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Listeriosis Alert

1 March 2019


The SAC Consulting vet labs have had a run of cases in the last two weeks associated with Listerial infection found in both cattle and sheep abortions and animals with neurological signs. This was a bit unexpected given the dry start to summer 2018, but a number of factors may have favoured listeria multiplication:

  • High dry matter silage tends to have a high pH (>4.5) due to reduced fermentation.  These drier silages are often more difficult to consolidate making it very hard to exclude all of the air from the clamp/bale to maintain anaerobic conditions.  Listeria bacteria are naturally present in the soil and multiply quickly in silage where air is present, being particularly problematic in poorly fermented silages that are not acidic enough.
  • This year soil contamination could be high if the sward was cut low to compensate for poor grass yields resulting in high ash content on silage analysis.  During cutting the dry conditions produced a lot of dust as well.
  • August and September were wet so later cut silage may have been of poorer quality.  Listeria bacteria also thrive on dead material in the bottom of a sward.

There are three main areas where listeria bacteria can cause issues:

  • Listerial encephalitis (neurological signs) – It takes 10 to 21 days for clinical signs to become apparent so cases may continue after suspect forage is removed.
  • Gastrointestinal/septicaemic – Localisation of ingested Listeria in the gut wall can result in clinical signs of scour within two days.
  • Reproductive – Foetal infection with Listeria can occur at any stage of gestation and abortion occurs from 7 days after infection.

Advice for Farmers

  • Sheep are the most susceptible, followed by pregnant cattle, with livestock destined for slaughter least affected by this silage.
  • Risk factors: high pH, dry silage (lack of fermentation), small amounts of air, visible spoilage, soil contamination (high ash content in forage analysis – above 10%)

Feed the best quality silage you can to the most sensitive stock particularly pregnant sheep and cattle. Discard and do not feed silage that is visibly mouldy. Dilute high risk forages with good forage, if this is not possible seek advice on alternative options for feeds.  and

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