General overview of Liver Fluke
Liver fluke is a common parasite of cattle in south eastern Australia. Liver fluke are different to some of the more common cattle round worms in that they have an intermediate host, a small snail (Lymnaea species) that they must pass through to complete their life cycle. Eggs passed out in the faeces of affected cattle infect the snail and multiply rapidly (from 1 egg to up to 4,000 larvae). These infective larvae (cercariae) have a tail which allows them to swim through moisture up a blade of grass, where they form a cyst that can survive for a year or more in the paddock and may also persist in fodder beyond harvest.
Once ingested the larvae travel through the intestinal wall and abdominal cavity to the liver. Juvenile fluke then migrates around in the liver for several weeks, causing tissue damage and bleeding. If the challenge is high this may cause acute disease with symptoms such as anaemia and weakness, with calves being most susceptible. The most significant economic losses from liver fluke are due to chronic disease, which occurs after the fluke matures and settles in the bile ducts. Affected ducts leak protein and blood, become thickened with fibrous tissue and eventually calcify. Heavy infections may lead to bottle jaw, weight loss, and diarrhoea. More frequently chronic disease will reduce milk production without causing obvious clinical signs. Production losses are thought to be in the order of 5-10% with low level infection, thus the potential losses are high as burdens increase.
Environmental conditions play a large role in the degree of infection of the pasture. The host snail generally prefers shallow, slow moving water bodies like irrigation channels, drains, springs, and swampy areas. Recent research in Victoria has indicated that leaking troughs are a high-risk area, so it follows that any location where there is moisture seepage is a potential habitat for the snail. The larval fluke stages require moisture to migrate and will die off in dry conditions. Reproduction is most rapid at about 25°C. At low temperatures (<10 °C) the snails become dormant and development of the larvae also stops. Hence the risk of new infections is variable according to seasonal and local conditions, but generally peaks from late spring through to autumn.
Treatment options vary in their ability to kill adult and juvenile fluke. Triclabendazole (TBZ) products such as Fasinex, Tremacide and Flukazole have the widest range of activity, and should be able to kill fluke down to 2 weeks of age. These products have a 21-day milk withhold and can only be used in young stock and dry cows. Chlorsulon containing products (Baymec Gold, Ivomec Plus) are only effective against adult fluke, but have nil milk withhold and thus can be used during lactation. Ideally dry stock should be treated with TBZ product in April-May when infective levels in pasture is high. A second treatment to reduce the adult fluke population is advised in August-September (e.g. Ivomec Plus). Heavily infected herds may need a third treatment of a TBZ product in January-February. Treatment of lactating cows is generally limited to TBZ at dry off and an optional adult treatment during lactation. Oxyclozanide (Nilzan) is also an option during lactation that is effective against adults but has been associated with negative side effects (milk drop, diarrhoea, nervous behaviour, recumbency). Unfortunately, after 40 years of use resistance to TBZ has been detected on some farms and should be considered when response to treatment is poor.
Tests available to assess fluke burdens on a herd level include faecal egg counts (specifically for fluke eggs), antibody levels in the bulk milk tank and a newer test that looks for fluke protein in faeces (CoproELISA antigen test). The CoproELISA test is becoming popular as a means of assessing herd prevalence and the severity of the infection within a cow. It can also be used to detect drench resistance by repeating the test a few weeks after treatment.
Eradication of liver fluke is near impossible due to the difficulties of controlling the host snail and larvae in the environment. A strategic approach of environmental management (e.g. fencing off drains/channels, fixing leaky troughs) combined with correctly timed treatments is necessary to control fluke levels on affected farms. As drug resistance develops to commonly used treatments an integrated approach will be even more critical.
Thank you to Dr Lauren Clyne BVSc (Hons) MVSc for writing this article for DairyTas.