By definition, a cataract is an opacity of the lens that obstructs passage of light and impedes vision. A cataract may appear as cloudiness or haze in your horse’s eye. Cataracts can develop for a variety of reasons and some types will cause more severe vision impairment than others. Cataracts can also lead to secondary complications, such as glaucoma – a painful eye condition. A horse may be born with cataracts (congenital cataracts), develop them at a young age (juvenile cataracts), or develop them as an adult. In general, this is an uncommon condition in the horse.
Congenital and juvenile cataracts are believed to be due to either hereditary condition or an abnormality during development inside the mare. This type of cataract is fortunately quite rare in horses. The possible hereditary nature of this type of cataract should be taken into consideration when breeding these horses and their close relatives.
The majority of cataracts in horses occur in adult animals. They most commonly develop secondary to diseases that cause intraocular inﬂammation, such as Equine Recurrent Uveitis (ERU, moon blindness). The second most common cause of equine cataract is trauma – both blunt and sharp trauma. All eye injuries should be evaluated by a veterinarian at the time of injury, as well as, at future rechecks to monitor for development of cataract.
The management of equine cataracts depends on the type and size of cataract present, as well as, the intended use of the horse. Cataracts that remain small enough that vision is not significantly impaired may not require treatment or require topical medical treatment only. A complete or mature cataract may be a surgical candidate. No medication will dissolve cataracts and cataracts are only resolved by surgical removal. Some equine cataracts can be removed by phacoemulsification and replaced by an intraocular lens, but this surgery is not an option for all horses. An in-depth discussion with a veterinary ophthalmologist can help to determine if your horse is a possible candidate. Foals with congenital or juvenile cataracts may benefit from early cataract removal surgery and have improved surgical outcomes. This higher rate of success is due to the smaller size of the foal eye and the decreased association between inflammation and cataract in foals.
The success rate for congenital/juvenile equine cataracts is approximately 85% in terms of vision while that for adult horses with acquired cataracts may be much less. Success rates for adult horses vary depending on the cause of cataract, chronicity, and any underlying disease. For instance, the surgical outcome for traumatically-induced cataracts in adults is highly favorable, while outcome for cataracts from chronic ERU is guarded (50% or even less chance of maintaining vision long-term).
Potential complications of lens removal in the horse may include: persistent or recurring intraocular inﬂammation, corneal ulcers, corneal cloudiness or edema, loss of vision, glaucoma, and retinal detachment. Leaving the cataract in the eye, unfortunately, has many of the same possible complications. While cataracts are not painful, many of these complications can cause discomfort and/or blindness.
Pre- and post-operative treatment should be discussed as well. If surgical removal is being considered, pre-surgical diagnostic testing is recommended prior to cataract surgery, including an ocular ultrasound and electroretinogram to evaluate retinal function and integrity.
Post-operative treatment should also be a consideration. Medical treatment with topical and oral medications for a month or longer may be required following the procedure to maximize chances of success. Immediately after surgery, medications will be given multiple times per day via a subpalpebral lavage system. Your horse should also wear a protective mask (Eye-Saver) for days to a week after surgery. During the post-operative treatment period, your horse may need to be kept out of bright light and will have marked exercise restrictions.
It is important to realize that a horse with an intraocular lens and a successful surgery is not considered to have normal vision. Fortunately, most horses seem to adapt quite well and are capable of returning to pre-surgical work. If a lens is not placed, your horse will be “far-sighted,” meaning the “close up” vision is poor. It is important that your horse be considered visually impaired and you, as the owner, are responsible for the safety of any other riders you allow on your horse. That said, many owners of visually impaired horses find them to be wonderful companions and athletic partners.