Multiple_congenital_ocular_abnormalities_(MCOA) in silver.pdf

University of Zurich

Zurich Open Repository and Archive

Winterthurerstr. 190

CH-8057 Zurich

http://www.zora.uzh.ch

Year: 2010

Multiple congenital ocular abnormalities (MCOA) in Rocky

Mountain Horses and Kentucky Mountain Saddle Horses in

Europe

Kaps, S; Spiess, B M

Kaps, S; Spiess, B M (2010). Multiple congenital ocular abnormalities (MCOA) in Rocky Mountain Horses and

Kentucky Mountain Saddle Horses in Europe. Pferdeheilkunde, 26(4):536-540.

Postprint available at:

http://www.zora.uzh.ch

Posted at the Zurich Open Repository and Archive, University of Zurich.

http://www.zora.uzh.ch

Originally published at:

Pferdeheilkunde 2010, 26(4):536-540.

S. Kaps and B. M. Spiess

Pferdeheilkunde 26 (2010) 4 (Juli/August)

Multiple congenital ocular abnormalities (MCOA)

in Rocky Mountain Horses

and Kentucky Mountain Saddle Horses in Europe

Simone Kaps and Bernhard M. Spiess

Equine Departement, Ophthalmology Service, Vetsuisse Faculty University of Zurich, Switzerland

Summary

The study describes the prevalence of multiple congenital ocular abnormalities (MCOA) in Rocky Mountain Horses and Kentucky Moutain

Saddle Horses in Europe. Materials and methods: 35 RMH und KMSH were examined between 1999 and 2010. Their coat color were

chocolate (24), seal brown (7), and one each of bay, black, chestnut and palomino. Ciliary body cysts (CBC) were found in 17/35 hor-

ses. Two (2/35) horses had multiple congenital ocular abnormalities consistent with anterior segment dysgenesis (ASD). None of the seal

brown, black or bay horses had ocular abnormalities, while 18/24 chocolate horses had lesions. One chestnut mare had also bilateral

CBC. Fourteen horses (40%) were unaffected. While the prevalence of multiple ocular abnormalities appears to be lower in Europe than

in the US, the prevalence of ciliary body cysts is almost the same. Prior to breeding, RMH und KMSH should be carefully examined for the

presence of CBC and MCOA.

Keywords: Mountain horse, eye, congenital anomaly, anterior segment dysgenesis

Multiple kongenitale okuläre Anomalien beim Rocky Mountain Horse und beim Kentucky Mountain Saddle Horse in Europa

Die Studie beschreibt das Auftreten multipler kongenitaler okulärer Anomalien und ihre Prävalenz bei Rocky Mountain Horses (RMH) und

Kentucky Mountain Saddle Horses (KMSH) in Europa. 35 RMH und KMSH wurde zwischen 1999 und 2010 ophthalmologisch untersucht.

Die Verteilung der Fellfarben war Chocolate (24) seal brown (schwarzbraun) (7), sowie je ein Brauner, Rappe, Fuchs und Palomino. Ziliar-

körperzysten wurden bei 17/35 Pferden gefunden. Zwei Pferde hatten multiple kongenitale okuläre Anomalien. Keiner der Braunen,

Schwarzbraunen oder Rappen zeigte Augenveränderungen. Hingegen hatten 18/24 der Chocolate, sowie eine Fuchsstute Veränderungen.

Vierzehn (40%) der untersuchten Pferde zeigten keine Veränderungen. Während die Prävalenz multipler kongenitaler okulärer Anomalien

geringer zu sein scheint, verglichen mit den Zahlen aus den USA, ist die Prävalenz der Ziliarkörperzysten vergleichbar. Vor einer Zuchtver-

wendung sollten RMH und KMSH auf Ziliarkörperzysten und multiple kongenitale okuläre Anomalien untersucht werden.

Schlüsselwörter: Mountain horse, Auge, kongenitale Anomalie, Dysplasie Vordersegment

Introduction

determining whether these abnormalities were a severe

defect, supported this study. As a consequence mandatory

eye exams for breeding animals of those breeds mentioned

above were installed. As some of the ophthalmic lesions are

of small size and of subtle nature these examinations have to

be performed by a board certified ophthalmologist (ACVO)

and filled out on eye examination forms provided by the Equi-

ne Eye Registration Foundation (EERF) located at Purdue Uni-

versity.

