———————————————————————————————————————————The Rocky Mountain Horse is a healthy, modest and hardy breed that can manage easily in any weather. It manages well during the cold intercontinental winters in USA, and one could be tempted to say that the breed is the American equivalent of the Icelandic Horse.
The Rocky Mountain Horse has a long life expectancy and both Old Tobe, the foundation stallion of the breed, as well as its offspring the five “sons of Tobe” that also served as founding stallions, lived to be more than 30 years old. Old Tobe himself lived to the age of 37 years.
In the beginning of the 20th century the breed endured a hard life as working horses, and the harsh winters along with the Great Depression in 30’ies and World War 2 contributed to the fact that only the strongest and most hardy individuals survived. Because of this the breed today sports a good health and are generally considered easy keepers.
The breed has, because of the high prevalence of the silver dapple coat colour, a disposition for an increased frequency of the hereditary eye disorder MCOA (Multiple Congenital Ocular Anomalies) – formerly known as ASD (Anterior Segment Dysgenesis). The increased frequency is in part also attributed to the so called “Founder Effect”, where a single individual is the ancestor of many individuals, that could inherit a certain genetic mutation for example.
———————————————————————————————————————————Like all other genes an individual always inherits two alleles – meaning versions – of a gene. The individual receives one allele from each parent. MCOA is a syndrome that arises when a horse has two alleles of the gene that codes for the silver dapple coat colour.
The silver dapple gene is a gene that is described a dilution gene, which only affects the eumelanine (black pigment). It is only black and bay horses that has the eumelanine pigment, and therefore it is only in these horse carriers of the silver dapple gene is evident. Red/sorrel horses are silent carriers, because the gene doesn’t affect the red pigment, pheomelanine. The gene, popularly known as Z, is called PMEL17 and is located on the horses 6th chromosome. The silver dapple colour arises from a mutation in this gene and it is co-dominantly inherited, which means for example that a horse can both carry a black and a silver dapple gene that are both expressed. A gene being dominant also means that a parent carrying one allele of the gene has a 50% chance of passing the gene on to an offspring, because the offspring receives one of the alleles randomly from each parent. Concurrently if one parent has two alleles of the gene the offspring will always receive an allele of the gene and it will be expressed because of its dominance.
It is not known whether the mutated silver dapple gene itself causes the MCOA syndrome or if it is a mutation in a gene right next to the silver dapple gene, that are to blame for the congenital malformations following the much desired silver dapple/chocolate coat colour with silver/flaxen mane and tail. MCOA is seen not only in the Rocky Mountain Horse breed, but also other horse breeds also possessing this colour variation – like the Icelandic Horse, Miniature horse and others. The Rocky Mountain Horse is undisputedly the horse breed with the highest prevalence of the special silver dapple colour though (roughly 55% of the registered RMH population), and it is also in this breed the highest prevalence of MCOA is recorded.
Most new sources point out that even horses with just one allele of the silver dapple gene (heterozygote) has minor anomalies in the anterior eye chamber – that is, small malformations in the form of uveal cysts. Heterozygote horses are regarded as a “cyst-phenotype”. In theory that means that a heterozygote horse will have small cysts in the anterior eye chamber, but it must be emphasized that in most cases the cysts are very hard to recognize clinically, because of their size (they are also termed micro-cysts). In all cases the malformations are so small, that horses of the cyst-phenotype aren’t clinically affected by their genetic disposition for MCOA. Horses with two silver dapple alleles – homozygotes for the silver dapple gene – has the MCOA-phenotype. This phenotype is characterized by pronounced congenital anomalies in or around the anterior eye chamber. The MCOA syndrome consist of more or all of the following anomalies:
- Uveal cysts (Uvea is the middle, pigmented layer of the three membranes that make up the eye “wall”. The cysts can be located in the temporal ciliary body (Temporal = part of the eye furthest away from the nose/medial eye corner. The ciliary body is both a muscle, that helps focusing the lens, and a gland producing inner eye fluid), peripheral retina and/or iris.)
- Cornea Globosa (The cornea appears a lot thicker and rounder, because of the greater curvature of the cornea. This is caused by a enlarged depth of the anterior eye chamber. Put frankly it looks like the horse has bulging eyes.)
- Little or no light reflex, as well as miosis (Contraction of the pupils. When the pupils are always small and contracted, it means that the horse is vision impaired in dim light and darkness, where light is scarce. The normal eye would enlarge the pupil to receive as much light as possible to the retina)
- Retinal dysplasia (Malformations in the retina) and retinal detachment
- Iris malformations, among others iris hypoplasia (inadequate formation of the iris), iridocorneal angle abnormalities (the angle between the iris and cornea is wrong) and iridocorneal adherences.
- Congenital cataract (unclear/opaque lens) and/or nuclear cataract (affects only the central part of the lens).
Clinical signs of MCOA syndrome. A. Oblique profile images of the lateral anterior segment of the right eye of a Rocky Mountain horse. A multiloculated cyst arising from the anterior ciliary body is present. B. Photograph of the right eye of a Rocky Mountain horse with ectropion uvea (the rim of the iris turn inside out), dyscoria (misshapen iris), cataract, and lens subluxation. The granula iridica (normal anatomical fringe-like feature in the rim of the iris) is hypoplastic, the pupil is misshapen, and complete circumferential ectropion uvea is present. Nuclear cataract of the nuclear-cortical junction is present. Vitreous (eye “gel”) is present in the anterior chamber between the iris and lens secondary to posterior ventral lens subluxation. C. Profile photograph of a Rocky Mountain Horse with Cornea Globosa. Note the anterior protrusion of the cornea and large corneal diameter. D. Profile photograph of a Rocky Mountain Horse with a normal cornea. Note the normal corneal curvature and diameter. (The image and image text is borrowed from this article www.ncbi.nlm.nih.gov/pmc/articles/PMC2653074/)
Heterozygous silver dapple horses are only diagnosed clinically with uveal cysts. There has been seen a few cases though with heterozygous horses also diagnosed with retinal dysplasia. Once again it must be emphasized that only homozygous silver dapple horses will have more or all of the above mentioned congenital abnormalities. Blindness is not diagnosed very often, but will always be associated with two MCOA alleles.
The MCOA syndromet is congenital as mentioned before and is by definition not a progressive illness. This means that the symptoms presented in the horse at birth will not get any worse (or better) during the horses lifetime. There has been seen certain tendency though, that the MCOA-phenotype is more prone to uveitis/moon blindness (state of inflammation in the uvea-membrane of the eye), which can lead to pain and also blindness in severe cases.
For the untrained eye it can be hard to recognize a MCOA horse. Often you will need ophthalmological equipment to be able to see the eye malformations. But some sources claim that there is a more pronounced colour dilution of the hairs along with more obvious dapples (see picture) on the body of a MCOA-phenotype than on the Cyst-phenotype. Apart from this there’s not really any other visible signs and typically only a genetic test will tell with 100% certainty wether the horse suffers from the syndrome or not.
Knowledge about this syndrome and how it is inherited is important for the breeding of the Rocky Mountain Horse, and you should really take care as a breeder to avoid breeding MCOA-phenotype horses. Most of the MCOA horses do fairly well despite their vision impairment, but it is in no way a desirable eye defect within the breed. Unfortunately it is an issue that comes with this beautiful and popular silver dapple colour in the breed, but if you are paying attention to this issue it won’t necessarily pose a problem for breeders and owners.