Listeriosis in sheep

What is listeriosis?

The bacterium Listeria monocytogenes causes listeriosis across the world. It is associated with serious disease in a wide variety of animals, including humans. Listeriosis is of major veterinary importance in cattle, sheep and goats. In the UK, it most commonly affects sheep. 

The gram-positive bacteria can survive in the environment for a long period of time. Grazing animals ingest the organism from pasture, with transmission of the disease via the faecal-oral route. 

Listeriosis is a zoonotic disease that can cause serious complications for pregnant women (visit NHS 111 for more information). Pregnant women are advised to avoid contact with sheep during lambing, and this disease is often why. It most commonly presents in 18–24 month-old sheep.

How do sheep get listeriosis?

Poorly stored silage can harbour the bacteria which cause listeriosis for many months.
Poorly stored silage can harbour the bacteria which cause listeriosis for many months.

The spread of L. monocytogenes on ruminant farms is seasonal. It tends to be associated with farm management practices, animal health, hygiene, and quality and storage of feedstuff. Flocks generally catch it around lambing season when sheep are housed and fed silage.

Outbreaks typically occur around 10-21 days after feeding poor-quality silage. L. monocytogenes thrives in the soil, in poorly made silage, and also in the faeces of healthy animals. Outbreaks of listeriosis in sheep fed silage usually affect less than 1% of the animals in a flock. The bacteria can survive for up to three months in manure, which can increase the risk of exposure over a long period.

What are the signs of listeriosis?

The most severe infections with listeria bacteria can lead to septicaemia (blood poisoning), meningitis (inflammation of the membranes surrounding the brain) and encephalitis (inflammation of the brain). Septicaemia can also lead to abortions in pregnant sheep.

Encephalitis is the most common form of listeriosis in sheep. Inflammation in the brainstem means that the disease presents through nerve dysfunction in affected sheep. The infection is typically limited to one side of the brain, and so affected animals can show lop-sided nerve function. 

Lack of appetite and disorientation, followed by a circling behaviour to one side are typical signs of listeriosis. The disease is sometimes called ‘circling disease’ as a result of this behaviour. Other signs are drooping ears, flaccid lips, lowered eyelids, and excessive salvation on one side. Sheep will often run into gates or corners, lean up against fences or lean against objects. This is a result of weakness affecting one side of the body. During the terminal stages, they are unresponsive to treatment. Seizures may occur during the final stages in some animals. 

What treatments are there?

Differences in expression of symptoms can make the diagnosis of listeriosis difficult. Listeriosis can be confirmed after death, but that is not always helpful. Of course, it pays to be aware of the signs and to keep track of any problems in a flock. Always speak to your vet if you suspect listeriosis.

Recovery of sheep from listeriosis depends upon early detection of illness and then prompt antibiotic treatment. Supportive therapies such as energy drenches can help if sheep haven’t been eating.

How do farmers control it?

Preventative measures are the most important factors. You should always take care when selecting the best silage for feeding, and mouldy materials should always be discarded. Try not to puncture wrapped silage bales during handling and storage and do not leave bales unwrapped for days before feeding.  It’s also good practice to avoid soil or faecal contamination of feed and water.

Although Listeria organisms can survive for up to three months in stored livestock manure, the bacteria do not survive in high temperatures. Fully effective control measures have not been established yet and currently no vaccine is available in the United Kingdom.


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