The term Mountain Horse is a collective name for three clo-

sely related gait horse breeds, the Rocky Mountain Horse

(RMH), the Kentucky Mountain Saddle Horse (KMSH), and the

Mountain Pleasure Horse (MPH). Basic characteristics are

medium-sized (14 to 16 hands) horses with gentle tempera-

ment, an easy ambling four beat gait and a solid body color

(Fig. 1). In the Mountain horse breeds the coat color “choco-

late” with white or flaxen mane and tail is very popular and

has become strongly associated with this horse although any

solid body color is accepted with no white marks above the

knee or hock except for facial markings. Mountain Horses

derived originally from Eastern Kentucky, US and are used

today as pleasure horses, as well as for trail, competitive or

endurance riding. In 1986 the American Rocky Mountain

Horse Association was founded and a registry was establis-

hed which has shown a steady and well-regulated growth in

the number of horses registered. Today, around 40 animals

are estimated to live in Europe, mainly in France, Germany

and England.

As their popularity increased, the first Mountain Horses were

imported to Europe from the US. Because of the low number

of breeding animals in Europe, ocular health has received

great attention. The purpose of this article is the description

and evaluation of the prevalence of congenital ocular

anomalies in RMH and KMSH in Europe.

Materials and methods

A retrospective study of the EERF sheets of all examined

Mountain Horses (RMH and KMSH) between November

1999 and March 2010 was performed. Included were horses

that were certified by the breed registry and their progeny that

Congenital ocular abnormalities in 514 Rocky Mountain Hor-

ses (RMH) in the US were described in detail. The Rocky

Mountain Horse breed association, which was proactive in

Pferdeheilkunde 26

Multiple congenital ocular abnormalities (MCOA) in Rocky Mountain Horses and Kentucky Mountain Saddle Horses in Europe

were deemed certifiable by the breed registry based on gait,

temperament, conformation and parentage. The animals

were examined ophthalmologically by slit lamp biomicrosco-

py (Kowa SL-14, Eickemeyer, Tuttlingen, Germany) and indi-

rect ophthalmoscopy (Heine Omega 100, Eickemeyer, Tutt-

lingen, Germany) with a 20 diopters condensing lens in a

darkened stable before and after dilation of the pupil with

1%tropicamide (Mydriaticum Novartis Ophthalmics, Switzer-

land). All examinations were performed by the same ophthal-

mologist (BMS). Eye examinations in Mountain Horses in

Europe have to be performed by an ACVO diplomate to be

accepted in the US.

temporally and were bilateral in 11/17 horses, involved the

left eye in 4 and the right eye in 2 horses.

Retinal dysplasia was found in 1/35 animal in combination

with CBC.

Multiple ocular anomalies were detected in 2/35 horses. In

these horses a telecanthus and exophthalmos were obvious.

On close examination cornea globosa, deep anterior cham-

ber (Fig.3), miotic and dyscoric pupil with flattened, circumfe-

rentially oriented granula iridica at the pupillary ruff, iris hypo-

plasia, and goniosynechiae were evident (Fig. 4). These two

Fig. 3 Close-up of a horse with multiple ocular abnormalities: a

cornea globosa, a deep anterior chamber ( © Dr. D. T. Ramsey)

Fig. 1 Typical representative of a Rocky Mountain horse with dark

chocolate coat color and white mane and tail.

Fig. 4 Miotic and dyscoric pupil with flattened, circumferentially

oriented granula iridica at the pupillary ruff and iris hypoplasia( © Dr.

D. T. Ramsey).

Fig. 2 A large translucent cystic structure (ciliary body cyst) arising

from the temporal part of the ciliary body is extending into the vitre-

ous cavity.

horses did not show CBC. Two horses showed unilateral focal

cortical cataracts but no signs of ASD (Table 1).

In 14/35 horses no ocular abnormalities were found. None

of the seal brown, black or bay horses had ocular lesions,

while 18 of the 24 chocolate colored horses (75%) had

lesions.

The horses included 23 RMH and 5 KMSH and 7

KMSH/RMH (registered with both breed associations). There

were 13 males and 22 females. Their age at the time of exa-

mination ranged from 2 months to 10 years. Among these 35

animals 24 had a chocolate coat color, 7 horses were seal

brown, 1 was a bay horse, 1 horse was palomino-colored,

one horse was a chestnut with flaxen mane and tail, and 1

horse was black (Table 1).

Discussion

In horses congenital ocular defects are rare and constitute

0,5 – 5,3% of the collective congenital defects . Very often the

exact cause is not determined. Only a few studies have repor-

ted results of ophthalmic examinations of large numbers of

horses of a specific breed . In 1999 Ramsey et al. described

congenital ocular abnormalities in 514 Rocky Mountain hor-

ses after the detection of a high prevalence of ophthalmic

lesions in that breed.

