Bovine fasciolosis

Liver Fluke

Bovine fasciolosis is an important disease of cattle, especially in the wetter western half of the British Isles. Calves and yearlings are most commonly affected, but any age of animal may be susceptible to the effects of infection. Climatic and hydrological factors play an important part in the epidemiology of the disease, and restrict the geographical distribution as well as affecting the severity of the disease encountered.
Eggs from the adult fluke are passed into the intestine to be voided in the faeces. One fluke can pass between 5000 and 20,000 eggs per day. When conditions are moist and warm, the fluke eggs hatch into small infective larvae (miracidia). Propelled by cilia, these larvae actively seek a snail host of the genus Lymnaea. The mud snail Lymnaea truncatulata is the host snail in the United Kingdom. In the digestive tract of the snail, they develop into young sporocysts which then follow one of two courses. The first is for the sporocysts to develop into rediae which enter the liver of the snail and produce daughter rediae; the second is to produce second generation sporocysts which cannot produce rediae. From the rediae, daughter rediae or second generation sporocysts, cercaria are produced. From one miracidium, six hundred or more cercariae are produced, emerging from the snails after about 5 to 10 weeks, depending on the weather. The cercariae swim to attach themselves to herbage where they lose their tails and secrete a tough cyst wall to become metacercariae. At 12-14 ° C, up to 100% of metacercariae can survive for six months, though only 5% survive for 10 months. For prolonged survival, the relative humidity needs to be above 70%.
Once ingested by cattle, the young fluke burrow through the gut wall and pass to the liver. They are voracious feeders and migrate through the liver parenchyma to reach the bile duct, where they mature. Egg laying takes place some 10-12 weeks after the initial infection.
Most development of the larval stages takes place during the spring and summer months and ceases during the winter. The 'summer' infection of snails results from the hatching of overwintering eggs or eggs passed in the spring. The metacercariae then appear on pasture from August to October. A smaller 'winter' infection of snails is due to the infection of snails in the autumn; larval development in the snails ceases during the winter months and commences again in spring resulting in pasture contamination with metacercariae in May and June.
Acute fasciolosis is uncommon in cattle. The clinical signs of chronic fasciolosis are variable and depend upon the number of metacercariae ingested. Weight loss is common. Anaemia, oedema and cachexia develop gradually. Diarrhoea and constipation have also been described (Torgerson, 1999).