Dicrocoelium dendriticum in Ruminants
(Lancet fluke, Lesser liver fluke)
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.
The first intermediate host is a terrestrial snail (Cionella lubrica, in the USA), 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 USA), with metacercariae forming in the abdominal cavity. One or two metacercariae in the subesophageal ganglion of the ant cause abnormal behavior in which the ants climb up and remain on the tips of the herbage where they attach themselves, which increases the probability of ingestion by the definitive host. Once ingested, the metacercariae excyst in the small intestine, migrate up the main bile duct, and then on to smaller ducts. They begin laying eggs ~10–12 wk after infection. The total life cycle takes ~6 mo.
In cattle, sheep, and goats there appears to be no immunity, 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 be seen 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 contain a miracidium and are very small (40 × 25 μm), 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 makes control of intermediate hosts almost impossible, because widespread chemical use has damaging ecologic effects on other similar organisms. 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 the nest, covering nests with tree branches to keep animals away from the base can also be useful. Effective anthelmintic treatments are available, but most must be administered at dosages higher than those recommended for F hepatica. Effective anthelmintic treatments 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.