Diagnosing and Treating Ocular Keratitis in Horses
Medical management of keratitis requires frequent drug administration, which a patient can rapidly grow to resent. Thus, Boggs recommends using a subpalpebral lavage system (seen here) to allow easier administration and better absorption of drugs.
Photo: Stephanie L. Church, Editor-in-Chief
Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 Western Veterinary Conference held in February.
Keratitis might have a simple definition--inflammation of the cornea--but the ailment itself is anything but easy to understand. During a presentation at the 2012 Western Veterinary Conference, held Feb. 19-23 in Las Vegas, Nev., Jacquelin Boggs, DVM, MS, Dipl. ACVIM,stressed just how serious the common equine disorder is if not treated promptly and aggressively.
"Ulcerative keratitis is a common condition in horses due to their large prominent eyes and can be potentially sight limiting if not diagnosed early and treated properly," explained Boggs, who is an equine technical services veterinarian for Pfizer Animal Health.
The clinical signs of keratitis are variable and numerous, Boggs said, but ocular pain is a "hallmark" that affects most, if not all, horses with the disorder. Other common signs include:
- Eye redness;
- Epiphora (watery eye);
- Corneal edema (when the surface of the eye appears whitish or bluish-gray);
- Ocular discharge;
- Corneal vascularization (an excessive amount of blood vessels in the cornea);
- Keratomalacia (softening, drying, and ulceration of the cornea); and
- Corneal neovascularization (proliferation of blood vessels), photophobia (an aversion to light), and pupil constriction.
Boggs noted that horse owners should consider all eye injuries emergent and a veterinarian should evaluate any abnormality as soon as possible to reach an accurate diagnosis and begin treatment.
To diagnose keratitis and assess the lesion's severity, Boggs said a veterinarian will perform a thorough ocular examination using proper restraint, sedation, and possibly nerve blocking. Painful eyes should be stained, she said, to determine if a defect in the cornea is present. Abrasions to the cornea are often difficult to see without stains, even with the proper lighting and equipment. Defects in the outer layer of the cornea allow the dye to diffuse into the middle layer, at which point lesions appear bright, fluorescent green.
In most cases a veterinarian can come to a diagnosis based upon clinical signs and ocular examination. While both bacterial and fungal keratitis can be confirmed with a positive culture, Boggs noted fungal culture in particular can take significant time to complete. Thus, if keratitis is suspected, the veterinarian should begin treatment before a positive culture is returned.
Many keratitis cases can be managed medically; however, surgery is an option if the condition does not improve with less aggressive treatment.
Medical management of keratitis requires frequent drug administration, which a patient can rapidly grow to resent, Boggs said. Thus, she recommends using a subpalpebral lavage system to allow easier administration and better absorption of drugs on a frequent basis. A subpalpebral lavage system uses flexible tubing, which is passed through the upper or lower eyelid and stitched into place, with medication administered via the other end of the tube.
Boggs relayed that she uses the "five A's of corneal ulcer treatment" approach for medical treatment of fungal keratitis. She coined this phrase to describe antibiotics, antifungals, atropine, anti-inflammatories, and anti-proteinases:
Antibiotic--"Triple antibiotic (ointment) or tobromycin are good first line drugs with broad spectrum activity that can be used as initial treatment or until cytology results can aide in a more tailored approach," she relayed. Boggs described additional antibiotics that can be used to target specific pathogens identified on cytology and culture.
"It is recommended that you use the most benign, broad-spectrum antibiotic and change if you are not getting improvement," she stressed.
Antifungals--Veterinarians have treated fungal ulcers successfully with a variety of topical antifungal ointments, Boggs noted. Common drug choices include miconazole, natamycin, fluconazole, voriconazole, clotrimazole, and itraconazole. Treatment with antifungals typically lasts several weeks, with ointment or solution being applied three to four times daily for the first few days to weeks.
"It is not uncommon to experience an increase in ocular inflammation with initiation of therapy as significant fungal death occurs," Boggs stressed. To this end, she recommends using non-steroidal anti-inflammatory drugs at the onset of treatment to combat additional inflammation.
Atropine--Often seen alongside keratitis is the painful condition of anterior uveitis (inflammation of the iris and ciliary body, which is a muscular ring located in the front part of the eye). Boggs advocated using atropine ointment to combat this potentially blinding condition and to reduce the horse's ocular pain level. Even if a horse doesn't show evidence of anterior uveitis, she recommends atropine treatment: "One should assume that every horse with a corneal ulcer has uveitis to some degree. Thus, topical atropine should be used."
Anti-Inflammatories--Eye conditions are painful, Boggs said, so she recommends using oral phenylbutazone (Bute) or oral or intravenous flunixin meglumine (Banamine) to control both pain and ocular inflammation. She noted that many ophthalmologists recommend using Banamine over Bute in these cases, as the drug appears more effective at controlling ocular pain and inflammation.
Anti-Proteinases--Used to decrease corneal healing time, anti-proteinases inhibit collagenase activity and prevent ocular protein from breaking down. Boggs noted that a horse's own blood serum can be a useful anti-proteinase and is relatively simple to obtain. Other adjunctive modalities that have been shown to inhibit proteases include 0.17% ethylenediaminetetraacetate (EDTA), topical 0.1% doxycycline, 5% N-acetylcysteine (NAC), and tetanus antitoxin. Because these compounds use different mechanisms to inhibit various families of proteases in tear film, a combination of these protease inhibitors could be beneficial for treating corneal ulcers in horses, said Boggs.
Discuss all anti-proteinase and adjunct treatments with a veterinarian before implementing them, she added.
Surgical Options--"Combined medical and surgical therapy may be warranted to address deep ulcers, nonresponsive ulcers, or melting corneal ulcers," Boggs said. She described several surgical options veterinarians can use in conjunction with medical therapy to treat keratitis:
- Grid Keratotomy--This procedure involves making grid lines on the surface of the cornea over and around the ulcer that allow healthy cells to develop and promote healing.
- Ulcer Debridement--This procedure simply involves removing dead and necrotic tissue from around the ulcer's edges.
- Conjunctival Flaps--This procedure involves using a flap of the horse's own conjunctiva (a delicate membrane lining the inside of the eyelids) to cover the serious or nonhealing ulcer to promote healing.
- Corneal Grafts--This procedure uses a cornea from a donor horse to repair damage to a recipient horse's eye.
- Amniotic Membrane Graft--This procedure is similar to a corneal graft, but using amnion (fetal membranes) harvested from mares is used to cover the cornea. The amnion is harvested, washed, and stored frozen with antibiotics until needed. At that point, the amnion is thawed and sutured into place on the eyeball.
In the event graft rejection occurs, Boggs recommended a full-thickness corneal transplant to correct the complication.
In some severe cases, complete enucleation (surgical removal of the eye and associated structures) is necessary, she noted.
The prognosis for affected horses depends on the type of keratitis, Boggs said. Horses with bacterial keratitis that receive prompt, appropriate treatment typically have a good visual prognosis, she said. Conversely, horses with fungal keratitis generally receive a guarded visual prognosis as "a sight-threatening situation is possible if appropriate drugs are not administered that can penetrate the cornea and reach therapeutic levels," she said.
While some cases of keratitis heal without issue, other cases require extensive care and still might not be curable. Involving a veterinarian early on in any suspected eye injury or disease can increase the chances of successful treatment.
About the Author
Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado.