Cerebellar abiotrophy and segmental axonopathy: two syndromes of progressive ataxia of Merino sheep

Clinicopathological and pedigree investigation of a novel spinocerebellar neurological disease in juvenile Quarter Horses in North America

BACKGROUND

In 2020, a novel neurologic disease was observed in juvenile Quarter Horses (QHs) in North America. It was unknown if this was an aberrant manifestation of another previously described neurological disorder in foals, such as equine neuroaxonal dystrophy/equine degenerative myeloencephalopathy (eNAD/EDM).

HYPOTHESIS/OBJECTIVES

To describe the clinical findings, outcomes, and postmortem changes with Equine Juvenile Spinocerebellar Ataxia (EJSCA), differentiate the disease from other similar neurological disorders, and determine a mode of inheritance.

ANIMALS

Twelve neurologically affected QH foals and the dams.

METHODS

Genomic DNA was isolated and pedigrees were manually constructed.

RESULTS

All foals (n = 12/12) had a history of acute onset of neurological deficits with no history of trauma. Neurological deficits were characterized by asymmetrical spinal ataxia, with pelvic limbs more severely affected than thoracic limbs. Clinicopathological abnormalities included high serum activity of gamma-glutamyl transferase and hyperglycemia. All foals became recumbent (median, 3 days: [0-18 days]), which necessitated humane euthanasia (n = 11/12, 92%; the remaining case was found dead). Histological evaluation at postmortem revealed dilated myelin sheaths and digestion chambers within the spinal cord, most prominently in the dorsal spinocerebellar tracts. Pedigree analysis revealed a likely autosomal recessive mode of inheritance.

CONCLUSIONS AND CLINICAL IMPORTANCE

EJSCA is a uniformly fatal, rapidly progressive, likely autosomal recessive neurological disease of QHs <1 month of age in North America that is etiologically distinct from other clinically similar neurological disorders. Once the causative variant for EJSCA is validated, carriers can be identified through genetic testing to inform breeding decisions.

Cerebellar Disease of Ruminants

Cerebellar disease can be congenital or acquired. Clinical signs of cerebellar disease include hypermetric gait in all limbs, normal to increased muscle tone, wide-based stance, swaying, intention tremor, and convulsions. Vestibular signs may be observed. Differential diagnoses for etiology include congenital (hypoplasia, abiotrophy, and lysosomal storage diseases), viral, bacterial, and toxic plants. Animals may present aborted as fetuses or stillborn, be affected at birth, develop disease at a few months old, or acquire the disease later in life.

A Novel Inherited Cerebellar Abiotrophy in a Cohort of Related Goats

Cerebellar abiotrophies, also known as cerebellar ataxias, are characterized by premature post-natal degeneration of cerebellar neurons. This report describes the clinical, magnetic resonance imaging (MRI), gross, histopathological and immunohistochemical features of a novel inherited cerebellar abiotrophy in a cohort of three closely related mixed-breed goats (Capra aegagrus hircus) in the southeastern USA. The animals all presented with early juvenile-onset ataxia, hypermetria, wide-based stance, head tremors and nystagmus. On MRI and at gross examination, there was moderate thinning of the cerebellar vermis and sharpening of the folia. Histologically, the vermis, paravermis and flocculonodular lobe had moderate to severe segmental loss of Purkinje cells with sparing of the hemispheres and secondary loss of granule cells and astrogliosis. Heritable cerebellar ataxias have been reported in many domestic animal species, but not, to the authors' knowledge, as a heritable condition in goats.

Cerebellar cortical abiotrophy in Wiltshire sheep

AIM

To investigate the nature of a neurological disease in Wiltshire sheep.

METHODS

Three affected lambs were examined, humanely killed and necropsied. Selected neurological tissues were examined by light and electron microscopy.

RESULTS

Primary neurological lesions were confined to the cerebellum and were characterised by loss of Purkinje cells and the presence of large hypertrophied dendrites of surviving Purkinje cells. These contained stacks of smooth endoplasmic reticulum. There was hyperplasia and cell swelling of Bergmann glia. Mild Wallerian-type degeneration affected white matter in the cerebellum and spinal cord.

CONCLUSION

The cerebellar lesions were of a degenerative and reactive rather than hypoplastic nature. These, and the history, suggest a genetic cause with putative inheritance as an autosomal recessive trait. Accordingly, the disorder is described as a cerebellar abiotrophy.

