Confined animals can suffer from a range of disorders that can impact both welfare and health. Neurological disease can be a severe and life-threatening issue. The major differential diagnoses for neurological disease in confined sheep include listeriosis, hypocalcemia, hypomagnesemia, nervous ketosis, and polioencephalomalacia (PEM).1 All of the common differentials are also nutritional in origin, which emphasizes the importance of proper diets, especially for confined animals.
Listeriosis (Listeria monocytogenes) causes meningoencephalitis, abortion and gastroenteritis. Clinical signs are associated with the affected cranial nerves, generalised proprioceptive deficits and altered mentation. Non-specific signs such as fever, lethargy and separation from the mob may be seen.1 Given the causative agent, severity of clinical signs and progression of the disease, listeriosis is challenging to treat and carries a poor prognosis.2
Hypocalcaemia often causes generalised signs of weakness and paresis or paralysis in severe cases. Hypomagnesaemia in sheep causes neuromuscular dysfunction, twitching, and convulsions which can progress to tetany. Ruminants are particularly susceptible to magnesium and calcium deficiencies, which may predispose confined animals on inappropriate diets to disorders related to their deficiency.3 The clinical signs of acute PEM include blindness leading to recumbency, seizures, and coma. Separation from the flock, staggering gait, recumbency, and tremors of the head and neck are common clinical signs in cases of nervous ketosis.4
In May 2019, a southern Riverina producer reported sporadic losses of 20 sheep over the previous month. The sheep were found dead after displaying neurological signs. The mob consisted of 2000 both pregnant and empty mixed-age merino ewes held in a confinement lot. Their diet was a total mixed ration of pit silage that had been produced 10 years ago with wheaten hay at a 30:70 ratio. There was a mixed limestone, salt and magnesium oxide loose lick available in each pen. The pregnant ewes were scanned for pregnancy five days prior to the call.
A fortnight later the property was revisited following five more deaths.
Management changes between visits included changing the diet and/or paddock of the two mobs of affected ewes – one mob was left in the original containment area but not fed silage, the other mob was moved to a new containment area but were again fed silage after a week without it in the diet. At this time, ewes consuming silage began to show similar clinical signs and there were deaths that prompted the second visit. The water source for both groups was also changed.
All affected sheep were 3-4 years of age. Clinical signs started with altered mentation, including dullness and separation from the mob. The producer also described signs of hyperaesthesia in affected sheep. No abortions or stillbirths were noted.
During the first visit: A twin-bearing merino ewe in body condition score (BCS) of 2-2.5/5 at an advanced stage of pregnancy was presented for examination. Clinical exam findings included injected mucous membranes, hyperaesthesia, lateral recumbency, bilateral loss of pupillary light response, extended hind limbs and flexed forelimbs, no tongue tone, apparent blindness and head turning. Post-mortem aqueous humour sample was taken for biochemical analysis, which showed a hypocalcaemia (1.19 mmol/L) and elevated beta‐hydroxybutyrate (0.63 mmol/L). Magnesium levels were normal.
During the second visit: A twin-bearing merino ewe in BCS of 2-2.5/5 was presented. Clinical exam findings included injected mucous membranes, mild pyrexia, right lateral recumbency with extension of hind limbs and flexion of forelimbs. No tongue tone and no pupillary light response detected. Blood was collected for complete blood count and biochemical analysis, significant findings included elevation of beta‐hydroxybutyrate (1.64 mmol/L) and elevated haptoglobin (1.4 g/L). Brain was extracted for transmissible encephalopathy exclusion.
The first visit ewe was euthanased for post-mortem examination. There was excess salivation, with hypostatic congestion of the left lung. There were two foetuses in third stage gestation present. Ruminal stasis evident by large amount of feed present with a rumen pH of six. Small intestine was hyperaemic. Pancreas was enlarged. Liver, integument, cardiovascular and urogenital system appeared grossly normal.
The second visit ewe was euthanased for post-mortem examination. Similar to the first ewe there was two foetuses present near full term, evidence of ruminal stasis with feed present. Otherwise there was no abnormalities noted grossly.
