The liver fluke life cycle begins when eggs are passed onto pasture in animal faeces. These hatch and infect a small mud snail (Galba truncatula), where they multiply and develop. Under suitable wet conditions, immature fluke called cercariae leave the snail and encyst on vegetation as metacercariae — the highly infectious stage.

Animals become infected when they graze contaminated pasture. The ingested metacercariae develop into juvenile flukes that migrate through the liver, causing significant damage, before moving into the bile ducts where they mature into adults. Adult fluke produce eggs that pass out through the digestive tract and continue the cycle.

It takes approximately 10–12 weeks from ingestion of metacercariae for adult fluke to be present in the bile duct.

Unlike gut worms, animals do not develop immunity to liver fluke, meaning all ages are susceptible.

Sheep

• Acute fasciolosis

Caused by large numbers of migrating juvenile fluke. Can lead to sudden death from internal bleeding or secondary clostridial infections (Black Disease). Signs may include extreme lethargy, abdominal pain, reluctance to move, and collapse. Most common in late summer and autumn.

• Sub-acute fasciolosis

Fewer migrating fluke cause rapid weight loss, poor fleece quality, inappetence, weakness and depression. Death can occur without treatment. Typically seen in autumn.

• Chronic fasciolosis

Caused by adult fluke in the bile ducts. More common in late winter and early spring. Signs include poor body condition, anaemia and bottle jaw (fluid swelling). Results from impaired liver function and inflammation.

Cattle

Cattle typically develop chronic disease, shown by poor weight gain and anaemia. Sub-clinical infection can reduce milk yield. Diarrhoea is possible but usually linked to secondary Salmonella infection.