Silage can pose a liver fluke risk, study shows

Liver fluke can survive in silage unless it is well fermented, a study carried out at Liverpool University has shown. 

Liver fluke is a parasite that affects cattle and sheep. Liver fluke cysts are shed onto pasture from infected mud snails and while it is well-known they can survive on pasture for many months, given the right conditions, there has until now been little understanding of whether they can survive in silage.

A research project, funded by the Biotechnology and Biological Sciences Research Council, the University of Liverpool and AHDB, aimed to identify the risk of liver fluke infection from feeding grass silage to livestock and what management factors could decrease the risk.

See also: Blood tests could help catch early liver fluke in sheep

About the research

Mini-laboratory silos were used to investigate if the cysts found on grass survived ensiling after two, six and 10 weeks.

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 above industry recommended values (between 5-6). 

Silage pH varies depending on dry matter and whether it is clamped or baled, but 3.8-5 should be the target.

The effects of different dry matters (DM) were tested alongside whether or not cysts survived under anaerobic fermentation or when silages were exposed to oxygen.

Silage pH and DM content alongside lactic acid, butyric acid and N-ammonia levels were measured.

Results

Regardless of grass DM, all fluke cysts ensiled under strict anaerobic conditions were dead after two weeks. Therefore, if these silages were fed out there would be no risk of infection to livestock.

However, if silages were exposed to oxygen, the results were different. After 10 weeks, most 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. 

Liverpool University PhD student Bethan John, who carried out the research and was supported by independent silage expert David Davies of Silage Solutions, said the results suggest acidity and lack of oxygen are important factors in killing liver fluke cysts in silage.

The results also demonstrate unequivocally that liver fluke cysts do not survive in silage under anaerobic conditions and are killed as early as two weeks after sealing.

Take home messages

  • Avoid harvesting grass from wet, marshy pasture to help reduce the risk of incorporating contaminated forage into a grass crop.
  • Rapidly wilt cut grass to achieve the advised dry matter content of 28-30% for clamp and 35-40% for baled silages to achieve optimal fermentation.
  • Ensure grass is properly consolidated and sealed, so anaerobic fermentation can begin.

About the lifecycle

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 its intermediate host, the mud snail, which is found in damp, muddy environments. Here it develops and multiplies.

Large numbers of fluke leave the snail and encyst on grass. They are ingested by sheep and cattle when they eat contaminated grass. 

Fluke cyst

© Sinclair Stammers/Science Photo Library

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