Results

Ciliary body cysts (CBC) were the most frequently detected

lesions (17/35 horses) (Fig. 2). They were always located

Pferdeheilkunde 26

S. Kaps and B. M. Spiess

Table 1

Relevant data of the 35 RMH and KMSH

OD right eye, OS left eye, OU both eyes, CBC ciliary body cyst, MC megalocornea, M miosis, EGI encrircling granulae iridicae, G goniosynechiae, C cataract, RD retinal dysplasia

Large, translucent cystic structures arising from the posterior

surface of the iris, ciliary body or peripheral retina were the

most frequently detected abnormality (48%), which is similar

to our findings. These cysts were only present in the temporal

part of the eye and extended into the vitreous. The size of the

cysts varies between 2-20 mm in diameter. In most cases the

cysts occur bilaterally.

Retinal dysplasia was the third most common (24%) ocular

abnormality detected by Ramsey et al. Most frequently a

dysplasia of the temporal part of the peripheral retina was

detected, which was characterized clinically as linear folds or

vermiform streaks. Retinal dysplasia was most frequently bila-

teral and was detected only in eyes of horses with ciliary body

or peripheral temporal retinal cysts. In our study retinal

dysplasia was found only in one horse presenting clinically

with unilateral linear folds, which were also associated with

CBC.

The second most common abnormality (37%) reported by

Ramsey et al. were single to multiple well-delineated, darkly

pigmented curvilinear streaks of the retinal pigmented epithe-

lium in the peripheral tapetal fundus. These curvilinear stre-

aks had their origin and ending near the temporal part of the

retina and always extended towards the optic papilla. The

streaks were mainly bilateral but not symmetrical and were

only detected in eyes of horses with cysts of the iris, ciliary

body or peripheral parts of the temporal retina. These fin-

dings are in contrast to ours, as we did not observe curviline-

ar streaks in any of the examined Mountain horses.

In 14% of the horse population studied by Ramsey et al. a

bilateral syndrome of multiple congenital ocular abnormali-

ties involving the cornea, nasal and temporal iridocorneal

angle, iris, lens, ciliary body and peripheral retina was obser-

ved. All horses with multiple ocular abnormalities also had

cysts of the iris, ciliary body, or peripheral retina, retinal

dysplasia, retinal detachment, and curvilinear streaks of the

retinal pigmented epithelium. Abnormalities of the iris inclu-

Pferdeheilkunde 26

Multiple congenital ocular abnormalities (MCOA) in Rocky Mountain Horses and Kentucky Mountain Saddle Horses in Europe

ded miosis, dyscoria, stromal hypoplasia of the anterior and

peripheral iris with occasional transillumination defects,

absence of a discernible iris collarette, and a visible sphinc-

ter pupillae muscle. Radially oriented deep stromal strands of

iris tissue extended from the papillary ruff towards the ciliary

zone of the iris. The granula iridica were flattened and cir-

cumferentially oriented at the pupillary ruff when compared

with the iris of a normal RMH. Typically, pupillary light respon-

ses were decreased or absent in horses with iris abnormalities

and the pupils failed to dilate with 1% tropicamide. According

to Ramsey et al. repeated topical administration of 1% tropi-

camide, 10% phenylephrine or 1% atropine did not induce

mydriasis or had minimal to no effect on the diameter of the

pupils. Megalocornea was found in 60% of horses with

MCOA and was always bilateral. The diagnosis of megalo-

cornea was made when a clear cornea with unremarkable

central corneal topography, grossly observable excessively

large optical diameter of the cornea with a short radius of

curvature and notably globular contour of the cornea with

atypical protrusion was seen in comparison to eyes of horses

with normal-appearing corneas. Generally, horses with

megalocornea had excessively deep anterior chambers with a

normal appearing lens-iris diaphragm. Concurrently, most

horses with megalocornea had macropalpebral fissures. Besi-

des, in 91% of horses with MCOA different degrees of ana-

tomical abnormalities of the iridocorneal angle were detec-

ted. These abnormalities were always bilateral and character-

ized by excessive mesenchymal tissue in the iridocorneal

angle. According to the study of Ramsey et al. (1999) cata-

racts were always present in horses with MCOA, and were

characterized by an immature, spherical, nuclear opacity that

was most dense at the anterior nuclear-cortical junction of the

temporal part of the lens. A posterior ventral subluxation of

the temporal part of the lens with concurring iridodonesis and

phacodonesis occurring always bilaterally was found in 25%

of the horses with megalocornea. Finally an abnormal pro-

minence of the anterior orbital rim with apparent hypertelo-

rism was detected occasionally in horses with multiple ocular

abnormalities.

animals is usually not impaired; exceptions are foals with

extensive retinal dysplasia that are already blind at birth or

horses with lens subluxations and progressive cataract forma-

tion (Ramseyet al. 1999b).

