One all-too-common call this autumn comes from frustrated sheep farmers reporting several sudden deaths in apparently healthy adult sheep.

Invariably, the sheep are discovered dead over a period of a few days, and frequently the number of deaths approaches double digits. Often the frustration is accompanied by assurances that they are “well dosed and well vaccinated”. So what could it be?

A postmortem examination on any available carcases is invaluable in answering the question. One particular diagnosis this autumn is increasingly common: acute liver fluke. This diagnosis is usually met with confusion, since flukicide dosing has usually been carried out in the past few weeks and would have been regarded as accounted for.

However, dosing failure is common and the use of unnecessary or inappropriate dosing products is widespread. For example, the use of an adult flukicide on a high-fluke-risk sheep farm in autumn is not going to provide sufficient protection to the flock.

In addition, there is resistance to some active ingredients, the most common of which is Triclabendazole. Careful product selection is essential to avoid development and promotion of resistance. Your vet will be of assistance in this decision.

In suspected cases of dosing failure, further investigation is important to determine the presence of resistance or identify other reasons for the apparent failure.

Fasciolosis, or liver fluke infestation, is an annual problem – particularly in autumn and winter. It occurs in three main clinical stages: acute, subacute and chronic forms.

Which form occurs depends on the numbers of infected snails ingested and the period of time over which they are consumed.

Disease can result from the migration of large numbers of immature flukes through the liver, or from the presence of adult flukes in the bile ducts, or both.

This year has undoubtedly presented an increased level of Fasciolosis. Indeed, it is in no way limited to sheep. Heavy infestations are evident in cattle, and it is also causing problems in many horses too.

The persistently wet summer and mild autumn have been a major contributing factor in developing the particularly high challenge this year.

Treatment and prevention will vary from farm to farm. The choice of product and frequency of use of that product will depend on many factors, such as the level of fluke challenge, the time of year, farming system, dosing history and withdrawal periods. Veterinary advice is important in choosing the most appropriate product and developing a wider control plan.

*Nick Garvey works at Old Church Veterinary Hospital, Donegal, a part of XLVets. Visit www.xlvets.ie for more.