Will liver fluke be a greater threat in 2015?

As Phelim O’Neill reports, losses due to liver fluke are estimated to cost the Irish agricultural industry over €90m each year. This stems from direct losses in condemnation of damaged livers and losses in animal performance. The threat of liver fluke is closely linked to weather conditions and in particular rainfall levels. Reports from veterinary practitioners suggest there could be a heightened risk of problems this autumn and winter, particularly on the western half of the country, given the high volume of rainfall in recent months.

Farmers on more marginal lands or located in high rainfall areas who are accustomed to dealing with a history of liver fluke burdens are likely to already have a preventative/treatment programme in place. In some circumstances this may need to be tweaked to deal with a higher risk period such as more frequent dosing in sheep or an earlier or repeat treatment where cattle are grazing on heavier or waterlogged soils. A greater risk category could be farmers located in areas of recent prolonged high rainfall that have very little previous history of problems and as such may be dismissing a liver fluke threat.

Risk period

The seasonal nature of liver fluke burdens has a direct influence on the optimum health programme devised. High levels of rainfall and mild temperatures are delivering an ideal environment for a high parasite population in waterlogged soils. With migration of fluke in the animals typically occurring from August to October, the greatest threat is initially likely to be from acute liver fluke.

This means that from the middle to the end of November onwards, a high percentage of fluke parasites in animals are likely to be in the adult or late immature stage of their life cycle (each stage takes about four weeks).

This is not an exact science. For instance, in cases where initial infestation does not occur until October, adult fluke will not be present until January. In years of high exposure, there may also be a continual risk of acute live fluke from August until early January as was the case in 2012 when heavy rainfall led to high mortality in sheep and significantly reduced performance in cattle. This stresses the importance of selecting a product that treats the particular stage in the fluke life cycle.

Product choice

There are numerous flukicides on the Irish market, with the flukicide ingredient included individually or as part of a combination product containing a flukicide and wormer. Combination products can work well if timed correctly to treat fluke and worms. A downfall of these products is that they are frequently used at inappropriate times where there is no threat of fluke or worms and as such can increase the rate of resistance occurring to one or both active ingredients.

As Table 1 shows, flukicides differ in the stage of the life cycle they target. There are five flukicides available as individual products on the Irish market. The other active ingredient, Albendazole, is only available in combination form and best known through the products Albex and Valbazen. Products containing the flukicide Triclabendazole look the best product for fluke control on paper as they are effective against all stages – early, immature and adult. However, research in Northern Ireland and anecdotal evidence on farms in the south show resistance has developed on some farms to this ingredient.

This is primarily the case on sheep farms. To reduce the rate of resistance developing, vets recommend varying product choice and changing to a different flukicide from year to year or between seasons on sheep farms. Special reference is made to the use of products containing Oxyclozanide as, while it is not licensed for its use, it is the only available product that is also effective against rumen fluke. Many vets recommend limiting its use in winter until eight to 10 weeks after housing to ensure all fluke are in the adult stage.

Dairy farmers should be aware that there are restrictions on what products they can use for treating liver fluke in dairy animals or pregnant animals that will produce milk for human consumption. Products containing oxyclozanide and albendazole are licensed for use while Fasinex 24% can be used with limitations to its use. Producers should consult with their vet and develop a health programme that adheres to flukicide restrictions in dairy herds.

Timing of treatment

As mentioned above, timing of treatment is farm-specific and will depend on the likely risk. In sheep flocks, the general advice is fluke treatment in outwintered ewes every six to eight weeks. In years with a very high exposure, this may need to be increased in frequency to four to six weeks although at this rate of treatment many vets advise trying to limit, if possible, an animal’s exposure for a limited duration over the winter months.

There is generally a trade-off in deciding if treating beef cattle/cows previous to, at or post-housing. There is generally no need to treat cattle for fluke before housing unless animals are being retained outdoors late in the year. The decision to treat animals at housing boils down to farm circumstances. Delaying treatment for four to six weeks post-housing expands the range of available products that can be used but it does come with a risk of reducing animal performance in the intervening period.

If in doubt, consult your vet and make use of all available information sources. This includes asking your factory for a liver assessment on animals being slaughtered. Faecal egg analysis can also play a role in determining the need for treatment. However, it does have limitations as it only diagnoses the egg count meaning an animal could have a heavy acute fluke burden that the faecal egg count will not show.

This is particularly risky with sheep as acute liver fluke can hit fast when exposed to a high population of larvae.

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