CASE NOTES

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Cerebellar abiotrophy in Merino sheep in the Southern New England region, NSW

Heidi Austin, District Veterinarian, North West Local Land Services and Leah K Manning, Elizabeth Macarthur Agricultural Institute, Department of Primary Industries and Regional Development, Menangle

Posted Flock and Herd August 2025

INTRODUCTION

Cerebellar abiotrophy (CA) is a neurodegenerative postnatal syndrome that affects many species, including humans, mice, horses, cattle, dogs, cats, goats, alpacas and sheep (Scott et al., 2018; Nicholas et al., 2025). Within each species, various forms of the syndrome are expected to be present. The condition causes degeneration of the Purkinje fibres of the cerebellum and the clinical signs - progressive ataxia, falling, a wide-based hindlimb stance, hypermetric gait, fine muscle tremors of the head and a reduced or absent menace response - reflect this.

Many forms of the syndrome have been reported as non-sex-linked autosomal recessive inherited diseases (De Lahunta, 1983). This has also been proposed in sheep (Johnstone et al., 2005, Milne and Schock, 1998). In ruminants, many reports of cerebellar ataxia have involved plant poisonings, including grazing pastures containing Romula rosea (onion weed) (Bourke et al., 2008).

Reported cases in sheep overseas were observed in pedigree Charollais (Milne and Schock, 1998) and Wiltshire sheep (Johnstone et al., 2005). Australian cases have been reported in Merino sheep from the Yass, Armidale (Harper et al., 1986) and Bathurst (Watt et al., 2006) districts in New South Wales (NSW). The age of onset varies amongst species with sheep in Australia clinically affected between three and six years of age, with an annual incidence of between 0.1 and 1% (Windsor et al., 2011).

This report outlines an investigation of CA in a fine wool Merino flock in the Southern New England region of North West NSW, following cases of neurological disease in the flock in September 2023.

HISTORY

In September 2023, the owner of a flock of 800 (numbers fluctuating between 450 at shearing 2020 and peaking at 820, shearing 2024) mixed age, mixed sex Merino sheep, reported that several sheep appeared to be 'drunk'. The description of clinical signs included a wide-based hindlimb stance, staggering, swaying and falling (Videos 1-4). There was an absence of weakness or weight loss. Affected sheep continued to eat, drink and produce wool and lambs. There was slow progression of clinical signs in affected sheep over one to two years, with severely affected sheep requiring euthanasia or dying of presumed misadventure. The flock currently comprises 670 sheep.

Video 1: Merino ewe exhibiting signs of ataxia and falling

Video 2: Merino wether exhibiting signs of head tremor

Video 3: Merino wether exhibiting signs of wide based hindlimb stance and swaying

Video 4: Merino wether, walking in yard exhibiting ataxia, swaying, hypermetric forelimb gait, knuckling

At shearing in September 2022, two sheep were noticed to be moderately affected (two of 750, 0.26%). The following year, 2023, when mustered for shearing, six more sheep were affected (eight of 800, 1%). The sheep had been grazing a hill paddock and when mustered, numbers were down by around 20. It was presumed by the producer that some of these had died due to the stagger syndrome (20 of 800, 2.5%). The increase in cases and the deaths prompted the producer to seek help.

The producer reported that only one cohort of sheep was affected, the 280 orange tag ewes and wethers, born September - October 2018, bought in as 10-15kg drought affected lambs in May 2019. In total there were eight known clinical cases (2.9%), and 20 deaths suspected to be due to the same condition (7.1%). The sheep were grazing native pasture in a mixed mob that also included 100 home-bred white tag sheep (born 2017) and 70 or their lambs (purple tag sheep, born 2020). None of white or purple tag sheep were clinically affected. The rams are sourced from flocks unrelated to the affected cohort. Clinical signs were first noticed when the orange tag sheep were four years old.

INVESTIGATION

Cases were defined as any sheep on the farm with a history of one or more of the following clinical signs: wide-based hindlimb stance, staggering, swaying and falling.

The first investigation was conducted on-farm in September 2023. The mob was examined in the yards, with several animals exhibiting clinical signs of staggers. The paddock in the vicinity of the yards was noted to be free of poisonous plants and did not contain Romula rosea (onion grass). The producer confirmed that there is no onion grass on the property.

One affected ewe, held in yards at the shearing shed with a smaller mob, was examined more closely and was noted to sway with a wide-based stance at rest. When moved, clinical signs were exaggerated with hindlimb ataxia, sideways staggering, falling and rolling. There was a reduced menace reflex, but it was otherwise bright and responsive with normal strength. It was elected to humanely euthanise the sheep and samples were submitted for Transmissible Spongiform Encephalopathy (TSE) surveillance. No gross lesions were found on autopsy.

On histology there were reduced Purkinje cells and degeneration of Purkinje cells with Bergmann gliosis. A diagnosis of suspected CA was made and a further sheep with consistent signs was euthanised and necropsied, with histopathology changes consistent with the first sheep.

A further six animals were identified over a period of ten months. Five of these were euthanised and sampled for further research to identify a possible underlying genetic variant/mutation for this suspected inherited condition, as a collaboration between Elizabeth Macarthur Agricultural Institute and The University of Sydney. All showed histological findings consistent with a diagnosis of CA. Of the affected sheep examined, six were wethers (75%), all of which were necropsied. One of the two ewes remained alive to raise her lamb and later died from a different cause, presumed to be haemonchosis. The lamb of the ewe is still alive and is, so far, unaffected.

