20 snail species have been identified as intermediate hosts for one or more Fasciola spp. Snail species may differ with respect to their suitability to serve as intermediate hosts for F. hepatica versus F. gigantica; host ranges for both Fasciola spp. are a subject of ongoing research.

Geographic Distribution

Fasciola hepatica is found on all inhabited continents, in more than 70 countries, particularly where sheep or cattle are raised. Human infections have been reported in parts of Europe, the Middle East, Latin America (e.g., Bolivia and Peru), the Caribbean, Asia, Africa, and rarely in Australia. Although the conditions for F. hepatica life cycle exist in the some parts of the United States, most of the reported U.S. cases of F. hepatica infection in humans have occurred in immigrants who became infected in other countries.

Fasciola gigantica is mainly found in tropical and subtropical regions. Human cases have been reported in parts of Asia and Africa, as well as in Hawaii and Iran.

“Intermediate forms” have been reported from areas, particularly in Asia, where both F. hepatica and F. gigantica are endemic. However, other non-sperm-producing forms with unusual ploidy and morphology occasionally have been reported in areas where the two species are not sympatric (e.g., the United Kingdom), which underscores the need for more research into atypical forms.

Clinical Presentation

Fasciola spp. infection in humans has two main phases, which may or may not be associated with symptoms or other clinical manifestations. During the early phase of the infection (usually referred to as the acute phase; also, the migratory, invasive, hepatic, parenchymal, or larval phase), the period when the larval fluke is migrating from the intestines and through the liver parenchyma, larval migration can be associated with inflammation, tissue destruction, and toxic/allergic reactions. Nonspecific symptoms/signs (e.g., abdominal pain, nausea, vomiting, hepatomegaly, malaise, fever, cough) and laboratory abnormalities (e.g., peripheral eosinophilia, elevated transaminase levels) may develop. Occasionally, larval flukes migrate to ectopic sites, such as the lungs, subcutaneous tissue, pancreas, genitourinary tract, eyes, or brain.

During the chronic phase of the infection (also referred to as the biliary or adult phase), clinical manifestations, if any, may develop months to years postexposure and include inflammation or blockage of bile ducts or the gallbladder (e.g., cholangitis, cholecystitis), which can be intermittent. Inflammation of the pancreas may also occur.