ANSWER: Dicrocoeliasis
Amanda Mansz
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2026;30(2):28.
Figures 1A. & 1B. Formalin-fixed liver tissue from a 3-year-old ewe. Note the reduction of normal hepatic parenchyma and replacement with bands of intersecting fibrosis imparting a nodular appearance. Red arrows indicate bile ducts with periportal fibrosis and pigmented flukes within bile duct lumens. Figure 1C. Darkly pigmented, 3 – 6 mm long fragments of adult flukes retrieved from bile duct lumens, consistent with Dicrocoelium dendriticum (AKA lancet liver fluke).
The lancet liver fluke lifecycle begins when embryonated eggs hatch after being swallowed by various genera of land snails (first intermediate host). Mother sporocysts produce a second generation of daughter sporocysts that produce cercariae that leave the snail in damp weather by being expelled from the snail’s lung, clumped in a slime ball. The slime balls are swallowed by an ant (Formica fusca or other spp.) where the cercaria encyst, and the lifecycle is complete when the definitive host (the ewe in this case) swallows the ant. The route of larval migration through the definitive host is likely from the gut to the liver via the common bile duct where larvae mature and flukes begin laying eggs 10–12 weeks after infection. The total life cycle takes approximately 6 months. Infection in sheep is often considered incidental; however, the severity of the hepatic lesions is dependent on the number of parasites. Long-standing heavy burdens can result in significant hepatic scarring. Typical histologic lesions are shown in (Fig. 2).
Dicrocoelium dendriticum can infect cattle, sheep and other ungulates. Infection of alpacas and llamas are more commonly associated with acute clinical decline. Rarely, humans (and other non-human primates) can be accidental definitive hosts.
Detection of eggs in feces using a flotation technique with a high specific gravity 1.30 to 1.45 is recommended. Preventing animals from grazing in the early morning or late evening, when ants are in tetany, and covering ant nests with tree branches to keep animals away from the base may also help reduce infections.
Figure 2. Histologic lesions induced in the liver by infection with the lancet liver fluke in a 3-year-old-ewe. H&E stain. A. Moderate periportal fibrosis and extensive biliary hyperplasia with adult flukes in bile ducts (black arrows). Dark brown pigmented eggs within the adult fluke present grossly as black discolouration in portal triads (yellow arrow). 10x. B. Bile duct surrounded by biliary hyperplasia and fibrosis (double-headed arrow) containing an adult fluke with a prominent sucker (blue arrow) in the bile duct lumen. 20x. C. Bile duct surrounded by biliary hyperplasia and fibrosis (double-headed arrow) with an adult fluke with testis and sperm (red arrow) and dark brown pigmented operculated eggs (yellow arrow). 20x.
Reference
1. Cullen JM, Van Wettere AJ. Liver and Biliary System. In: Jubb, Kennedy, and Palmer's Pathology of Domestic Animals, 7th ed. Maxie MG, ed. Elsevier, 2026;2:325.