All sheep examined showed similar signs of ataxic gait, intermittent falling, knuckling and head tremor, often with forelimb hypermetria. Clinical signs were exacerbated with stress and exercise/being yarded. Sheep were bright, alert and responsive. Clinical signs are shown in Videos 1, 2, 3 and 4. For two sheep in which a detailed neurologic examination was performed, no abnormalities were detected in cranial nerve examination; menace response and palpebral reflex were present bilaterally, there was normal eye movement with no abnormal nystagmus, pupil size was symmetric, and there was good jaw tone and tongue movement. Withdrawal reflex and pain perception was present on all four limbs. However, there was a marked delay to absence in the knuckling reflex in all four limbs. The cutaneous trunci reflex was present.

HISTOPATHOLOGY FINDINGS

Brains were unremarkable on gross examination. Histopathology was similar for all seven sheep in which brains were examined (Figure 5). Within the cerebellar cortex, there was a noticeable reduction in number of Purkinje cells. Remaining Purkinje cells were multifocally eosinophilic and shrunken (degeneration). Multifocally, in some areas of lost or degenerate Purkinje cells, there were increased numbers of glial cells ('Bergmann gliosis'). In some sheep, there were small numbers of ectopic Purkinje cells within the granular layer. One sheep had mild multifocal areas of gliosis, with focal mild necrosis and a separate protozoal cyst (likely Sarcocystis sp.) within the cerebrum, however the cerebellar changes were interpreted to be the most likely cause of clinical signs.

Figure 5: Histopathology image from an affected wether, haematoxylin and eosin. There are multifocal segments of absent Purkinje cells. Remaining Purkinje cells are often hypereosinophilic, shrunken and/or small.

All four sheep for which spinal cord was examined had evidence of mild to moderate Wallerian degeneration in the white matter, characterised by dilated myelin sheaths (often empty, sometimes containing remnant eosinophilic material), swollen axons ('spheroids'), and dilated myelin sheaths containing gitter cells ('digestion chambers').

DISCUSSION

This ten-month investigation was undertaken at the request of the producer, concerned by the increasing number of clinical cases in the flock. Of the 280 orange tag cohort, eight animals (2.8%) were examined and seven of these were subsequently euthanised for necropsy. The producer suspected a further 20 deaths of sheep in the paddock resulted from the condition, however, this was not confirmed.

Investigation of two initial clinical cases through Australia's National Transmissible Spongiform Encephalopathies Surveillance Project (NTSESP) identified CA as the likely cause of the stagger syndrome, leading to inclusion of subsequent animals in ongoing research.

CA is difficult to manage in a flock as clinical signs may develop after rams and ewes have produced lambs (Windsor, 2011). This was a concern for the producer, questioning how many affected animals might be expected in the future. As CA is considered a non-sex-linked autosomal inherited genetic condition in sheep and the rams have been sourced from different properties to the affected cohort of sheep, clinical cases in subsequent generations are less likely.

Current research aims to further describe the histologic phenotype of cerebellar abiotrophy in these Merino sheep through special stains, immunohistochemistry and electron microscopy, as well as whole genome sequencing and data analysis to identify a possible causative variant and assess if this segregates with disease. Once a causative variant is identified, this will allow development of a diagnostic DNA test to allow efficient, affordable ante-mortem testing of suspected affected Merino sheep, as well as inform future breeding management for affected flocks.

Reporting of any suspected inherited diseases to the Anstee Hub for Inherited Diseases (AHIDA), an online portal for surveillance and reporting of inherited diseases in Australian animals (hosted by the Sydney School of Veterinary Science) is encouraged. Anonymous reporting is possible, submitters who provide contact information can request to be connected to geneticists who may be able to offer additional advice or collaborative research.

ACKNOWLEDGEMENTS

The commitment of the producers to the investigation is greatly appreciated. Due to the donation of their time and submission of animals for examination and necropsy, ongoing research is possible.

The authors would like to acknowledge Imke Tammen (The University of Sydney), Brendon A O’Rourke, Anthony Chamings, Katie Eager, Pedro Pinczowski and Zoe Spiers (EMAI) for their contributions to the investigation, as well as the laboratories at EMAI.

REFERENCES

1. Scott EY, Woodlard KD, Finno CJ & Murray JD (2018) Cerebellar Abiotrophy Across Domestic Species The Cerebellum 17:372-379 doi.org
2. Nicholas FW, Tammen I & Sydney Informatics Hub (2025) Online Mendelian Inheritance in Animals (OMIA) [dataset] omia.org   doi.org
3. De Lahunta A (1983) Veterinary Neuroanatomy and Clinical Neurology. Second Edition. Saunders
4. Johnstone AC, Johnson CB, Malcolm KE & Jolly RD (2005) Cerebellar cortical abiotrophy in Wiltshire sheep New Zealand Veterinary Journal 53(4):242-245 doi.org
5. Milne EM and Schock A (1998) Cerebellar abiotrophy in a pedigree Charollais sheep flock Veterinary Record 143:224-225
6. Bourke CA, Bunker EC, Reece RL & Whittaker SJ (2008) Cerebellar ataxia in sheep grazing pastures infested with Romulea rosea (onion grass or Guildford grass) Australian Veterinary Journal 86:354-356
7. Harper PA, Duncan DW, Plant JW & Smeal MG (1986) Cerebellar abiotrophy and segmental axonopathy: two syndromes of progressive ataxia of Merino sheep Australian Veterinary Journal 63:18-21
8. Watt B, Carr M, Spiers Z & Dawood K (2015) Two cases of neurogenetic progressive ataxia in Merino sheep: segmental axonopathy and cerebellar abiotrophy Flock & Herd Case Notes www.flockandherd.net.au Retrieved 20 January 2025
9. Windsor PA, Kessell AE & Finnie JW (2011) Review of neurological diseases of ruminant livestock in Australia. VI: postnatal bovine, and ovine and caprine, neurogenetic disorders Australian Veterinary Journal 89:432-438