Equine Recurrent Uveitis (ERU)

Equine Recurrent Uveitis (ERU) is the most common cause of blindness in horses. It is thought to affect 3-15% of the horse population worldwide. With multiple causes, ERU may originate from environmental factors, ocular infection of Leptospira, a genetic predisposition or a combination of these factors. Characterized by episodes of inflammation of the middle layer of the eye, ERU leads to the development of cataracts, glaucoma and eventually complete loss of vision.

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Appaloosas, European Warmbloods, draught breeds and Standardbred trotters are most commonly affected by ERU. Foreseeably, these diverse breeds present different modes of genetic inheritance for ERU. For example, in Appaloosa horses a Single Nucleotide Polymorphism (SNP) located on TRPM1 (the same gene responsible for the characteristic spotting of the breed) indicates a higher chance of disease development.  Additionally, certain mutations within the MHC region of the genome, which encompasses genes regulating immune response, indicate disease severity.

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In a population of German Warmbloods, a SNP on ECA 20 located near interleukin genes IL-17A and IL-17F proved significant in predicting horses affected by ERU. The “G” allele, rather than the more common allele “A”, showed a strong correlation with ERU. Horses with two copies of the "G" mutation are 80% more likely to be affected by ERU while those with only one allele (A/G) are thought to be at relatively moderate increased risk, but are "carriers" and can transmit this “G” risk allele to their offspring.

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ERU Risk: ECA20

GG = Increased risk

GA = Possible increased risk/carrier

AA = Normal risk

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Gene or Region: ECA18

Severity  

Reference allele: G

Mutant allele: A

Affected Breeds: Many

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A second SNP on ECA 18 located within an intron of PLEKHM3 and downstream of CRYGA-CRYGF,  significantly correlated to ERU severity. A mutation from G to A increased the severity of disease in 69% to 78% of the cases.

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Thus a horse inheriting two copies in this region (A/A) was shown to be associated to be at increased symptom severity should they acquire ERU via infection or otherwise (independent of genetic "risk" for any heritable form of ERU). Studies indicate that horses with two copies of the "A" mutation are likely to suffer more severe symptoms than the average horse with the same disease. Horses with only one allele (A/G) are thought to be at moderately increased risk & carriers due to their ability to produce affected offspring.

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ERU Severity: ECA18

GG = Normal severity

GA = Possible increased severity/carrier

AA = Increased severity of symptoms

Publications

Witkowski L, Cywinska A, Paschalis-Trela K, Crisman M, Kita J. (2015) Multiple etiologies of equine recurrent uveitis--A natural model for human autoimmune uveitis: A brief review. Comp Immunol Microbiol Infect Dis. 2016 Feb;44:14-20

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Kulbrock M, Lehner S, Metzger J, Ohnesorge B, Distl O (2013) A Genome-Wide Association Study Identifies Risk Loci to Equine Recurrent Uveitis in German Warmblood Horses. PLoS ONE 8(8): e71619. doi:10.1371/journal.pone.0071619

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