On October 1st, 2014, Zoetis announced the renewed US availability of Terramycin® ophthalmic ointment (oxytetracycline with Polymyxin B Sulfate), after a 2+ year hiatus since Pfizer Animal Health stopped production in June 2012. Although oxytetracycline has since been available through compounding pharmacies, the renewed availability of the commercially manufactured form warrants a review of its indications and uses in our small animal patients.
The manufacturer’s product information states that “Terramycin is indicated in dogs and cats with superficial ocular infections, such as conjunctivitis, keratitis, pink eye, corneal ulcer, blepharitis and bacterial inflammatory conditions that may occur secondary to other infectious diseases. In dogs, it is also indicated for ocular infections due to secondary bacterial complications of distemper.”
Key benefits are stated to be: “broad-spectrum effectiveness against both Gram-positive and Gram-negative organisms, including Pseudomonas aeruginosa. The combined antibacterial effect of oxytetracycline hydrochloride and polymyxin B is at least additive, and in many instances, an actual synergistic action occurs.”
As a topical tetracycline antibiotic, Terramycin® is most useful against Chlamydophila and Mycoplasma conjunctivitis in cats. A cautionary note must be made in that anaphylaxis has been documented in cats to topical Polymyxin, and while rare, this can be fatal. In a report of 61 cats with anaphylaxis to topical ophthalmic antibiotics, 21% (13) of the cases had received topical oxytetracycline and polymyxin B. An overall incidence of anaphylaxis cannot be determined, but is thought to be extremely uncommon.
In general, feline herpesvirus keratoconjunctivitis presents very similarly to Chlamydophila felis infection, and feline herpesvirus is the more common infection. Therefore oxytetracycline (typically dosed QID since it is bacteriostatic) may be used concurrently (to protect against secondary bacterial infection) with viral therapy, as empiric therapy when a case has failed viral therapy, and of course in documented PCR-positive Chlamydophilal infections.
In dogs, oxytetracycline can be utilized (as labelled) as a broad-spectrum antibiotic for corneoconjunctival bacterial infections, although in severe infections such as Pseudomonas or other “melting” ulcers, a bactericidal antibiotic with corneal penetration, such as a fluoroquinolone, would be considered to be superior. However, there is no substitute for corneoconjunctival bacterial culture with an antimicrobial sensitivity panel to direct therapy. For broad-spectrum coverage against infection in non-complicated, non-infected superficial corneal ulcers, triple antibiotic (neo-poly-bac) ointment or (neo-poly-gram) drops are more commonly used alternatives.
A special consideration for tetracyclines is the anti-collagenase properties that may enhance corneal healing and reduce proteinase destruction (“melting”) of the cornea. Oxytetracycline has been shown in vitro to stimulate transforming growth factor-beta activity, thus increasing transcription factor expression and enhancing corneal epithelial migration. It might therefore be used to support corneal healing, in conjunction with or instead of other anti-proteinases such as autologous serum or oral doxycycline.