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Original Article
Prevalence of ophthalmic disease in blue-eyed horses
B. E. Bergstrom, A. L. Labelle*, M. E. Pryde, R. E. Hamor and K. E. Myrna †
Department of Veterinary Clinical Medicine, University of Illinois Urbana-Champaign, Urbana, USA; and†Department of Small Animal Medicine and Surgery, University of Georgia, Athens, USA.
*Corresponding author email:
Keywords: horse; heterochromia; uvea; squamous cell carcinoma; eye
A perception exists amongst referring veterinarians and the lay
public that blue-eyed horses have increased frequency of
ocular disease. The aim of this retrospective study was to
assess the prevalence of ocular disease in horses with blue
or heterochromic eyes relative to those with brown eyes.
The medical records of horses presenting to either the
Comparative Ophthalmology services or Equine Medicine/
Surgery services at 2 institutions were reviewed. Signalment,
ocular and nonocular diagnoses were recorded. Ocular
disease was divided into 4 categories: adnexa, cornea,
intraocular/orbit and squamous cell carcinoma (SCC). Owners
were contacted by telephone to confirm iris colour.
Chi-square analysis was used to compare group proportions. A
total of 164 eyes of horses with ocular disease and 212 eyes of
horses without ocular disease were included. Blue eyes were
equally common in the ocular disease and nonocular disease
groups (P = 0.265). There was no significant difference in the
proportion of blue- and brown-eyed horses when comparing
the adnexal and corneal (P = 0.548), corneal and
intraocular/orbit (P = 0.379) and adnexal and intraocular/orbit
(P = 0.843) categories. A significant difference was detected in
the proportion of blue-eyed horses between the adnexal (P =
0.000), corneal (P = 0.033), intraocular/orbital (P = 0.000) and
SCC categories, with a higher proportion of blue-eyed horses
in the SCC than in the other 3 groups. Horses with blue or
heterochromic irides are more likely to develop ocular SCC
than horses with brown irides, but are not more likely to have
adnexal, corneal or intraocular/orbital disease or to be
presented for evaluation of ophthalmic disease. Veterinarians
should consider the results of this study when answering
questions from the lay public regarding the predisposition of
blue-eyed horses to ocular disease.
Melanocytes, derived from embryonic cells of neural crest
origin, are found in the skin, hair, eye, inner ear and
leptomeninges and are responsible for pigmentation (Bellone
2010). The variation of coat colour primarily depends on the
amount and type of pigment granules in the melanocytes
(Toth et al. 2006). Coat and skin colour are intimately
associated with iris colour in horses. Structurally, the iris is
composed of melanocytes, vascular, muscular and neural
tissue, as well as a collagenous stroma. Iris colour is influenced
by the thickness, density and pigmentation of the anterior
melanocytic border layer of the iris, as well as the amount of
pigment present in the iridal stroma and posterior epithelium
(Crispin 2000). An increase in the density of iridal melanocytes
results in a nearly uniform dark brown iris colour (Crispin 2000).
Most solid colour horses have brown irides. In colour-dilute horses such as Paints, Appaloosas and cremellos, the iris may
be blue or heterochromic (containing both blue and brown
areas) in colour due to a decrease in the density of iridal
melanocytes (Carastro 2004). Coat colour can be associated with ocular disease.
Normal melanocyte migration and cell signalling are essential
for ocular development; lack of melanocytes or abnormally
programmed melanocytes may lead to a variety of
developmental ocular disease (Bellone 2010). Appaloosa
horses with a white spotting pattern known as leopard
complex are predisposed to congenital stationary night
blindness (CSNB) characterised by impaired vision in dim light
(Sandmeyer et al. 2007; Bellone 2010). Chocolate or silver
dapple coat colours in Rocky and Kentucky Mountain horses
and American miniatures are associated with multiple
congenital ocular anomaly (MCOA). This syndrome involves
potentially devastating ocular abnormalities including
temporal ciliary epithelial cysts, iridal hypoplasia,
macrocornea, focal temporal retinal degeneration related to
ciliary cysts and, rarely, retinal detachment (Ramsey et al.
1999; Grahn et al. 2008; Plummer and Ramsey 2011).
Amongst the general horse-owning public there exists the
perception that blue-eyed horses have a predisposition to
ophthalmic disease (Labelle 2012, 2013). To the authors’
knowledge, no relationship between iris colour and ocular
disease has been documented in horses. The purpose of this
study was to assess the prevalence of ocular disease in horses
with blue or heterochromic eyes relative to those with brown
eyes. We hypothesised that horses with blue or heterochromic
eyes would not have an increased prevalence of ophthalmic
Materials and methods
Medical records from the veterinary teaching hospitals at the
University of Illinois and University of Georgia Colleges of
Veterinary Medicine were retrospectively reviewed. Cases
presenting to the ophthalmology services or equine
surgery/internal medicine services at the 2 locations over a 5
month period (approximately 1 January 2013–1 June 2013)
were eligible for inclusion. Medical records were reviewed and
signalment, iris colour as well as final systemic and ophthalmic
diagnoses were recorded. Horses were then divided into 2
groups: horses with ophthalmic disease presenting to the
ophthalmology service and horses free of ophthalmic disease
presenting to the equine surgery/internal medicine service.
Horses free of ophthalmic disease were excluded if any
ophthalmic abnormality was detected during hospitalisation. Ocular disease was classified into one of 4 categories:
adnexa, cornea, intraocular/orbit or squamous cell
carcinoma (SCC). For the purposes of this study, adnexal SCC
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