Corneal lesions are a common occurrence in pinnipeds maintained in captivity and are also found in free-ranging animals. In captive display animals, of corneal lesions have been associated with various factors including housing animals in fresh water, ultraviolet light, poor water quality, and infectious agents. This report describes the development of corneal lesions in California sea lions (Zalophus californianus) that were undergoing rehabilitation, and its possible association with the use of ozone as a disinfectant in the filtration system.
From May 20 to September 12, 2002, 26 California sea lions (17 male, nine female) ranging in weight from 19 kg to 140 kg and one 71 kg male yearling elephant seal (Mirounga angustirostris) developed corneal lesions. During this time, a total of 153 stranded pinnipeds were housed at The Marine Mammal Center. None of the animals affected had any corneal lesions at the time of stranding. Corneal opacity developed very quickly in affected animals. Some animals that appeared normal in the evening would be found the next morning (eight hours later) with severe bilateral corneal edema. Corneal lesions were bilateral in 17 of the 27 (63 percent) cases. Corneal opacity was often associated with blepharospasm and lesions progressed to bullous keratopathy and severe corneal ulceration. Severe lesions resulted in temporary blindness in several animals.
There was no difference in the bacteria cultured from corneal and conjunctival swabs from five sea lions with lesions and four sea lions with no lesions that were swabbed on arrival at The Marine Mammal Center. Cytology results from four animals indicated acute neutrophilic necrotizing keratitis with no indication of an associated infectious agent. Thirty-nine anesthetic procedures were carried out on affected animals for treatment and diagnosis, including fluorescein dye tests, unilateral and bilateral tarsorrhaphies, keratectomies, corneal biopsies, subconjunctival injections, and sonography. Biopsy results from four sea lions included superficial stromal scarring, post-necrotic epithelial dysplasia, edema, fibrosis, and keratomalacia with lesions consistent with healing after mechanical or chemical ulceration. Water quality issues were strongly suspect.
The animals were all housed in a closed, recirculating, fresh water system with primary disinfection provided by ozone injection and supplemental bromine and chlorine, as required based on weekly coliform counts. Water temperatures varied between 18 and 26°C. Measured bromide concentrations varied between 0 to 1.2 ppm and pH varied from 7.2 to 8.0. Dissolved ozone is measured secondarily using oxidation-reduction potential (ORP) before inflow pipes reach the pools. Measured ORP varied from 350 to 500 mV during the outbreak of corneal lesions. Ozone was also measured using an indigo blue spectrophotometric test at the pools. The indigo blue test was negative for ozone at all pools tested. Ammonia levels were measured as high as 30 mg/L. The occurrence of eye lesions did not correlate to any measured indicators of poor water quality. However, as lesions continued to develop, the ozonation system was reexamined and it was discovered that the degassing towers were improperly designed and did not allow sufficient water agitation to dissipate ozone prior to contact with animals in pools. In addition, ozone was injected into small-diameter pipes that flowed into large-diameter pipes prior to flowing into pools. This may have potentially created gas pockets in larger pipes where ozone could accumulate and discharge into pools, causing caustic damage to corneas. The ozone system was fixed and no further lesions occurred.
Treatment of affected animals included systemic antibiotics and non-steroidal anti-inflammatory agents, as well as subconjunctival injection of gentamycin or enrofloxacin. Third eyelid tarsorrhaphy was performed in 11 animals that had moderate to severe corneal ulcers. Partial keratectomies of thickened corneal tissue were done in three animals to accelerate healing. Animals with corneal lesions were also placed in a salt water system to aid corneal healing. Three animals died of unrelated causes during treatment; however, the lesions in all other animals eventually resolved, and the animals were released. The cause of the corneal lesions remains unknown. However, the lack of evidence for an infectious etiology, poor correlation with other water quality problems, combined with no further incidence of corneal lesions after the ozonation system was fixed seem to indicate that ozone may have been at least partially responsible for the outbreak. Ozone is a strong oxidizing agent and has the potential to create caustic corneal lesions with a very short tissue contact time.
Corneal healing was improved by tarsorrhaphy and placing the animals in salt water. Partial keratectomy did not seem to affect healing rate. This outbreak highlights the difficulty of pursuing the causes of corneal lesions in pinnipeds. Of particular concern are the difficulties in measuring ozone in solution due to how quickly it dissipates, the limitations of poolside tests, and relying on ORP for monitoring ozone levels.