The foal's eyes are fully developed at birth. Disorders of the foal eye might be noted at birth, or they can be inherited or acquired after birth. Low tear film production, a round pupil, reduced corneal sensation, and a temporary lack of some neurologic eye reflexes are found in all newborn foals, but the values become adult-like with time. These temporary problems can affect healing of the eye if it's injured while they are still present, so it's important to understand them. Following are the most common foal eye problems; some occur at birth, and some are a result of disease or injury after birth.
Microphthalmos, a congenitally small globe (eyeball), is common in foals and can occur in one or both eyes (see photo on page 67). This differs from phthisis bulbi, in which a normal-sized globe shrinks from severe injury. Most microphthalmic eyes of foals are blind and can be associated with other ocular abnormalities such as cataracts. A small eyelid opening and prominent nictitans (third eyelid) might be seen in affected foals. Thoroughbreds appear to be at an increased risk for microphthalmos. There is no therapy for this condition.
Strabismus refers to a misalignment of the eyeball from its normal position, resulting in an inability of one eye to attain binocular vision with the other one. In the foal, the position of the pupil and eye are deviated slightly down and toward the midline, with the eye reaching the normal adult position by one month of age. Congenital strabismus is reported in Appaloosa horses and mules. Surgical correction is needed in some severe cases, and the owner of a horse with severe strabismus is advised to consult a veterinary ophthalmologist.
Hair loss, skin abrasions, and depigmentation are all associated with fly-bite blepharitis (eyelid inflammation) in foals. To avoid fly bites around the eyes, fly repellants can be applied to the skin or slow-release insecticide strips can be attached to the mane or halter. Bacterial and parasitic infections of the eyelids can also occur in foals.
Treatment consists of topical and systemic antibiotics in most cases.
Rolling inward of the lower or upper eyelids (entropion) can occur in foals as a primary anatomic condition, or be secondary to microphthalmos, dehydration, malnutrition, prematurity, or scarring following eyelid trauma (see photo on page 67).
The entropion might cause increased tearing, eyelid squinting, conjunctivitis (inflammation of the conjunctiva that lines the inner eyelids), or corneal ulcers. The treatment in young foals with entropion is to roll out or evert the eyelid margin with temporary sutures until the causative mechanism has resolved. Permanent reconstructive entropion surgeries should be reserved for larger, older foals.
Traumatic eyelid lacerations and forehead trauma can occur in foals. Upper eyelid lesions are more serious than lower eyelid injuries, as upper eyelid movement distributes the tear film to wet the eye, preventing exposure keratitis (inflammation of the cornea).
Preservation of the eyelid margins (edges) is critical and removal of eyelid margins following trauma should be avoided. The rich blood supply to the eyelids generally allows for quick healing and functional surgical repair of eyelid lacerations.
Nasolacrimal System Malformation
Congenital absence of any portion or all of the nasolacrimal (tear drainage) system must be differentiated from acquired obstruction of the tear drainage system. The clinical signs are a unilateral or bilateral, chronic, mucoid (resembling mucus), and eventually mucopurulent (mucus and pus) ocular discharge in a young horse.
As infection develops, there can be significant discharge from the eye. Some foals with nasolacrimal system congenital defects aren't identified until the clinical signs of infection become severe at one to two years of age. Surgical creation of a new drainage system is required to correct this problem.
Heterochromia iridis is a variation in the normal dark brown iris color, and might be a combination of white and blue iris color with brown corpora nigra ("wall eye," see photo on page 67), or white iris color with brown corpora nigra known as a "china eye." This normal variation is common in Appaloosas, palominos, chestnut, grays, and spotted and white horses, and is not an indication of disease.
Incomplete development (hypoplasia) of the iris can occur, which leaves holes in the iris. Aniridia (the complete absence of the iris) has been reported in Thoroughbreds, Quarter Horses, and Belgians. Aniridia in foals is usually bilateral and results in the inability of the foal to regulate light entering the eye so that he squints severely in bright light. The use of fly masks for animals bothered by light is recommended.
Fractures of the orbit (bony eye socket) due to trauma from kicks or accidents might require surgical correction in foals.
Congenital cataracts are a frequent eye problem in foals. The lens is normally clear, but a lens opacity is a cataract. Cataracts can be very tiny with no effect on vision, or they can involve the entire lens, resulting in blindness. Most cataracts in foals are in both eyes. Heritable, traumatic, nutritional, and post-inflammatory etiologies have been proposed as causes for foal cataracts. Hereditary cataracts have been reported in Belgians, Thoroughbreds, and Morgans.
