Sylvaine Lacrosse BVetMed MRCVS, Molecare Veterinary Services
Listeriosis is a bacterial disease seen in many species, including humans, and is caused by the bacterial organism Listeria Monocytogenes. Generally associated with spoilt silage, the disease in sheep is often seen over winter or lambing, when sheep are housed and fed silage.
Circling sheep and rapid deaths are what most farmers think of when mentioning Listeria. Whilst this is true, the disease presents itself in many different forms, not just circling, and an accurate prompt diagnosis is often key to saving affected sheep.
It should also be noted that Listeria is in fact zoonotic, meaning it can be spread between animals and humans. Whilst the risk is very low, consequences to pregnancy can be devastating. Therefore, as a general rule, which is not only applicable for listeriosis, pregnant women should stay away from sheep during the lambing period.
What does listeriosis look like?
Encephalitis (inflammation of the brain) is the main cause of clinical signs of listeriosis. Inflammation usually affects one side of the brain, hence why we typically see unilateral signs such as circling, excessive salivation and paralysis of the affected side. With the brain being affected, infected sheep are often inappetent, disorientated and lethargic. These animals can be found pushed up into hedges or gates, eventually leading to recumbency and death.
Experts suggest that the bacteria reach the brain through blood circulation and with the organism being present in the blood, other clinical signs can be observed such as septicaemia, abortions and mastitis.
What can listeriosis be confused with?
Differential diagnoses include:
-Pregnancy toxaemia (twin lamb disease);
-Cerebrocortical necrosis (thiamine deficiency);
-Gid (tapeworm cyst in the brain);
-Hepatic encephalopathy (caused by liver pathology).
As shown above, there are plenty of other diseases which cause nervous signs in sheep, all with very different treatments which usually require prompt intervention. This is why the diagnosis is very important; so do not hesitate to speak to your vet for action or advice if you have any doubts.
What causes listeriosis?
The culprit organism for listeriosis, Listeria monocytogenes, is only part of the reason as to why outbreaks occur. The bacteria is drawn to organic matter, living in plant matter and soil, especially that found in spoilt silage. Spoilage in silage causes the pH to increase (become less acidic), favouring growth conditions and encouraging bacterial multiplication. Listeria monocytogenes can also be found in soil and faeces from healthy animals, therefore high stocking rates can lead to close grazing and consequently, ingestion of the bacteria.
How should listeriosis be treated?
With listeriosis having a particularly poor prognosis, one factor which has been shown to improve this is how quickly the disease is treated.
Treatment for listeriosis involves high doses of antibiotics, which is one of the reasons to involve your vet, as the dose rate would be off-label and potentially dangerous. The treatment of choice for Molecare Farm Vets would be high doses of penicillin, but other antibiotics such as oxytetracyclines have also been shown to be effective.
It is additionally important to nurse sheep when they are affected, ensuring they are still meeting their daily energy requirements, especially pregnant ewes. The disease can cause inappetence, therefore providing the ewe with fluids and propylene glycol can prevent concurrent pregnancy toxaemia.
Recovery rate can be up to 30% with early treatment, however signs such as recumbency drastically reduce prognosis, so it is important to know at what point the animal needs to be humanely euthanised.
How to prevent listeriosis?
The easiest way to prevent listeriosis is by avoiding feeding silage to sheep. However, with winter housing and having to feed ewes throughout pregnancy, we appreciate this is often not very realistic or possible. Therefore, effort should go towards preventing silage spoilage by avoiding soil and faecal contamination and preventing any air exposure throughout the storage period, whether clamped or baled.