Fasciola hepatica

Author: MBLOGSTU

Introduction

The hepatobiliary fluke Fasciola hepatica is estimated to infect over 17 million people worldwide and cause significant morbidity and mortality in livestock. Also known as liver rot, infection occurs by ingestion of metacercariae from contaminated freshwater vegetables such as watercress and water chestnuts.

F. hepatica is a hermaphroditic trematode whose life cycle includes an intermediate host (snail) and a definitive host (mammals). The adult worm is leaf-shaped, measures up to 30–13 mm, and has an average lifespan in humans of up to 10 years. It contains a ventral oral sucker, uterus, testes, intestinal system, and surface spines and resides in the biliary ducts, where it secretes an average of 9,000–25,000 eggs per day. The operculated ova are yellowish-brown and measure 140–75 µm.

Epidemiology

Fascioliasis is endemic in many regions and is often associated with sheep and cattle raising. Human infections have been reported in over 40 countries across Europe, North Africa, Asia, South America, and the Western Pacific.

Endemicity depends on a warm and humid climate, necessary for sustaining both the snail intermediate host and the animal reservoirs (usually sheep and cattle), combined with ingestion by humans of metacercariae from raw freshwater vegetables. Infections are often seen in the fall or winter, may occur in familial clusters, and are typically found in rural settings.

Although previously considered rare, recent surveys indicate higher prevalence rates, with reports of 3% in Portugal, 9% in Peru, and 7% in Egypt—the highest being 28% in one community in Bolivia.

Pathogenesis

In the acute stage, the pathologic findings include focal hemorrhage and inflammation in the duodenum. Once the flukes enter the liver, they digest hepatic tissue and cause inflammation, hemorrhage, and dilatation of intrahepatic bile ducts. They also produce subcapsular cavities and surface liver nodules measuring 5–15 mm in diameter.

Migration tracks from some nodules extend 1–2 cm into the liver parenchyma and contain cellular debris, Charcot–Leyden crystals, and eosinophilic inflammation; flukes may even be observed within these tracks.

The gall bladder is often involved with the presence of nodules and adhesions. The resulting inflammation can lead to fibrosis, thickening, and dilatation of the extrahepatic bile ducts and the gall bladder.

Clinical Manifestations

Fascioliasis presents with a wide spectrum, ranging from an asymptomatic state to significant gastrointestinal complaints. The clinical illness is generally divided into two phases, with a minority of patients developing extraintestinal manifestations.

Acute Hepatic Phase

In the acute phase, after the excysted metacercaria penetrates the duodenal wall and enters the peritoneal cavity, it migrates through the liver over 2–4 months. The classic triad of symptoms comprises abdominal pain, fever, and hepatomegaly.

Chronic Obstructive Biliary Phase

Once the fluke reaches the biliary tree, the chronic phase begins. There may be an asymptomatic latent period lasting months to years, followed by symptoms related to bile duct inflammation and mechanical obstruction. Patients can develop cholangitis and cholecystitis, although the percentage progressing to these complications is not well defined.

Ectopic Manifestations

Occasionally, flukes migrate to extrahepatobiliary sites such as the skin, stomach, pancreas, cecum, or lungs. A condition known as “halzoun” was once attributed to pharyngeal fascioliasis from ingesting raw liver contaminated with adult worms.

Laboratory Diagnosis

A combination of stool, serologic, and radiologic studies is utilized for the evaluation of fascioliasis.

Stool Analysis: Sensitivity for ova detection can vary from 0% to 100% depending on the technique used, infection intensity, and phase of infection (egg secretion is absent in the hepatic phase).

Serologic Tests: Several tests with high sensitivity are available, including enzyme-linked immunosorbent assay (ELISA), indirect hemagglutination (IHA), immunoelectrophoresis (IE), and electroimmunotransfer blot (EITB) for research purposes.

Radiology

Radiologic findings in fascioliasis are not pathognomonic but can be suggestive and useful for determining the extent of organ involvement. Ultrasound abnormalities have been reported in 0–93% of patients.

Molecular Techniques

DNA-based techniques are not yet commonly used in the clinical diagnosis of fascioliasis. In Japan, however, parasite DNA analysis has helped identify Japanese Fasciola species as F. gigantica rather than F. hepatica.