Fascioliasis Hepatica

Fascioliasis is a helminthic infection resulting from exposure to the trematode Fasciola hepatica or Fasciola gigantica. They are also known as common liver flukes. Their definitive hosts are sheep, goats and cattle,. At least 20 species of snails have been identified as intermediate hosts for Fasciola. More than 70 countries have detected Fasciola hepatica. Fasciola gigantica is mainly found in tropical and subtropical regions. 

Fasciola can also infect people. Human fascioliasis most often results from the consumption of freshwater plants such as watercress and water chestnuts, to which the infective metacercariae attach themselves. Once they are ingested, the metacercariae emerge from a cyst in the intestine. Within a period of 2 to 24 hours, they migrate into the peritoneal cavity as immature flukes. After 48 hours, they penetrate Glisson’s capsule of the liver. Over a period of 7 weeks, the flukes migrate through the liver parenchyma, resulting in necrosis and an eosinophilic infiltration. During this larval stage, the clinical symptoms and signs include right-upper-quadrant abdominal pain, nausea, weight loss, fever, and eosinophilia. These acute symptoms of infection generally last 2 to 3 months.

Dermatologic manifestations of acute infection include urticaria and migrating cutaneous nodules as a result of larval migration to the skin. Imaging of the liver shows characteristic hypoattenuating tracks extending from the liver capsule into the parenchyma due to larval penetration and migration. 

In the chronic, or biliary, stage of fascioliasis, adult hermaphrodite flukes release eggs in the hepatic and common bile duct of the host. This latent stage, which can last for decades, may be characterized by biliary obstruction, ascending cholangitis, acute pancreatitis, or mucosal erosion and hemobilia.

During the acute phase, immature larvae do not release eggs and are therefore not detected on stool examination. Since antibodies to Fasciolidae can develop within 2 to 4 weeks after cyst ingestion, serologic testing is the mainstay of diagnosis in patients with an acute infection. 

Eggs can be detected by light microscopy during the chronic phase of infection. Eggs can be detected in feces, duodenal fluid, or biliary drainage. F. hepatica and F. gigantica eggs are morphologically indistinguishable. Adult flukes may be detected with endoscopic retrograde cholangiopancreatography (ERCP). Migrating larval flukes may be seen in histologic sections.

CDC has developed a CLIA-approved immunoblot assay for the diagnosis of Fasciola infection, which is based on a recombinant F. hepatica antigen (FhSAP2)*. A positive reaction is defined as the presence of a band at ~38 kDa. The sensitivity of the assay is 94% and the specificity is 98%  for humans with chronic Fasciola infection. This assay has not yet been validated for acute Fasciola infection.

References

Micic D et al. Hiding in the Water. N Engl J Med 2020; 382:1844-1849.

Shin, SH, et al, Development of two FhSAP2 recombinant–based assays for immunodiagnosis of human chronic fascioliasis. Amer J Trop Med Hyg, 2016; 95(4), 852-855.