by Noelle La Croix, DVM, Dip. ACVO
Antibiotics to Treat Corneal Ulceration
There are a number of antibiotics that are available to treat corneal ulceration. The appropriate antibiotic will specifically kill the opportunistic or contaminating bacteria within the corneal ulcer. At our present level of laboratory science, it normally takes a few days before a veterinarian receives the results of bacterial culture and sensitivity tests. Typically the results will indicate an antibiotic’s minimal inhibitory concentration (MIC) within the bloodstream to effectively treat an infection. Unfortunately these systemic concentrations do not necessarily equate with effective concentrations within the tear film. The concentration of antibiotic in the tear film is supposedly higher than can reached in the blood stream but for shorter periods. In this article I will review some concepts which will help you choose a primary antibiotic to treat a corneal ulceration.
Corneal antibiotic therapy is designed to kill (or inhibit the growth of) bacteria with the potential to colonize exposed corneal stroma. Most antibiotics (regardless of bacterial sensitivity) supress bacterial adhesion to denuded stroma. However, in most veterinary cases the stroma has already been colonized with bacteria by the time of first presentation. It is therefore prudent to choose an antibiotic with appropriate bacterial sensitivity even at this early stage.
The primary suspects for corneal infections are the aerobic species Staphylococcus aureus, Streptomyces and Pseudomonas. Both S. aureus and Streptomyces are Gram-positive organisms, whereas Pseudomonas is Gram-negative. At first glance, with such a diverse flora, no one antibiotic family appears to be ideal. Combination antibiotics (neomycin-polymyxin-bacitracin, or neomycin-polymyxin-gramicidin) are therefore popular as first choices to treat canine corneal ulcers. Neomycin is bactericidal for Gram-negative species, polymyxin is bacteriostatic for Pseudomonas, and bacitracin/gramicidin are bactericidal for many Gram-positives. Treatment of human corneal injuries often begin with a fluoroquinolone (a Gram-negative and Gram-positive bactericidal antibiotic). However, these broad-spectrum antibiotics tend to promote the growth of resistant species in humans.
Neomycin and polymyxin have also been associated with a limited number of anaphylactic reactions in cats. Therefore an antibiotic such as erythromycin (effective against Staphylococcus, Streptococcus, Chlamydia, and Mycoplasma) is a better primary choice in the primary treatment of a feline corneal ulceration.
Interestingly, when treating corneal ulcers the instillation frequency of an antibiotic is often more important than its specificity. Many antibiotics are completely lost from the tear film within 45 minutes post-instillation. In humans, the FDA recommends that the fluoroquinolone ciloxan (ciprofloxacin) is instilled every 15 minutes for the first 6 hours, then every 30 minutes for the first day, then every hour for the next day, and then every 4 hours until an ulcer is healed. In veterinary medicine this schedule is clearly impractical. Most owners will not to instill antibiotic drops every 1 to 2 hours on the first day of treatment, however its wonderful if they will treat their pet with this frequency. In cases with uncomplicated superficial ulcerations, 3 to 4 times daily administration can be sufficient (but also requires re-evaluation for corneal infection within 48 hours). An ulcer is considered to be ‘complicated’ if uveitis, stromal losses, and/or a yellow-white cellular infiltrate of the ulcer are appreciated (Figure 1). These signs will dictate an increased antibiotic instillation frequency (every hour) or the choice of an alternative/supplemental antibiotic (a fluoroquinolone and/or tobramycin).
Corneal ulcer treatment can also include autologous serum administration to prevent corneal melting. This may also necessitate referral and treatment by a veterinary ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: A melting corneal ulcer in a 12 -year- old m/c Shih Tzu that is an excellent case for referral.