A Complete Guide to Listeriosis in Sheep

Listeriosis is a serious bacterial disease that affects sheep and other livestock. It is caused by Listeria monocytogenes, an organism that thrives in damp, cool conditions and is closely linked to the feeding of spoiled silage. The disease is of particular concern because it not only causes illness and death in sheep but is also zoonotic, meaning it can infect humans.

For sheep farmers in the UK, listeriosis is an important condition to understand, especially in flocks that rely on silage as winter forage. This guide provides a detailed overview of listeriosis in sheep, including causes, symptoms, diagnosis, treatment, prevention, and its wider impact.

What is Listeriosis?

Listeriosis is caused by Listeria monocytogenes, a bacterium that:

• Is found widely in soil, water, faeces, and rotting vegetation.
• Survives and multiplies in poorly fermented or mouldy silage.
• Can persist at low temperatures, even in refrigeration.

In sheep, infection usually follows ingestion of contaminated silage, though it can also occur through oral wounds, eye injuries, or contaminated bedding.

Causes and Risk Factors

Listeriosis in sheep is most often associated with:

• Poor-quality silage: particularly silage with a high pH, soil contamination, or mould growth.
• Rotting feed: spoiled hay, grass, or bedding.
• Soil contamination: mud and dirt in feed troughs or silage clamps.
• Stress: lambing, poor weather, transport, or overcrowding can weaken immunity.
• Environmental conditions: damp, cold weather encourages bacterial growth.

Forms of Listeriosis in Sheep

Listeriosis can appear in different forms depending on how the infection develops.

Neurological Form (Encephalitis)

• The most common form in adult sheep.
• Bacteria travel along cranial nerves into the brain.
• Clinical signs include:
  • Circling in one direction.
  • Head tilt and facial paralysis (drooping eyelids, lips, or ears).
  • Depression and loss of appetite.
  • Incoordination, stumbling, and collapse.
  • Convulsions and death in advanced cases.

Septicaemic Form

• More common in lambs.
• Bacteria spread throughout the bloodstream.
• Signs include fever, depression, weakness, diarrhoea, and sudden death.

Reproductive Form

• Occurs in pregnant ewes.
• Causes abortions, stillbirths, or weak lambs.
• Placental and foetal tissues are highly infectious to other animals and humans.

Diagnosis

Diagnosis is usually based on a combination of clinical signs, history, and sometimes laboratory testing.

• History: feeding silage, especially if poor quality.
• Clinical signs: circling, head tilt, abortion storms, or sudden deaths.
• Laboratory confirmation:
  • Culture of Listeria monocytogenes from brain tissue, placenta, or foetal material.
  • Post-mortem findings may include brain lesions or evidence of septicaemia.

Treatment

Listeriosis requires immediate veterinary attention. The earlier treatment begins, the better the chances of survival.

• Antibiotics: high doses of penicillin, oxytetracycline, or ampicillin are commonly used.
• Anti-inflammatories: NSAIDs may help reduce brain inflammation and pain.
• Supportive care: nursing, fluids, and provision of easily accessible feed and water.
• Isolation: affected sheep should be separated due to zoonotic risk and to reduce stress.

Sheep with advanced neurological disease, particularly if recumbent, often have a poor prognosis and may need to be euthanised for welfare reasons.

Prevention

Feed and Forage Management

• Avoid feeding poor-quality silage to sheep.
• Discard silage that is mouldy, spoiled, or contaminated with soil.
• Feed silage quickly once exposed to air and avoid long storage in troughs.
• Use high-quality forage or hay where possible, especially for pregnant ewes.

Housing and Environment

• Maintain clean, dry housing with good drainage.
• Remove soiled bedding regularly.
• Clean water troughs frequently.

Reproductive Health

• Isolate aborting ewes immediately.
• Dispose of placental material and foetuses safely and hygienically.
• Use gloves and disinfect equipment when handling potentially infected material.

General Management

• Minimise stress by avoiding overcrowding and unnecessary handling.
• Monitor closely during late pregnancy and lambing, when risk is higher.
• Implement good biosecurity to prevent introduction of infection from outside sources.

Zoonotic Risk

Listeriosis is a zoonotic disease, meaning it can infect humans.

• Particularly dangerous for pregnant women, the elderly, newborns, and those with weakened immune systems.
• Infection in humans is usually through consumption of unpasteurised dairy products or direct contact with infected material.
• Farmers and handlers should wear gloves when dealing with aborted material, practise strict hygiene, and never eat or drink while handling animals.

Economic and Welfare Impact

• Welfare: Listeriosis causes neurological disease, abortions, and death, making it a major welfare concern.
• Economic losses: abortion storms, loss of lambs, reduced milk production in dairy flocks, veterinary costs, and loss of breeding ewes.
• Prevention vs cost: Proper silage management and hygiene are inexpensive compared to the heavy losses associated with outbreaks.

When to Seek Veterinary Help

Seek veterinary assistance immediately if:

• Sheep show circling, head tilt, or other neurological symptoms.
• Multiple abortions occur within a short time.
• Sudden deaths happen after silage feeding.
• You need advice on silage quality control and disease prevention.

Conclusion

Listeriosis in sheep is a severe bacterial disease caused by Listeria monocytogenes, strongly linked to feeding poor-quality silage. It can present as neurological disease, septicaemia, or abortion, and often results in high mortality if not treated quickly.

Prevention through good silage management, clean housing, stress reduction, and strict hygiene is essential. For sheep farmers in the UK, vigilance during silage feeding periods, especially in winter and spring, is key to protecting flock health and minimising losses.