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Liver Fluke in Cattle

Cause

The liver fluke, Fasciola hepatica, infects a number of animals including cattle, sheep, goats and horses. Unlike other parasites of grazing animals, the liver fluke life cycle requires an intermediate host, the mud snail (Galba truncatula). These snails are only present in damp, muddy areas such as the borders of permanent water bodies, wet flushes (often identifiable through presence of rushes and other water loving plant species), ditches, boggy areas etc. often referred to as “flukey” pastures. Mud snails and the external life stages of liver fluke are only active when temperatures rise above 10^oC, meaning development of liver fluke on pastures in the UK generally occurs from mid- to late-Spring through to early Autumn. The development time of liver fluke within the mud snail host can be prolonged, taking six weeks or more depending on weather conditions. Consequently, pastures are usually at their most infectious from August to October, although due to the long-lived nature of the infective “cyst” stages, and survival of infected mud snails over winter, pastures can be infective for most of the year.

Once ingested by cattle, the juvenile flukes migrate to, and then through the liver, causing tissue damage as they go before reaching the liver bile ducts, where they mature to adults and begin producing eggs.

Unlike roundworms, all ages of cattle are susceptible to infection with liver fluke, as protective immunity does not develop against this parasite. Due to the biology of the external life stages, and it’s mud snail host, risk is greater when there has been sustained warm, wet weather. Unsurprisingly, fluke is a bigger problem in parts of the UK with suitable weather conditions, although it also appears to be increasingly common in areas previously considered lower risk as a result of changing weather patterns.

Additionally, and again unlike roundworms, both sheep and cattle are susceptible to infection with liver fluke, meaning pastures grazed by one species can act as a source of infection for the other, including drug resistant strains of the parasite.

Figure 1. Life cycle of liver fluke

Image: COWS Control of liver and rumen fluke in cattle https://www.cattleparasites.org.uk/liver-fluke-and-rumen-fluke/

Welfare and Economics

Liver fluke is thought to cost over £40 million to the UK cattle industry each year. Economic losses occur through clinical disease, loss of milk yield, reduced growth rates and other production impacts. Fluke-infected animals may also be more susceptible to infection with other diseases e.g. Salmonella and Black disease, particularly in unvaccinated animals.

Figure 2. Fluke damage to the liver is easily identifiable post mortem. Image: Phil Scott

Clinical Signs and diagnosis

Cattle tend to accumulate fluke burdens towards the end of the grazing season. In large numbers, juvenile stages can cause liver damage resulting in acute disease typically in early autumn, although this is less common in cattle than it is in sheep.

More usually, infection progresses to its chronic stage, becoming apparent in late-autumn and winter with weight loss and poor performance. Occasionally, signs of severe protein deficiency are seen with bottlejaw or brisket swelling. Diarrhoea may also be seen with chronic fluke infection, possibly due to lowered immunity to other infectious diseases. Many chronically infected animals will show no obvious signs of disease, yet their productivity may be adversely affected. These animals are also an important source of pasture contamination if left untreated. At slaughter, chronically infected livers are easily identifiable due to damaged areas and bile ducts becoming calcified and gritty.

Figure 3. Weight loss during winter is a sign of chronic fluke in cattle

Many diseases other than chronic fluke infection cause weight loss and poor performance. A veterinary diagnosis is therefore required for clinical cases.

Migrating and adult fluke and associated liver damage are easily identifiable at postmortem. Abattoir reports may be the first indication of a fluke issue seen by farmers.
Faecal samples can be used to test for either fluke eggs or by copro-antigen ELISA. Unlike antibody testing (below), these will only detect current infection. Copro-antigen testing can detect infection from as early as 4-6 weeks and is useful for testing of individual samples. Egg counts can be performed on individual or pooled samples, but will only detect infection if adult fluke are present and shedding eggs, which only occurs from 12 weeks after infection onwards. Egg counts are most useful in late winter/ early spring, although for the reasons discussed a single negative result should not rule out fluke infection.
Screening blood samples for fluke antibodies are useful to assess if a herd has been exposed, particularly in youngstock in their first grazing season if tested in late summer/ early autumn. Antibody testing can detect fluke infection from as early as 2 weeks, but it is important to note these levels do persist for several months following treatment, meaning positive results in adult cattle may not necessarily indicate a current infection.
For dairy cattle individual or bulk milk tank sampling can be used to test fluke antibody levels at an individual or herd level.
Signs of liver damage consistent with fluke can be identified on diagnostic blood biochemistry samples.

Prevention and control

Integrating prevention strategies with targeted flukicide treatment is important to control this disease and prevent resistance to flukicides, particularly for dairy farms with limited treatment options. Treatment with the correct flukicides reduces pasture contamination for the following season.

Avoiding grazing the highest risk, “flukey” pastures during peak risk periods for fluke, particularly in wet years. These can be assessed through the NADIS fluke forecasts published in August and again in October-December. Where possible, fencing off and isolating likely snail fluke habitats on pastures will decrease exposure.

Figure 4. Prevent fluke exposure in peak season by avoiding high risk pasture.

Strict biosecurity and quarantine treatments are essential to prevent introducing the parasite to fluke-free farms with suitable snail habitat. Prevention of entry of triclabendazole resistant fluke with bought-in animals is also important, particularly if the farm has sheep too, either co-grazing or winter grazing. Remember fluke can be brought in with cattle or sheep. For cattle, the COWS group have published specific quarantine guidelines to avoid the introduction of both liver fluke and triclabendazole-resistant fluke onto farms.

Treatment

Treatment selection for cattle should be used to prevent clinical disease and losses associated with subclinical chronic fluke infections. Fluke maturity remains an essential consideration when choosing a product. Triclabendazole resistance is reported in the UK, and its use should be reserved for situations where it is necessary to treat young immature stages such as acute stage infections. Often in beef cattle, treatment can be delayed until several weeks after housing and a different product will be just as effective. In dairy cattle it is also important to consider milk withdrawal periods and possibly planning treatments where needed during the dry periosd, since relatively few flukicide products are licenced for use in lactating animals.

It is also important to note that pour-on preparations of triclabendazole (e.g. Cydectin triclamox) are not effective against very immature fluke unlike the oral forms, whilst for use as a flukicide albendazole must be used at a higher dose than is required for use against roundworms. As with roundworm treatment selection, always aim to use the narrowest spectrum possible and only use a combination product if both are indicated. Ultimately, product selection and timing of administration is complex and should be discussed and incorporated into the herd health plan.

Table 1. Flukicide actives by fluke age. From: Adapted from Noah Compendium of datasheets 2020

Table 2. Flukicides licensed for dairy cattle. From: Adapted from Noah Compendium of datasheets 2020

Figure 5. Products to treat fluke in lactating dairy cattle are very limited