Samples from both ewes were collected and sections of the brain stem, cerebellum, caudal cerebellar peduncles, cerebral cortex, kidney, liver, spleen, lung and heart were examined. The findings from other tissues outside the central nervous system were fairly generalized and non-specific.
Brainstem findings in both cases included meningoencephalitis, necrosuppurative multifocal subacute marked with microabscess, spheroids and lypmhohistiocytic perivascular cuffs.
On routine culture of brainstem from the second ewe there was pure growth of Listeria monocytogenes.
A diagnosis of listeriosis was made based on isolation of Listeria monocytogenes from the brainstem. Multifocal areas of necrosis associated with marked infiltration of gitter cells along with lymphohistiocytic perivascular cuffing is consistent with an infectious aetiology.
Hypocalcaemia, hypomagnesemia and nervous ketosis were considered as differential diagnoses in this case given the signalment of affected ewes and as many of the ewes were in late gestation.3,5 Hypocalcaemia and hypomagnesemia were less likely differentials as there was only a small number of sheep initially affected compared to those at risk, there was dietary supplementation of calcium and magnesium, and there was an absence of pathognomonic clinical signs for each disease process (muscle weakness – hypocalcaemia, muscle twitching - hypomagnesemia). Nervous ketosis remained a differential as both ewes that were sampled had an elevated beta‐hydroxybutyrate. However nervous ketosis tends to generally carry a slower more insidious onset of progression, apparent blindness and gait weakness, which were not observed in affected ewes in this case. The hypocalcaemia observed in the first ewe, and the elevated ketone bodies in both ewes is most likely secondary to inappetence of affected ewes in this case.
Treatment to rule out PEM as a differential was not attempted due to the delayed presentation of both ewes for diagnostics. PEM was therefore difficult to exclude in the absence of laboratory testing. Further diagnostic work including culture of samples, which was conducted in this outbreak investigation, is often warranted for a definitive diagnosis.
Pathogenic organisms and mycotoxins are can be found in silage.6 Listeria monocytogenes, a facultatively anaerobic bacteria can multiply at pH 4.5-9.6, and temperatures between -0.4°C and 45°C. Significant numbers of Listeria may be found in the top layers of silage even if oxygen exposure is limited.6,7 Regardless of removal of contaminated material these pockets of Listeria can then be a source of future proliferations and infections once re-exposed to oxygen on feeding.7,8
The two most common disease syndromes in sheep caused by L. monocytogenes are abortion and meningoencephalitis. However, it is rare for both manifestations to occur in the same flock.9 The path of entry of L. monocytogenes into the body plays a role in the disease syndrome that is observed. If the organism enters the body via ingestion this route of entry allows penetration of the intestine to reach systemic circulation. From there Listeria monocytogenes can proliferate in the liver and spleen surviving intracellularly among resident macrophages.8 On multiplication to sufficient numbers, septicaemia leads to the development of gastroenteritis and abortion syndromes, the latter more common in sheep. Routes of infection causing meningoencephalitis syndromes include ascending infection of the trigeminal (cranial nerve V) nerve following buccal mucosal lesions. In this case as different forms of roughage made up the entire diet, trauma to the buccal mucosa could have played a role in the development of meningoencephalitis. Risk factors causing immunosuppression such as climatic changes and pregnancy make sheep more susceptible to disease.9
Due to the severity of the disease and sudden onset, aggressive antibiotic treatment (penicillin and tetracyclines are effective) is recommended early in the course of the disease.10 Pharmacological treatment was not attempted in this case – initial control measures were to remove the ewes from confinement where possible or remove silage from the ration and change the water source until a definitive diagnosis could be made. After the recommencement of feeding the silage and recurrence of clinical signs prompting the second visit and positive L. monocytogenes culture, silage was again removed from the ration and ewe losses ceased.
Testing a representative sample of feedstuffs for proliferation of Listeria monocytogenes prior to feeding would occur in ideal conditions but is not practically feasible as feed samples collected may not reflect the true distribution of the pathogen. Listeria bacteria being ubiquitous in the environment means that positive cultures may also occur in samples that are in fact safe for livestock consumption. Ultimately, careful preparation and handling of ensilaged material can help to prevent future outbreaks. Careful management of silage may be challenging in the short term but may save the producer financial and mental stress in the long term.