Losses of adult sheep due to ovine Johne's disease in 12 infected flocks over a 3-year period

OBJECTIVE

To measure the biological and financial impact of ovine Johne's disease (OJD) mortalities on 12 infected flocks within the endemic area of southern New South Wales over a 3-year period.

DESIGN AND POPULATION

An observational study was conducted over a 3-year period from 2002 to 2004 on sheep from 12 OJD-infected flocks from southern NSW. Flocks ranged from between 3,500 and 20,000 sheep. At the start of the study owner estimates of OJD mortality were 5% or greater.

METHOD

Annual mortality rates were estimated from farm records provided by owners. The proportion of OJD mortalities was assessed after histological examination of tissues collected from dead and moribund sheep during 5-day necropsy inspections conducted in autumn, winter, spring and summer in 2002. The financial impact was estimated using a gross margin analysis for each of the three study years and by placing a financial value on the necropsied sheep.

RESULTS

On the 12 farms, the average OJD mortality rate was 6.2% (range 2.1% to 17.5%) in 2002, 7.8% (range 1.8% to 14.6%) in 2003 and 6.4% (range 2% to 11.9%) in 2004. The average decrease in gross margin due to OJD infection on a farm in 2002 was 6.4% (range 2.2% to 15.4%), 8.5% (range 3.1% to 15.8%) in 2003 and 7.4% (range 1.5% to 15.4%) in 2004. This equates to an average reduction in annual income of $13,715 per farm per year. OJD losses accounted on average for two thirds of the total estimated financial loss associated with sheep deaths.

CONCLUSION

This study demonstrates the significant biological and financial impact of OJD on sheep flocks. These findings are of relevance to all Australian sheep flocks infected or at risk of OJD infection.

Segmental axonopathy of Merino sheep in New Zealand

AIM

To investigate an axonopathy of Merino sheep that caused progressive hindlimb ataxia and slight to moderate paresis, with the purpose of understanding its pathogenesis.

METHODS

Tissues were fixed in buffered paraformaldehyde or paraformaldehyde and glutaraldehyde, processed into wax and epoxy resin, respectively, and examined by light and electron microscopy. Fresh frozen spinal cord and trigeminal nerve roots were subjected to homogenisation, centrifugation and two-dimensional electrophoresis. Selected protein spots were identified using matrix-assisted laser desorption ionisation (MALDI) mass spectrometry. RESULTS. By light microscopy, there were large pale foamy spheroidal axonal swellings affecting peripheral as well as central axons. By electron microscopy, these were shown to contain many membrane-bound vesicles. The main abnormalities in expressed proteins involved cytoskeletal elements and myosin heavy chain, the latter interpreted as associated with the molecular motor myosin Va.

CONCLUSIONS

The disorder is the same as that described in Merinos in Australia as segmental axonopathy, and believed to have an inherited aetiology. The lesions and protein changes indicate abnormalities of the cytoskeleton, its relationship with the myelin sheath, and myosin Va molecular motor. The consequence appears to be abnormal axonal transport and inability to maintain the integrity of axons and their myelin sheaths.

The impact of malnutrition and other causes of losses of adult sheep in 12 flocks during drought

OBJECTIVE

To establish the range and cost of losses in Merino flocks in southern New South Wales during drought conditions by determining the cause of death, morbidity or wasting in adult sheep.

DESIGN AND POPULATION

Pathological studies were performed on 392 dead or moribund adult sheep from 12 Ovine Johne's disease (OJD)-infected flocks during 2002 and a further 58 sheep culled due to wasting from one of these flocks in 2003. Flocks ranged between 3,500 and 20,000 sheep.

METHOD

The most likely cause of death, morbidity or wasting was determined following consideration of the environment in which the animal was found, clinical and gross pathological findings, plus histopathology of tissues collected during necropsy.

RESULTS

A most likely cause of death, morbidity or wasting was determined for 362 sheep in 2002 and 58 sheep in 2003. OJD contributed to the death of 250 sheep in 2002, and wasting of 48 sheep in 2003. Of the sheep that died or were euthanased due to other causes, malnutrition was a contributing factor in the death of 70 sheep (63%) in 2002 and 2 sheep (20%) in 2003. Losses were not evenly distributed across flocks, with 57% of mortalities caused by malnutrition in 2002 occurring in one flock. Malnutrition accounted for 18% of the annual cost of all deaths among adult sheep in 2002 with an average cost of $16,882 per farm. Losses not attributed to malnutrition included a range of infectious and non-infectious disorders. These included balanoposthitis, clostridial enterotoxaemia, cutaneous myiasis, endoparasitism, enteritis, intestinal adenocarcinoma, misadventure, peritonitis, periparturient death of ewes, photosensitisation, pneumonia, post-shearing stress and squamous cell carcinoma of the perineum.

