Listeria sp. are ubiquitous environmental bacteria and are also found in the faeces of normal animals. They will multiply in silage following entry of air and when pH is ≥ 5.5. Spoiled silage is traditionally blamed for most outbreaks of listeriosis but in some cases has not always been fed. A wet winter will leave muddy conditions which could increase disease risk through ingestion of soil.
There are three main areas where listeria bacteria can cause issues:
- Listerial encephalitis (neurological signs) – symptoms include a head tilt and circling as well as paralysis on one side of the face. It is usually caused by listerial bacteria entering areas of damaged oral mucosa, gaining access to the nervous system, ascending to the brain stem and producing micro-abscesses. 2 to 6 weeks can elapse between infection and the development of clinical signs; therefore cases may continue after suspect forage is removed.
- Gastrointestinal/septicaemic – Localisation of ingested Listeria in the gut wall. Clinical signs of scour, pyrexia, dullness and anorexia can be seen within 2 days of infection.
- Abortion – Foetal infection with Listeria can occur at any stage of gestation. Very occasionally pinpoint abscesses are seen on the foetal liver in cases of listerial abortion. Abortion can occur from 5 to 12 days after infection.
The three disease syndromes are usually seen separately.
Advice for farmers:
- Check silage analyses for high pH and/or high ash content (>100g/kg DM) indicating large amounts of soil contamination at harvest e.g. cut very low, molehills or wet weather at harvest.
- Avoid feeding silage that is either obviously spoiled or from punctured bales. The top layer of the pit and the outer layer of bales are highest risk.
- Feed the best quality silage you can to the most sensitive stock particularly pregnant sheep and cattle.
- Dilute high risk forages with good forage, if this is not possible seek advice on alternative options for feeds.
- Clean away uneaten silage before adding more.
- Make silage accessible to sheep avoiding trampling by muddy feet.
- If rolls are fed directly onto the ground change the area where feeding takes place.
Mary Young, firstname.lastname@example.org
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