Healthy foals with cataracts, no uveitis, visual impairment, and the personality to tolerate the administration of eye medications are candidates for cataract surgery. The most common technique for removal of cataracts in foals is phacoemulsification, which involves sound waves focused on the cataract, which cause it to shatter and be aspirated from the eye. Prognosis for vision is good if the cataract surgery is done by an experienced ophthalmologist. Recent advances in the surgical technique have increased the success rate to nearly 80% in foals which undergo surgery at less than six months of age.
Conjunctivitis and Subconjunctival Hemorrhage
Conjunctivitis caused by environmental irritants (hay, sand, dirt, and ammonia) is common in foals--sick foals (that spend a lot of time lying down) are especially at risk. Conjunctival inflammation secondary to pneumonia is usually found in older foals (one to six months). Clinical signs are tearing and redness of the conjunctiva. Broad-spectrum antibiotic ophthalmic ointments or eye lubricants are used as therapy. Corticosteroid ophthalmic preparations are useful if no corneal ulcer is present.
Subconjunctival hemorrhages resulting from birth trauma generally resolve in seven to 10 days and don't require treatment. Traumatic hemorrhages are generally quite large and must be differentiated from the small hemorrhages found with a blood clotting disorder.
Corneal Ulcers in Foals
Corneal ulcers are abrasions or defects in the epithelium of the cornea, and are always emergencies as they can cause blindness. Corneal ulceration in foals requires early clinical diagnosis and appropriate medical and perhaps surgical therapy. Both bacterial and fungal ulcers can be present. They might cause a mild, early clinical course, but require prompt therapy if serious ocular complications are to be avoided.
Neonatal foals, especially those under intensive care, should be monitored daily for corneal ulcers. In general, clinical signs in foals are not as obvious as in the adult horse. Foals with corneal ulcerations will generally be only mildly painful, exhibiting slight squinting and increased tearing (remember, the cornea of the young foal is not as sensitive as in the adult horse). The eyelashes might be pointing downward rather than perpendicular to the corneal surface. The corneal surface of superficial ulcers will appear dull, cloudy, and roughened. The corneal stroma (the second layer of the cornea) might appear to be "melting" in rapidly progressive, infected ulcers.
Infection should be considered a strong possibility in every corneal ulcer in the foal. Fungal involvement should be suspected if there is a history of corneal injury with plant material (such as a tree branch), or if a corneal ulcer has received prolonged antibiotic and/or corticosteroid therapy with poor or no improvement.
The application and retention of fluorescein dye can give the veterinarian the diagnosis of a corneal ulcer. It is strongly recommended that the veterinarian obtain corneal cultures and scrapings for cytology (cell analysis) in order to initiate appropriate medical therapy.
Medical therapy is almost always the major thrust to ulcer control in foals, albeit tempered by judicious use of surgical procedures. First, bacterial and fungal growth must be halted and the microbes killed; second, anterior uveitis (more on this in a moment) must be controlled to prevent blindness. Treatment frequently needs to be sustained for weeks, or occasionally for months.
Superficial ulcers can be treated with x x broad-spectrum antibiotics such as polymyxin, bacitracin and neomycin, or chloramphenicol (gloves should be worn when using this antibiotic). Deeper ulcers involving the corneal stroma often require more specific antibiotics in high concentrations. Gentamicin, tobramycin, and amikacin are usually effective against pathogenic bacteria in ulcers. Miconazole, itraconazole, fluconazole, and natamycin have been used successfully as topical treatments for fungal ulcers in foals.
In foals, as in adult horses, iridocyclitis (anterior uveitis) is the usual and expected sequel to ulcerative keratitis. Anterior uveitis in horses with corneal ulcers should be treated with both topical atropine and systemic anti-inflammatories.
Severe corneal inflammation secondary to bacterial (especially Pseudomonas) or fungal infection can result in sudden, rapid corneal liquefaction ("melting") and rupture of the eye. The appearance of grey, mucoid, gelatinous corneal melting should prompt emergency action. Blood serum from the foal or his dam, or other drugs, might be quite effective in arresting corneal melting when used as an hourly topical treatment.
To augment lost corneal thickness and strength, deep corneal ulcers threatening rupture of the eye might require conjunctival flap placement. A conjunctival flap is the movement and suturing of the conjunctiva to the corneal ulcer to cover it.
