by Noelle La Croix, DVM, Dip. ACVO
Antibiotics to Treat Corneal Infections
Antibiotics can effectively treat corneal infections and prevent further complications (e.g.; corneal rupture, loss of vision). Simplistically, the appropriate topical antibiotic is chosen based upon specific bacterial susceptibility. Melting corneal ulcers with unabated infections may rupture within 24 hours of presentation. However, the identification and antibiotic sensitivities of infective bacteria are typically obtained 2 to 3 days after initial culture. Antibiotics are therefore initially chosen to combat the most common bacteria found in corneal infections. Each case’s unique history of antibiotic sensitivity, ocular cytology, and frequent re-evaluation will then refine the prescribed antibiotics.
An uncomplicated corneal ulcer is described as superficial and lacks a cellular infiltrate. These ulcers are generally treated with a topical triple antibiotic (bacitracin, neomycin, and polymyxin-b) 3 times daily, and re-evaluated within 24 to 48 hours of initial presentation. The triple antibiotic is effective against a broad-spectrum of bacteria, and is used prophylactically to prevent the establishment of further corneal infections. Neomycin is an aminoglycoside effective against Gram negative (e.g.; Escherichia coli, Enterobacter spp.) and Gram positive (e.g.; Staphylococcus aureus)bacteria. Pseudomonas spp. are sensitive to the cationic detergent polymyxin-b. Numerous Staphylococci and Streptococci are sensitive to bacitracin and/or gramicidin. In addition, the adhesion of all bacteria to the corneal stroma (and the subsequent establishment of a bacterial biofilm) is inhibited by the presence of any antibiotic. Therefore, even bacteria with antibiotic resistance routinely fail to establish or maintain colonies within these treated wounds.
Corneal infections are commonly characterized by a break within the corneal epithelium associated with an underlying white-to-yellow cellular infiltrate. Corneal infections may also exhibit a loss of corneal stroma, corneal edema, limbal vascularization, flare within the anterior chamber, miosis, and blepharospasm (signifying ocular pain). It is appropriate to treat any suspected corneal infection aggressively.
The most common bacteria cultured from canine corneal infections include Streptococci (mainly beta-hemolytic varieties), Staphylococcus pseudintermedius (some are methicillin-resistant or MRSP), P. aeruginosa and Enterococcus spp. About 90% of these bacteria are sensitive to chloramphenicol with the exception of P. aeruginosa. Both P. aeruginosa and S. pseudintermediusare sensitive to tobramycin and ciprofloxacin, but MRSP and Enterococcus spp. are not. Combining chloramphenicol with tobramycin (or ciprofloxcin) should kill over 99% of bacteria found in corneal infections. However, chloramphenicol has been shown to rarely induce aplastic anemia in humans. An owner’s accidental ingestion of even minute amounts of their pet’s prescribed chloramphenicol could be life-threatening. Due to this toxicity, chloramphenicol should not be prescribed for trivial infections, prophylaxis, or when infections respond to other antibiotics. If prescribed, owners should wear gloves while administering topical chloramphenicol to their pets.
Proper treatment for a corneal infection begins with corneal cytology, Gram staining, and aerobic culture. Bacterial identification from aerobic cultures typically requires 2 to 4 days. Anaerobic bacteria are rarely identified in canine corneal infections and require 7 to 10 days of culture. Most treated corneal ulcers will also resolve within this time period so the expense of anaerobic culture is rarely justified.
The results of cytology and Gram staining should refine the prescribed antibiotics. The replication of Gram positive bacteria (e.g., Streptococci) can be repressed by moxifloxacin that inhibits DNA gyrase and topoisomerase. The growth of Gram negative bacteria is effectively controlled with tobramycin that inhibits protein synthesis. Ciprofloxcin kills Pseudomonas spp. but is supplied in an acidic pH of 4.5 that elicits pain when introduced into most animal eyes. Unfortunately, alkylating topical ciprofloxacin with sodium bicarbonate (to diminish the pain response) causes precipitation of the drug.
Cytology may also reveal fungal hyphae indicating corneal mycosis that necessitates topical application of an antifungal medication (e.g., fluconazole). Corneal mycosis is rarely found in small animals. Seasonal and geographic variations that result in warmer and moister environments can promote the growth of fungi in corneal wounds.
Antibiotics (moxifloxacin and tobramycin) are prescribed hourly to induce asepsis when keratitis accompanies corneal infection. Melting of the cornea can be prevented with the addition of protease inhibitors (autologous serum, EDTA, and/or doxycycline). Flushing the cornea with a povidone-iodine solution (a complex of polyvinylpyrrolidone and elemental iodine without additional detergents) for 5 minutes will help destroy some bacterial proteins. Topical atropine can be used to prevent the development of synechiae. Oral prednisone (0.25 to 0.5 mg/kg) and/or NSAIDs will act as anti-inflammatories to control uveitis, and tramadol can be prescribed for pain. This drug regimen typically requires hospitalization. An E-collar is applied to the patient to prevent self-inflicted trauma. While hospitalized, the patient’s overall health (appetite, hydration, and comfort) should be periodically assessed. Systemic concerns always supersede any ophthalmic treatments.
A corneal infection should be re-evaluated every 8 to 12 hours. A corneal infection is progressing if the stromal defect increases in size or is associated with further inflammation, necrosis, and pain. These worsening infections (Figure 1) usually require surgical correction (e.g., corneal debridement followed by conjunctival, corneal, or amniotic grafting) by a veterinary ophthalmologist.
Signs that an infected cornea is healing include (but are not limited to) a decrease in size, a clearing of cellular infiltrates, resolution of necrosis (corneal melting), and increased animal comfort. Ulcers may appear to deepen as cellular infiltrates, necrotic debris, and infection leave the cornea. Deep defects may then require surgical correction to prevent corneal rupture. The frequency of topical antibiotic administration can be decreased as a corneal infection regresses. The extended use of antibiotics can prevent re-epithelialization of the corneal stroma. However, the benefits of antibiotics in treating corneal infections generally outweigh their potential toxicity.
Corneal infections can often be resolved by treatment with appropriate antibiotics. Effective antibiotics are chosen based upon clinical signs, bacterial culture, ocular cytology, and frequent patient re-evaluation. Corneal infections require attentive monitoring and may require alterations in antibiotic and other drug regimens. Serious infections that fail to resolve medically may require surgery 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: Worsening infections usually require surgical correction (e.g., corneal debridement followed by conjunctival, corneal, or amniotic grafting) by a veterinary ophthalmologist.