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Dicrocoelium dendriticum is slender and 6–10 mm long and 1.5–2.5 mm wide. It is found in many countries and infects a wide range of final hosts, including domestic ruminants. Another species, D hospes, is common in 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 cercariae are ingested by the second intermediate host, which is an ant (Formica fusca, in the USA), and encyst in the abdominal cavity. One or two metacercariae in the sub-esophageal ganglion of the ant cause abnormal behavior in which the ants attach themselves to the herbage, which in turn increases the probability of ingestion by the final host. The young flukes do not migrate through the liver tissue but reach the bile ducts from the intestine and begin laying eggs ~10–12 wk after infection.
There appears to be no immunity, and heavy infections may accumulate (up to 50,000 flukes in a mature sheep). Cirrhosis develops, 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. The eggs contain a miracidium and are very small (40 × 25 μm), lopsided, and yellowish brown.
The complex life cycle makes control of intermediate hosts almost impossible, because widespread chemical use has damaging ecologic effects on other similar organisms. Effective anthelmintic treatments in both cattle and sheep are albendazole at 15 mg/kg in a single dose or 2 doses of 7.5 mg/kg on successive days, or netobimin at 20 mg/kg.
Last full review/revision March 2012 by Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP
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