As already mentioned, two different ocular phenotypes of the

disease exist: 1) Large fluid-filled cysts arising from the tem-

poral ciliary body and/or peripheral retina and retinal dyspla-

sia, and 2) MCOA consisting of ciliary and/ or retinal cysts,

along with megalocornea, dyscoria, nuclear cataracts, lens

subluxation, retinal dysplasia, iris hypoplasia, abnormal drai-

nage angles and macropalpebral fissures.

In Rocky Mountain Horses, a codominant autosomal inher-

itance model of a single gene with incomplete penetrance in

approximately 12 percent of the time has been found by ana-

lyzing collected pedigree data (Ewartet al. 2000, Andersson

et al. 2008). Due to the inheritance model CBC are expres-

sed in heterozygous animals and the MCOA in homozygous

animals (Ewartet al. 2000, Anderssonet al. 2008).

Animals in small breed registries (as the RMH) generally deri-

ve from a small gene pool. Many RMH for example descend

from a single foundation sire. Gene pools also can be selec-

tively limited by the intensive breeding of individuals with des-

ired characteristics (e.g. coat, mane and tail color). This

selection of phenotypic characteristics especially in the Moun-

tain Horses may have resulted in a concentration of the

abnormal gene and subsequently a higher incidence of eye

abnormalities.

Several studies concerning mainly RMH have been perfor-

med. Though the intraocular pressure was not significantly

different between RMH with cornea globosa and RMH with

clinically normal eyes, corneal thickness in the central and

temporal portions were significantly increased in horses with

cornea globosa (Ramsey et al. 1999a). Horses with cornea

globosa refract near emmetropia due to a reduction of the

thickness of the crystalline lens and the depth of the vitreous

chamber (Ramseyet al. 1999b).

In our study only 2 of 21 horses (5.7%) showed MCOA. Both

of them had megalocornea, miosis, circling granula iridica

and goniosynechiae. In contrast to the findings of Ramseyet

al. (1999), only in one horse bilateral anterior cortical cata-

racts were found. In this particular horse a focal cataract in

the posterior cortex was also seen in one eye. In one horse a

focal pinpoint capsular opacity was noted, as single ocular

finding which, due to its localization, was considered as being

related to the MCOA complex. In contrast to the findings of

Ramseyet al. (1999) in both of the horses with MCOA neit-

her ciliary body cysts nor retinal dysplasia were noted. These

lesions might not have been present in the European Moun-

tain horses or they might have been overlooked due to exten-

sive and drug-resistant miosis.

Previous studies have shown that horses with chocolate coat

color and white or flaxen mane and tail have the highest dise-

ase incidence. This popular color combination among the

Mountain Horses is related to the “silver dapple gene”. It has

been hypothesized that ASD is closely linked to a dominant

gene at the “Silver Dapple” locus (Anderssonet al. 2008).

This gene is a color-diluting gene, which dilutes only the

black parts of the animal. It will typically dilute a black mane

and tail to flaxen and a black body to a shade of brown or

chocolate (Brunberg et al. 2006). Genetically, a chocolate

colored horse is a black horse with the color-diluting gene at

the “Silver Dapple” locus. Red-based horses (chestnut, cre-

mello, palomino) may carry the gene but will not show it.

Another characteristic of silver dapple is that as the horses

mature and age they commonly get more and more dark hair

in their manes and tails.

The ophthalmic abnormality of Mountain Horses is termed

Anterior Segment Dysgenesis (ASD). This is a congenital and

inherited defect in the formation and development of the

anterior part of the eye during the early embryonic stages,

though some horses may also have abnormal development

of the posterior part of the eye. The affected horses are alre-

ady born with this condition, which is not painful and in the

majority of cases bilateral. The functional vision in affected

ASD is not unique to the Mountain Horses and has been

documented in other horse breeds (Icelandic horses, Shetland

Ponies, Miniatures horses) with chocolate coat color as well

(Ramsey2004, Ekestenet al. 2009). Having this in mind the

Pferdeheilkunde 26

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: