Although liver fluke is thought to be a reasonably common parasite, the significance of the disease is variable depending on geographical location and cumulative effects of the fluke on the liver. Liver fluke is known to be widely distributed throughout the North Island but it causes clinical disease in only some areas.
The life cycle of the liver fluke Fasciola hepatica is slightly different to normal parasites as it involves a small aquatic snail as an ‘intermediate host’. The adult flukes ‘graze’ on the lining of the bile ducts creating an inflammatory reaction and scarring. This results in protein loss and anaemia, and can compromise liver function if severe.
Fluke snails live in areas of reasonably clean, slow moving water that does not dry up in summer. The snails predominate from late summer through to early winter, so this is when the most juvenile flukes are available to livestock. Ingestion occurs when stock grazes in the wet areas where the snails live, so often fluke infections can be worse after dry summers.
The liver fluke infects the liver of many animals, but is mostly an issue in sheep. Although cattle are able to develop quite a strong resistance to fluke infection, sheep do not generate immunity and can potentially suffer ongoing liver damage from repeated infections. Thus we normally see fluke ‘disease’ in older ewes as their livers become more scarred over time.
Most of the white drenches on the market today will kill adult liver fluke; however immature flukes are only killed by a couple of specific drugs. If fluke genuinely is an issue, treatment is recommended in autumn/early winter to ensure removal of both adults and immatures. The ideal timing of these treatments depends on how early fluke infection starts on your property.
A diagnosis of liver fluke as the cause of ill-thrift should be based on careful examination of the underside of the liver. Adult flukes may be seen in the bile ducts (the big white ‘veins’ on the underside of the liver) but the presence of one or two adult flukes in a 6 year-old ewe may have little to do with the reason she has lost weight. The scarring can be a bit trickier to define as the bile ducts are quite thick-looking normally.
Alternative methods of diagnosis include:
Chronic disease will present as ill-thrift, anorexia and anaemia. Severe disease may also cause bottle-jaw. Do not neglect the other potential causes of these signs: chronic intestinal worm challenge, Johnes disease, molar tooth problems, old pneumonia lesions, liver damage from other causes and small intestinal cancers are just some of the reasons we see for wasting ewes.
In summary, be wary of diagnosing fluke solely on the visual appearance of sheep, and get us to give you a hand to nail down the cause of any tail-end (post-mortems can be very useful). If fluke is an issue, we can help you target any treatment to the most appropriate time.