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Dicrocoelium dendriticum in Ruminants

(Lancet Fluke, Lesser Liver Fluke)


Lora Rickard Ballweber

, DVM, DACVM, DEVPC, Department of Microbiology, Immunology, and Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University

Reviewed/Revised Nov 2021 | Modified Oct 2022

Dicrocoelium dendriticum is slender (6–10 mm long × 1.5–2.5 mm wide). It is found in many countries and infects a wide range of definitive hosts, including domestic ruminants. Another species, D hospes, is common in West Africa, and D chinensis is found in East Asia.

The first intermediate host is a terrestrial snail (Cionella lubrica, in the US), from which cercariae emerge and are aggregated in a mass of sticky mucus (slimeball). The slimeballs of cercariae are ingested by the second intermediate host, which is an ant (Formica fusca, in the US), and metacercariae form in the ant's abdominal cavity. One or two metacercariae reside in the subesophageal ganglion of the ant, causing abnormal behavior in which, as temperatures fall, the ant climbs up onto the tips of herbage. Temperatures < 15°C , cause tetany in the ant's mandibular muscles making it unable to let go. This increases the probability that the ant and its metacercariae will be ingested by the definitive host, particularly during early-morning and late-evening hours. Once ingested, the metacercariae excyst in the small intestine, and juvenile flukes migrate up the main bile duct and then on to smaller ducts. They begin laying eggs ~10–12 weeks after infection. The total life cycle takes ~6 months.

Cattle, sheep, and goats appear to have no immunity to D dendriticum, and heavy infections may accumulate (up to 50,000 flukes in a mature sheep) with minimal pathologic or clinical changes. Cirrhosis can develop, and the bile ducts may be thickened and distended. Economic loss is due primarily to condemnation of livers. Clinical signs are not obvious but may occur in massive infections. Infections in alpacas and llamas are associated with an acute decline in condition, recumbency, hypothermia, and anemia. Liver enzyme values tend to be within normal limits. Severe pathologic changes occur within the liver and bile ducts, including cirrhosis, abscesses, and granulomas.

The eggs of D dendriticum contain a miracidium and are very small (40 × 25 mcm), lopsided, and yellowish brown. Fecal flotation with a solution of high specific gravity (1.30–1.45) is recommended to detect D dendriticum.

The complex life cycle of D dendriticum makes the control of intermediate hosts almost impossible; in addition, chemical use has damaging ecologic effects. However, keeping ducks, turkeys, or chickens to eat the snails can effectively reduce the intermediate host populations in small areas. Because infected ants are usually found within 30–50 cm of the base of their nest, covering ant nests with tree branches to keep animals away from the base can also be useful. Preventing animals from grazing in the early morning or late evening, when ants are in tetany, may also help reduce infections. Effective anthelmintic treatments are available; however, most must be administered at dosages higher than those recommended for Fasciola hepatica. Effective anthelmintic treatments (>90% reduction) in both cattle and sheep are albendazole at 15–20 mg/kg in a single dose or two doses of 7.5 mg/kg on successive days, or netobimin at 20 mg/kg. Praziquantel (50 mg/kg) has been shown to decrease egg-shedding by ~90% in llamas.

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