Corticosteroid therapy by all routes is contraindicated in the management of corneal ulcers. Even topical corticosteroids following apparent healing of an ulcer to reduce the size of a corneal scar or to decrease corneal blood vessel formation can be disastrous if bacteria or fungi remain in the cornea, since steroids interfere with corneal healing.
Diminished corneal sensitivity in foals might be associated with delayed healing of corneal ulcers.
Anterior Uveitis in the Foal
Systemic disease can cause blinding iridocyclitis (inflammation of the iris and ciliary body) or anterior uveitis in the foal (see photo above), so attempts to diagnose the cause early and treat it aggressively are imperative. Iridocyclitis can be immune-mediated, or due to invasion of an organism that creates an infection. Organisms associated with equine anterior uveitis are Salmonella spp., Rhodococcus equi, Escherichia coli, Streptococcus equi, Actinobacillus equuli, adenovirus, and equine arteritis virus.
Tearing and squinting can be seen in eyes with iridocyclitis or anterior uveitis. Corneal cloudiness, a small pupil, conjunctival redness, and white and red blood cells inside the eye are observed in eyes with iridocyclitis.
It is imperative to immediately employ a fluorescein dye test to differentiate a painful eye with anterior uveitis caused by an ulcer from an eye with anterior uveitis and no corneal ulcer. While corticosteroids are the treatment of choice for anterior uveitis, they can lead to the rapid destruction of an eye with a corneal ulcer.
The major goals in treating foal anterior uveitis are to preserve vision and decrease pain. Treatment includes systemic antibiotics and non-steroidal anti-inflammatory drugs, and topically administered atropine, antibiotics, and corticosteroids.
Depending on the cause, the overall prognosis for anterior uveitis is usually guarded, as anterior uveitis can be potentially severe with a strong possibility of loss of sight.
Glaucoma is an elevation in intraocular pressure (IOP) that is detrimental to vision. The abnormal rise IOP eventually results in optic nerve damage and blindness. Glaucoma can be an offshoot of
Congenital glaucoma has been reported in foals and is associated with abnormal development of the iridocorneal angle. No particular breed predisposition has been reported for glaucoma in foals.
The clinical signs of glaucoma include corneal cloudiness, linear bands going in any direction in the cornea, and a fixed and dilated pupil. The entire eye will enlarge if the IOP remains elevated.
Therapy for foal glaucoma is aimed at preserving vision and minimizing discomfort. It is a difficult disease to manage in the foal, with little to no chance of preserving vision unless the condition is detected early and treated surgically. While glaucoma can be treated medically or surgically, laser surgery has the best potential for preserving vision.
Diseases of the Retina and Optic Nerve in Foals
Variations of the normal foal retina are numerous. First, let's briefly look at anatomy--the retinal vessels are small and extend only a short distance from the optic disk. Foals have a round optic disk with smooth margins that is located in the nontapetal retina.
Tapetal color is related to coat color, with most foals having a blue-green tapetum, although combinations of red, orange, and blue can be found. Small dots or "Stars of Winslow" are distributed in a uniform pattern throughout the tapetum. White foals, Appaloosas, and Paint foals might not have a tapetum, with resultant exposure of the blood vessels of the choroids. This gives the eye a red "eye-shine" at night.
Congenital abnormalities of the foal retina are uncommon, but when they do occur, retinal detachments cause blindness and might be unilateral or bilateral. The detached retina can be seen through the dilated pupil as a floating veil of opaque tissue in the vitreous. If bilateral, the foal will be blind and rarely leave the mare's side.
Chorioretinitis is inflammation of the foal's retina and choroid. This problem might be noticed in foals born to mares suffering from respiratory and other systemic diseases in late gestation.
Congenital stationary (nonprogressive) night blindness of Appaloosas and Quarter Horses has been reported in the United States. Affected foals are born with this hereditary disease and continue to have it as adults, but it does not progress. The retina appears normal on examination, but vision is severely diminished in dim light. Vision in the daytime is normal, with most affected horses being safe to ride.
Optic nerve atrophy can occur secondary to severe trauma to the eye or to inflammation. Foals with optic nerve atrophy will be blind, have pale optic disks with no retinal vessels, and have fixed and dilated pupils.
If you think your foal might have one of these eye problems, get him evaluated by a veterinary ophthalmologist as soon as possible to ensure the best outcome.
Editor's note: This is the third article in a series of eye articles by Dr. Brooks. See the first article, "Eye Anatomy and Physiology," article #2797 at www.TheHorse.com for more information on eye anatomy.
About the Author
Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.