Listeriosis in sheep

Sheep Diseases

Listeriosis

Also known as: Circling Disease, Encephalitis – Listerial

Listeria monocytogenes is found in soil, feedstuffs and faeces. Silage may become a source of infection if soil containing the organism is included. If possible, feeding sheep silage should be avoided.

Listeriosis is caused by the contamination of feedstuffs by the bacteria Listeria monocytogenes, which is widely distributed in nature and is found in soil, feedstuffs and faeces from healthy animals (Wardrope and Macleod, 1983). Listeria can survive for up to three months in stored livestock manure (Nicholson et al., 2005). Silage may become a source of infection if soil containing the organism is included (Low and Donachie, 1991, Wagner et al., 2005). Improperly prepared silage having a pH value greater than 5.4 and silage which has become spoiled due to prolonged exposure to air (Clarkson and Faull, 1987; Low and Renton, 1985) will enable the organism to multiply. Close grazing with soil ingestion may also be a cause. The incubation period is approximately 3 weeks following the introduction of silage (Low and Renton, 1985). Outbreaks of listeriosis in sheep fed silage usually affects less than 1% of the animals in a flock (Scott, 1992).

The prevalence of L. monocytogenes on ruminant farms is seasonal, and associated with farm management practices, animal health and hygiene and feedstuff quality and storage. It is often seen around lambing time when sheep are housed and fed silage or haylage.  The epidemiology and transmission of L. monocytogenes differ between small-ruminant and cattle farms (Nightingale et al., 2004; Nightingale et al., 2005).

Encephalitis is the most common form of listeriosis in sheep, although abortion, septicaemia and, less commonly, mastitis (Winter et al., 2004) and keratitis also occur (Low and Donachie, 1991). It is postulated that the organism reaches the brain by way of circulating blood, or branches of the trigeminal nerve that terminate in the oral cavity, nasal cavity or conjunctiva (Barlow and McGorum, 1985).

Because of the location of the inflammation in the brainstem, medulla oblongata and pons, signs of cranial nerve dysfunction are prominent in affected sheep. Typical signs of listeriosis are initially inappetence and depression, followed by circling to one side (Gill, 1937). With unilateral facial paralysis and excessive salivation on the side affected. Individuals will often run into gates or corners and lean up against fences, progressing to recumbency and death. Subtle differences in expression of these symptoms can make the diagnosis of listeriosis difficult. Central nervous system (CNS) fluid analysis may be used as an aid to diagnosis (Scott, 1992), in conjunction with haematology and biochemistry to check for other causes of CNS signs.

Death usually occurs within a few days and is preceded by recumbency and paddling of the forelimbs (Low and Donachie, 1991).

Listeria is a non-notifiable zoonotic disease, which can occur through contact with animals. Pregnant women are particularly at risk and should avoid contact with sheep during lambing. It is very uncommon for a human pregnancy to be affected, however the consequences can be extremely serious. Listeriosis can cause miscarriage and stillbirth or cause newborns to be born severely ill. Further information is available here: https://www.gov.uk/government/news/pregnant-women-advised-to-avoid-animals-that-are-giving-birth–2

Control and Prevention of Listeriosis

Awareness of risk factors may be used to develop control measures to reduce animal disease and introduction of L monocytogenes into the human food chain (Nightingale et al., 2005). Preventative measures are the most important factors. If possible, avoid feeding silage to sheep and certainly do not feed poor quality silage. If silage is to be fed to sheep, the inclusion of soil in the clamp or big bales must be avoided. Exposure to air should also be avoided (Low and Renton, 1985). Uneaten silage should be cleaned away from the sheep and animals that have aborted should be isolated.

Although Listeria can survive for up to three months in stored  livestock manure (Nicholson et al., 2005) and following application on land, the bacteria do not survive when high temperatures are achieved in manure storage  (Hutchinson et al., 2004).

Listeriosis is a zoonotic disease that can cause serious complications for pregnant women. It is therefore advised that pregnant women avoid all contact with ewes while they are lambing.

Treating Listeriosis

Treatment response of ovine listerial meningoencephalitis cases is generally considered to be poor (Low and Renton, 1985; Low and Donachie, 1991, Braun et al., 2002)). Treatment should only be instigated if the ewe is not recumbent. A number of different antimicrobials have been recommended by various authors, since L. monocytogenes is sensitive to a wide range of agents including penicillins and tetracyclines. Generally the impression is that the choice of antibiotics may be less important than the speed of diagnosis and therapy after the onset of clinical signs. Anti-inflammatories may also be useful. In heavily pregnant ewes, propylene glycol and rehydration therapy should be started concurrently to prevent a severe energy deficit and pregnancy toxaemia.

Listeriosis and Welfare

Affected sheep should only be considered for treatment if they are in the early stages of the disease; otherwise, they will have to be humanely killed. The veterinary surgeon must make this decision.

Good Practice Based on Current Knowledge

• If possible, do not feed silage to sheep
• If silage is fed to sheep, make sure it is of high quality and without soil contamination or prolonged air exposure
• If silage is fed to sheep, remove leftovers after feeding
• It is important that affected sheep are treated with antimicrobials as soon as possible after diagnosis
• Recumbent animals must be humanely destroyed
• Movement of livestock bedding waste from animal pens to a secondary store, and storage under conditions conducive for increased temperature, is a simple and cost-effective method to rapidly lower levels of zoonotic agents in solid farm wastes