Liver fluke cysts are shed onto pasture from infected mud snails and we know they can survive for many months given the right conditions, but can they survive in silage? This question has been frequently asked but with no proven answer up until now. A research project funded by the Biotechnology and Biological Sciences Research Council, the University of Liverpool and the Agricultural and Horticultural Development Board (AHDB) and carried out by PhD student, Bethan John, aimed to identify the risk of liver fluke infection from feeding grass silage to livestock and what management factors could decrease the risk. David Davies (Silage Solutions), a silage expert, supported the project with advice based on his extensive experience in silage production.

What risk does liver fluke pose to cattle and sheep?

Liver fluke is one of the most important pathogens affecting sheep and cattle in the UK.  Heavy infections kill sheep rapidly whilst moderate infections lead to chronic disease and production losses.  Liver fluke is estimated to cost the UK cattle industry approximately £23 million annually through reduced growth rates and milk yield and is the leading cause of liver condemnation in the abattoir. Liver fluke is becoming increasingly common, in part because of our changing climate, with milder, wetter winters and less predictable summer weather.  Control of liver fluke relies almost exclusively on the use of specific veterinary medicines known as flukicides. Previous research has shown that resistance to one particular flukicide, triclabendazole, is common and widespread across the UK.  This means farmers are forced to use other flukicides that are less effective against the highly pathogenic youngest stages of the parasite. Increased use of the other medicines may lead to resistance developing to those as well.  It is therefore important that new ways are investigated to reduce levels of infection in sheep and cattle, so that farmers can reduce their reliance on flukicides.

The liver fluke life cycle depends on an intermediate host, a tiny mud snail, found in damp, marshy or muddy environments.  Parasite eggs are shed in the dung of infected animals and washed out into marshy areas by rainwater; when the eggs hatch, the parasite burrows into the snail where it develops and multiplies.  When it is ready, large numbers of fluke leave the snail and encyst on grass or other vegetation surrounding the damp ground where the snails live.  These parasite cysts infect sheep and cattle as they eat the contaminated grass. 

Looking for answers

Mini-laboratory silos were used to test the effect of ensiling on cyst survival. Different grass dry matter (DM) contents alongside anaerobic fermentation and aerobic deterioration conditions were investigated.  Fluke cysts were added to the grass at the start of the ensiling process and were recovered from each silo and tested for viability after two, six and ten weeks storage. Alongside visual indicators of aerobic deterioration (such as mould growth), silage pH and DM content alongside lactic acid, butyric acid and N-ammonia levels were measured.

The results showed that regardless of grass DM content, all fluke cysts, ensiled under strict anaerobic conditions were dead after two weeks. In other words, if these silages were fed out there would be no risk of infection to livestock.

For silages exposed to oxygen, the results were different. After ten weeks in silos, the majority of cysts were killed, but 30% of cysts were still viable in low DM (20%) content silage.  The pH of these deteriorated silages was >9.0, far higher than the pH in well-made silages.  These results suggest acidity as well as oxygen availability are important in killing liver fluke cysts in silage.

In these experiments, the grass was not treated with additives prior to ensiling, so only natural fermentation occurred, and the pH of the anaerobic silages produced was slightly above industry recommended values (pH 5 - 6).  Nevertheless, these results demonstrate unequivocally that under anaerobic conditions, liver fluke cysts do not survive in silage and are killed as early as two weeks after sealing.

What this means for your silage

Ideally, avoid harvesting grass from wet, marshy pasture to help reduce the risk of incorporating contaminated forage into a grass crop.  Rapidly wilting cut grass to achieve a recommended DM content of 28 - 30% for clamp and 35 - 40% for baled silages is important to achieve optimal fermentation. In practice, aerobic deterioration events may happen if bale wrap is pierced or when defective oxygen barriers are used to cover a clamp. The key to reducing the risk of liver fluke transmission from silage, is to ensure cut forage is properly consolidated and sealed so anaerobic fermentation can begin as soon as possible.

AHDB’s Animal Health & Welfare Scientist, Rebecca Small, says: “This research promotes good management of your silage, and shows that the risk of liver fluke infection is minimal provided optimum fermentation is achieved during silage production.  This will not only ensure that you have a good source of feed, but it also minimises the potential transmission of liver fluke from pasture to housed animals, subsequently decreasing the risk to stock.”