AFBI forecasts high liver fluke risk for NI livestock this winter

The Agri-Food and Biosciences Institute (AFBI) has forecast a high risk of liver fluke infection across Northern Ireland for autumn and winter 2026-2027, and is telling farmers to review their control programmes now.

The forecast rests on a climate-based modelling system operated by AFBI staff. An AFBI spokesperson said: "From May to September 2026 the average monthly rainfall was 86.56mm." That figure sits below 2025 levels but exceeds 2024, and it represents an 8.03% increase above the average monthly rainfall for the summer months in Northern Ireland.

Temperature data reinforces the concern. "The mean monthly temperature from May to September 2026 was 14.21˚C," the spokesperson said. AFBI states this exceeds the levels recorded in each of the previous five years and sits 9% above the Northern Ireland summer mean.

The result: "Relating to the above, the Ollerenshaw index for sites monitored in NI was greater than 400," the spokesperson added. "This level prompts a forecast of high prevalence of fluke infection in stock, if appropriate measures are not taken."

Why the weather drives the parasite

AFBI explains that a mean day/night temperature of 10˚C or higher is necessary both for the intermediate host snail to breed and for fluke larvae to develop inside those snails. A temperature of 10˚C is also the minimum at which fluke eggs develop and hatch.

Given the 2026 rainfall and temperature figures, AFBI concludes that snail multiplication will have continued unabated on poorly drained pastures that stay wet year-round. "Thus, there is a high probability of liver fluke infection in the autumn and winter 2026-2027," the spokesperson said.

What does the disease cost?

Liver fluke presents in two forms. Acute disease strikes sheep when large numbers of immature flukes migrate through the liver; it is often fatal and carries serious welfare implications. Chronic fluke is more common, affects both sheep and cattle, and usually appears in winter and spring, though infection can persist all year.

The production losses are measurable. AFBI warns fluke infection can cut milk yield in dairy cows by 5-15% and reduce growth rates in fattening lambs and cattle. In dairy cattle it can also predispose animals to metabolic conditions such as ketosis and infectious diseases including salmonellosis. AFBI considers the same likely in sheep.

Migrating fluke can also open the door to the clostridial infection Black disease. AFBI advises farmers to ensure cattle and sheep in fluke-affected areas are fully vaccinated against it.

Which treatments and when?

AFBI advises reducing access to snail habitats on wet, poorly drained ground, or housing sheep or moving them to clean pasture. "However, in most cases, control will be based on the strategic use of anthelmintics, employing a product effective against the life cycle stages likely to be present in the flock or herd at the time of treatment," the spokesperson said.

That matters most in autumn, when acute infection hits sheep and both sheep and cattle are still picking up infection. A product effective against both immature and mature flukes is needed now. Treating out-wintered sheep once or twice in autumn, with a possible follow-up in January, plus a treatment against adult flukes in early spring, should significantly reduce the on-farm burden.

Winter and spring treatment of chronic adult infections in cattle and sheep helps cut pasture contamination with fluke eggs. "Use of an anthelmintic with activity mainly against adult flukes may be sufficient in these circumstances," the spokesperson said. "However, the flukicide programme used has to be on a 'know-your-farm' basis and no one set of recommendations will cover all flocks or herds."

Resistance warning

Resistance to fluke treatments is prevalent in Northern Ireland. On some farms, triclabendazole products — the only flukicide licensed in the UK and Ireland effective against the immature stages that cause acute fasciolosis in sheep — have been used almost exclusively for years. On those premises, triclabendazole may now be less effective, including for treating acutely ill animals.

Farmers can check treatment effectiveness by taking dung samples three to four weeks after dosing and submitting them through a vet for laboratory examination.

Rumen fluke also on the radar

Stomach (rumen) flukes have become common in sheep and cattle in Northern Ireland, especially in fluke-prone areas. Adult rumen flukes are less damaging than liver flukes, but heavy infections of immature flukes may cause diarrhoea, ill-thrift and, exceptionally, death in young animals. Farmers who suspect a problem should contact their vet for laboratory testing and treatment advice.

The AFBI veterinary laboratories at Stormont and Omagh can test dung and blood samples submitted by veterinary surgeons. Farmers should watch dung sample results three to four weeks after their autumn dosing to confirm their flukicide still works.