Symptoms of the Disease/Condition.
Liver fluke can infect sheep, cattle, horses, pigs, goats, alpacas, deer, and a range of wild animals including wallabies and rabbits. Humans can also be infected by eating watercress from creeks in fluke-infested country.
Adult flukes live in the liver and bile ducts and can cause severe production losses and death in stock. Liver fluke has a complex life cycle with specific water snails as an intermediate host. Liver fluke is therefore more common on farms where there is lots of slow-moving water for the snails and a climate that suits the development requirements for both the snails and the liver flukes.
Liver fluke infections tend to be more common on the east and west coasts of the North Island and the west coast of the South Island.
Grazing animals ingest the metacercariae, which release immature flukes in the small intestine. The young flukes penetrate the intestinal wall, make their way to the liver, and then migrate through the liver tissue for 6–7 weeks before entering the bile ducts to become adults. Egg production starts 8–10 weeks after infection. Adult fluke can live for several years and produce over 20,000 eggs per day.
In sheep and cattle the "acute" disease may not be obvious and animals may show abdominal pain or become jaundiced followed by sudden weakness and death. This is uncommon in NZ but must be considered as a cause of acute deaths when very hungry stock have access to snail/fluke infected wet environments. The sudden and massive ingestion of metacercariae leads to an invasion of the liver by the immature flukes which effectively “chew the liver to bits”.
The "chronic" form is more common, in which case sheep and cattle lose condition, have poor milk production, show chronic diarrhoea, develop a soft fluid swelling under the jaw ("bottle jaw"), become weak and may die. Closer examination will reveal pale eye linings and gums caused by the loss of blood. In NZ the chronic form probably has the greatest potential impact.
Diagnosis is best confirmed by autopsy. There is a blood and faecal test available and surveillance from the slaughter premises can provide evidence of the extent of liver fluke infection in animals from the property.
Cost/Impact on Herd/Farm Revenue.
Damage may initially be caused by the immature stages of the fluke migrating through the animal's liver ("acute" liver fluke disease). These immature stages develop into adults which cause significant blood loss and liver damage in heavy infestations ("chronic" liver fluke disease). The liver lesions seen in sheep generally apply to cattle but the reactive fibrosis (scarring) is more marked and affected bile ducts show marked calcification.
Relatively small numbers of flukes can have a significant impact on wool production and pre-weaning lamb growth rates.
The effects of liver fluke infection on the productivity of adult cattle and the benefits from treatment are uncertain. Studies to determine the significance of subclinical infections in grazing cattle are inconclusive. Young cattle are more susceptible to liver fluke infection than older cattle.
It appears that nutrition levels, environmental conditions, age of the cattle, and level of infection play critical roles in determining the significance of infection. Cattle previously exposed to infection develop a substantial level of resistance to reinfection.
Liver fluke infection can make the animals more susceptible to the Clostridial disease called “Black disease”.
Management and Control.
There is a range of liver fluke drenches that are effective against liver fluke in cattle but they vary according to the age of the flukes.
Prevention has to rely on restricting access to high-risk areas of the farm. Attempts to eradicate the disease have generally been unsuccessful because both snails and liver fluke need to be eradicated at the same time. All potential liver fluke hosts need to be considered – sheep, cattle, deer, goats, horses, and rabbits to name a few!