Listeriosis is primarily a winter-spring disease, most commonly but not exclusively, associated with silage feeding. Disease is caused by Listeria monocytogenes, a bacterium that lives in a plant-soil environment.

The less acidic pH of spoiled silage (pH >5.0) enhances multiplication of Listeria monocytogenes. The number of sheep clinically involved in an outbreak is usually less than 2 per cent but, in exceptional circumstances, may reach 10 per cent in a flock. Outbreaks occur around 14-21 days after feeding poor quality silage, whilst removal or change of silage in the ration often halts the appearance of listeriosis but cases can still occur for a further two weeks or so.

Typical farm occurrence includes:

  • Sheep aged 18 to 24 months
  • Poorly-conserved/stored silage
  • Outbreaks affects 2 per cent of sheep; rarely much higher incidence
  • 14-21 days after silage feeding commences

Clinical presentation
Bacterial infection is limited to one side of the brain in most animals; hence the one-sided appearance of nerve paralysis. Initially, affected animals do not eat/come to the feed trough, are depressed, disorientated and may propel themselves into corners, into fences, or under gates and feed troughs etc.

Affected sheep may lean against objects due to weakness affecting one side of body, with knuckling of the foreleg. There is profuse, almost continuous, salivation with food material impacted in the cheek of the affected side. There is a drooping ear, deviated muzzle, flaccid lip and lowered eyelid on the affected side.

Clinical signs may include:

  • Not eating
  • Depressed, disoriented
  • Propel themselves into corners, into fences, under gates and feed troughs
  • Lean against objects
  • Profuse salivation
  • Food material impacted in the cheek of the affected side
  • Drooping ear, deviated muzzle, flaccid lip on the affected side
  • Lowered eyelid on the affected side.

Differential diagnoses
Your veterinary surgeon may also consider the following diseases:

  • Pregnancy toxaemia in heavily pregnant ewes during the last four weeks of pregnancy
  • Peripheral vestibular lesions (middle ear infections)
  • Brain abscesses
  • Gid (coenurosis, tapeworm cyst in the brain)

Diagnosis of listeriosis is based upon a thorough veterinary examination.

Recovery of sheep from listeriosis depends on early detection of illness by the shepherd, together with prompt and aggressive antibiotic treatment prescribed by the veterinary practitioner. High doses of antibiotic are required to achieve appropriate concentrations within brain tissue to kill the bacteria. The overall recovery rate in sheep can be up to 30 per cent when sheep are presented early in the clinical course.

Propylene glycol, or a concentrated oral rehydration solution containing dextrose, should be administered as per the manufacturer's data sheet to prevent development of a severe energy deficit and the possibility of pregnancy toxaemia. Fresh palatable foods and clean water must always be available. A topical antibiotic eye ointment should be applied twice daily.

Outbreaks occur more than 10 days after feeding poor quality silage. If silage is being fed, use of that particular silage should be discontinued whenever possible. Spoiled silage should be discarded routinely or fed to growing cattle because of their much lower risk of disease. Clean feed troughs daily and discard refusals

Silage feeding:

  • Discard spoiled silage (or feed to cattle)
  • Clean feed troughs daily
  • Avoid soil/manure contamination of feed troughs from tractor wheels etc
  • Discard refusals
  • Clean water troughs regularly
  • Repair punctured wrapped bales immediately

The use of additives for grass silage is likely to produce a more acid pH which discourages multiplication of L. monocytogenes. Silage clamps must be rolled continuously during filling then sheeted to prevent entry of air. A block cutter operating along a short silage face limits air entry and secondary fermentation once the clamp has been opened. Every effort must be taken not to puncture wrapped silage bales during handling and storage, with all punctures sealed immediately. Stores of wrapped silage bales must be fenced against farm stock and vermin.

Silage making:

  • Use silage additives
  • Silage clamps must be rolled continuously
  • Silage clamps must be sheeted to prevent entry of air
  • Use a block cutter
  • Cut across a narrow silage face
  • Seal punctures immediately
  • Fence against farm stock and vermin