CONCLUSION

Almost one third of mortalities in OJD-infected flocks during drought were unrelated to OJD and could be reduced by improving nutritional and disease management practices. The importance of close supervision of the flock is highlighted so that early management intervention can be instituted, including the culling of cases of welfare concern.

Neurodegenerative diseases in domestic animals: A comparative review

Neurodegenerative diseases are characterised by selective damage to specific neurons in the nervous system. Interest in such diseases in humans has resulted in considerable progress in the molecular understanding of these disorders in recent decades. Numerous neurodegenerative diseases have also been described in domestic animals but relatively little molecular work has been reported. In the present review, we have classified neurodegenerative disease according to neuroanatomical criteria. We have established two large groups, based on whether the neuronal cell body or its axon was primarily affected. Conditions such as motor neuron diseases, cerebellar degenerations and neuroaxonal dystrophies are discussed in terms of their clinical and neuropathological features. In the most studied disorders, we also present what is known about underlying pathomechanisms, and compare them with their human counterparts. The purpose of this review is to re-kindle interest in this group of diseases and to encourage veterinary researchers to investigate molecular mechanisms by taking advantage of current diagnostic tools.

Ovine neuroaxonal dystrophy in New Zealand

A condition of neuroaxonal dystrophy of Coopworth sheep is described. This was characterised clinically by progressive ataxia from weaning with collapse of hindquarters and ultimately death. One per cent to 10% of the lamb flock on three commerical farms were affected over several years. Histopathological features were bilateral spheroid formation in specific brain stem nuclei and in the dorsal horn (especially in and about Clarke's column) along the length of the spinal cord. The condition appears virtually identical to that recorded earlier in Californian Suffolk sheep and that seen previously in the Romney, Perendale and Coopworth breeds in New Zealand. The cause was undetermined but it is suggested there may be an inherited component, as has been postulated in the Suffolks.

Congenital defects of the ruminant nervous system

Abnormalities of the nervous system are common occurrences among congenital defects and have been reported in most ruminant species. From a clinical standpoint, the signs of such defects create difficulty in arriving at an antemortem etiology through historical and physical examination alone. By first localizing clinical signs to their point of origin in the nervous system, however, a narrower differential list can be generated so that the clinician can pursue a definitive diagnosis. This article categorizes defects of the ruminant nervous system by location of salient clinical signs into dysfunction of one of more of the following regions: cerebrum, cerebellum,and spinal cord. A brief review of some of the more recognized etiologies of these defects is also provided. It is important to make every attempt to determine the cause of nervous system defects because of the impact that an inherited condition would have on a breeding program and for prevention of defects caused by infectious or toxic teratogen exposure.

Cerebello-olivary and lateral (accessory) cuneate degeneration in a juvenile American Miniature horse

A 12-month-old American Miniature horse colt was presented to the Virginia Tech Veterinary Teaching Hospital with a 7-month history of progressive ataxia. Physical examination revealed a head intention tremor, base-wide stance, and ataxia. Necropsy findings were confined to the brain. There were bilateral areas of liquefactive necrosis and cavitation corresponding to the dorsal accessory olivary and lateral (accessory) cuneate nuclei. Cerebellar folia of the dorsal vermis were thin. Microscopically, the cerebellar cortex was characterized by patchy areas of Purkinje cell loss with associated variable thinning of the molecular and granule cell layers and astrogliosis. Dorsal accessory olivary and lateral cuneate nuclei were cavitated and had mild glial response around their periphery. Additionally, a focus of necrosis and neuropil vacuolization was found in the right putamen. These findings indicate the presence of a neurodegenerative disorder centered, but not confined to, the cerebellum and its connections in this American Miniature horse colt.

Progressive ataxia associated with degenerative thoracic myelopathy in Merino sheep

A neurological disorder in Merino sheep, characterised clinically by progressive posterior ataxia and microscopically by Wallerian degeneration in thoracic segments of the spinal cord, is described. Animals of both sexes were affected, with the earliest onset of disease being at 5 months of age. Most affected animals died before 2 years of age. The clinical, pathological and epidemiological features suggest that this degenerative thoracic myelopathy is a previously unrecognised entity differing from other reported causes of ataxia in sheep in Australia.