Degenerative disease of the cerebellum classically described in the Arabian horse, characterised by progressive loss (abiotrophy) of the Purkinje neurons of the cerebellar cortex. It produces ataxia, hypermetria and intention tremor from the first weeks or months of life. The animal remains alert and in good general condition, but the incoordination substantially limits its safety and its athletic function. No treatment is available, so affected animals are withdrawn from riding and breeding.

It appears sporadically but recurrently in the Arabian population. Carrier prevalence study in South African Arabians: 5.1% (95% CI 2.5-9.1). Outside the Arabian it is exceptional. Limited data.

- Cerebellar ataxia with a wide-based stance
- Hypermetria (overshooting) of the limbs, more evident when turning or going downhill
- Intention tremor of the head and neck
- Alert animal, with preserved appetite and general condition
- Signs visible from birth or, more often, between 6 and 16 weeks of life
- Subsequent stabilisation of the condition without recovery of the deficit

Cerebellar abiotrophy has been known in the Arabian for decades, with cases described in North America, Europe and Australia. Neuropathological studies defined it as a primary degeneration of the Purkinje cells with thinning of the cerebellar molecular layer. In 2011 the phenotype was linked to an ECA2 marker and a shared region of homozygosity of ~142 kb was described; sequencing identified a SNP in exon 4 of TOE1, located ~1.2 kb from the start of MUTYH (transcribed on the opposite strand) and next to a possible GATA2 binding site. Later studies have described differential expression of a specific MUTYH isoform in the cerebellum of affected horses, although TOE1 and MUTYH are still considered candidate genes and causality has not been formally demonstrated. The test is used to classify animals as clear, carrier or affected for breeding purposes.

- Classify breeding animals as clear, carrier or affected before the first breeding season
- Never mate carrier with carrier: 25% risk of affected foals in each breeding
- A carrier can be mated with a clear animal, but keep only clear offspring for breeding if your goal is to eliminate the allele
- Affected animals must not be used in breeding or as athletes
- Record the results and monitor lineages with recurrent cases of ataxic foals

The differential diagnosis includes cervical ataxia of traumatic or compressive origin, meningoencephalitis (for example, EPM in endemic areas), poisonings and, in Arabian foals, lavender foal syndrome (LFS), which presents with seizures and coat dilution. Definitive confirmation is neuropathological (loss of Purkinje cells). The genetic test is based on the TOE1/MUTYH candidate variant, so it must be interpreted together with the pedigree and the phenotype.

1. Brault LS, Cooper CA, Famula TR, Murray JD, Penedo MC. Mapping of equine cerebellar abiotrophy to ECA2 and identification of a potential causative mutation affecting expression of MUTYH. Genomics. 2011;97(2):121-129. PMID: 21126570
2. Tarr CJ, Thompson PN, Guthrie AJ, Harper CK. The carrier prevalence of severe combined immunodeficiency, lavender foal syndrome and cerebellar abiotrophy in Arabian horses in South Africa. Equine Vet J. 2014;46(4):512-514. PMID: 24033554
3. Scott EY, et al. Variation in MUTYH expression in Arabian horses with Cerebellar Abiotrophy. Brain Res. 2018;1678:330-336. PMID: 29103988
4. De Lahunta A, Glass E. Veterinary Neuroanatomy and Clinical Neurology (texto de referencia en neurología veterinaria).