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Equine Ophthamlmic Medications

EQUINE OPHTHALMIC MEDICATIONS

CAUTION: An intact cornea serves as a barrier preventing penetration of topical medications into intraocular tissues. Ointments are toxic to intraocular tissues and are contraindicated if the cornea is perforated, since leakage into intraocular tissues may occur (1).

CAUTION: Topicalgentamicin should be avoided when the cornea isperforated. Toxic e ects on the corneal endothelium and the retina and ciliary epithelium have been reported when those tissues were exposed to high concentrations ofgentamicin (2).

ANTIBIOTICS

Fortified antibiotic ophthalmics for topical administration can be compounded by combining the injectable form of certain antibiotics with artificial tears. A compounding pharmacist can recommend and prepare an appropriate concentration of various antibiotics. Some fortified topical antibiotics that have been used in treating corneal infections include penicillin G, ampicillin, ticarcillin, cefazolin, gentamicin, tobramycin, and amikacin. These antibiotics can also be prepared at a concentration suitable for subconjunctival administration.

ANTI FUNGALS

The ideal topical antifungal drug should be non-irritating and non-toxic to the eye, penetrate the eye well, and should be highly effective against at least one significant ocular pathogen. It is easier to achieve bactericidal antibiotic levels in corneal tissue than fungicidal levels; therefore, fungal infections are more difficult to treat. In fact, most antifungal drugs only reach fungistatic levels in the cornea and rely on the body’s defense mechanisms to eliminate the fungus. Natamycin is the only commercially available topical antifungal product and is approved for human rather than veterinary use. Natamycin has a wide spectrum of activity against the common fungi that cause equine keratomycosis but maybe cost-prohibitive in many cases. A compounded product of 1 percent itraconazole and 30 percent dimethyl sulfoxide applied topically has been shown to reach therapeutic corneal drug concentration and to be effective against a variety of equine keratomycoses in vitro and in vivo. Reducing the concentration of DMSO to 5 to 10 percent minimizes the significant eye irritation/stinging caused by the more concentrated preparation and appears to be equally effective according to anecdotal reports. It can be compounded as an ophthalmic ointment or as a solution. Other antifungal drugs used topically for the treatment of corneal infections include nystatin, amphotericin B, clotrimazole, thiabendazole, and flucytosine (1).

REFERENCES:

(1) Hamor, RE, Whelan, NC. “Equine infectious keratitis,” Vet Clin North Am Equine Pract 1999 Dec; 15(3):623-46.
(2) Peiffer, RL, Jr., and Petersen-Jones, SM. SmallAnimal Ophthalmology, 2nd ed. Philadelphia: WB Saunders, 1997.