A cataract is an opacity or cloudiness of the lens. A horse may be born with cataracts (congenital cataracts), develop them at a young age (juvenile cataracts), or develop them as an adult.
There are multiple causes of equine cataracts. Congenital and juvenile cataracts are probably due to either heredity or accident of development inside the mare. These are fortunately rare in horses. Consideration should be taken as to the wisdom of breeding these horses and their close relatives. Most cataracts in adult horses develop secondary to diseases that cause intraocular inflammation like Equine Recurrent Uveitis or ERU. Trauma is another possible cause.
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 usually require no treatment, although it is recommended to monitor for progression and complications by periodic ophthalmic examinations by a veterinary ophthalmologist.
While your veterinarian or veterinary ophthalmologist may prescribe ocular medications to treat eyes with cataracts, no medication will dissolve cataracts or improve vision. However, foals with congenital or juvenile cataracts resulting in visual impairment may greatly benefit from early cataract surgery. Cataracts in the adult horse have also been successfully removed, although lens surgery performed on foals is statistically more likely to be successful than in adult horses for a variety of reasons.
The lens of an adult horse is harder and larger than that of a foal lens, thereby making the surgery technically more difficult and with increased risk of complications. Because adult onset cataracts occur most frequently due secondary to ERU, it must be considered that there is likely already substantial concurrent and vision- impairing damage to other ocular structures besides the lens (which won’t be addressed and may even be worsened by cataract surgery).
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 underlying disease. For instance, surgical outcome for traumatically induced cataracts even in adults is highly favorable, while outcome for cataracts from chronic ERU may be guarded (50% or even less chance of a long-term visual outcome).
Potential complications of lens removal in the horse may include: persistent or recurring intraocular inflammation, corneal ulcers, corneal cloudiness or edema and retinal detachments. Any of these complications may lead to discomfort, further treatment, visual impairment and/or blindness of the eye.
Pre-surgical diagnostic testing is recommended prior to cataract surgery, including an ocular ultrasound and electroretinogram to evaluate the posterior segment of the eye which cannot be readily examined through the cataract.
Surgical removal of the lens requires intensive medical treatment with topical and oral medications for a month or longer following the procedure to maximize chances of success. Placement of a subpalpebral lavage system at the time of surgery is usually performed for ease of treatment. A protective mask may be placed to protect the eye and lavage system from damage. During the treatment period, the horse should be kept out of bright light and have severely limited exercise.
It should be noted that even after successful lens removal, vision will never be normal. Without a lens, your horse will be substantially “far-sighted,” meaning “close up” vision is poor. Fortunately, most horses seem to adapt to this and are capable of functioning adequately. Additionally, some veterinary ophthalmologists are beginning to implant artificial lenses to reduce this effect. Expected scarring of ocular structures can reduce vision somewhat, as can pre-existing ocular disease and surgical complications. A horse that has had cataract removal is never considered “sound,” because vision is diminished from that of a normal eye, and he may not be suitable for some occupations.
If you have any questions about Equine Cataracts, please do not hesitate to call us at Eye Care for Animals.
