IDENTIFICATION OF THE ETIOLOGIC AGENT FOR ULCERATIVE DISEASE IN KOI (CYPRINUS CARPIO)

Identification of the Etiologic Agent for Ulcerative Disease in Koi (Cyprinus carpio)

University of Georgia College of Veterinary Medicine, Departments of Medical Microbiology and Paristology and Small Animal Medicine, Athens, GA. 30605

Abstract: An outbreak of dermal ulceration in Koi, similar to erythrodermatitis in carp, is having a substantial economical impact on the ornamental fish industry. Affected fish develop ulcers primarily along the body walls and head. Secondary bacterial infection and death are common sequela to the ulcers. Surviving fish are commonly scarred and may be stunted. At present the etiologic agent for this disease is not documented within the scientific literature and there is debate and antedoctal speculation as to its cause. During the spring of 1998 a group of imported Japanese Koi were presented to the University of Georgia by a wholesale facility that had been experiencing periodic outbreaks of dermal ulcerations for the past two years. These fish were cultured for potential bacteria. A gram-negative rod was isolated from an ulcer and was subsequently characterized by the National Fish Health Research Laboratory as an atypical strain of Aeromonas salmonicida. This isolate was able to reproduce the ulcerative disease in subsequent experimental trials.

In May of 1998 a group of 8 Japanese Koi were donated to the University of Georgia for the purpose of attempting to solve a recurring problem a local retail fish dealer had been having for two to three years. The main complaint was that the fish would develop prominent ulcers primarily along their body wall and, therefore, loose their inherent value. Lesions also had been seen on the operculum, head and fins. Ultimately these fish would either die or recover with only supportive care. Based on historical information obtained from this dealer and other Koi hobbyists, there did not seem to be any reliable effective treatment for the disease. At this time, there was also no way to predict which fish would be susceptible to the disease. Upon presentation, all fish with ulcers were isolated from those without lesions. Grossly, the fish were in variable states of body condition, ranging from emaciation to good. The size, number and extent of the ulcerations were also variable (Figs. 1-6). No fish had more than three ulcers and at there worst penetrated below the dermis and bone was visible. The attitude of the fish seemed to be correlated with body condition. Those Koi that were heavily ulcerated and emaciated also seemed to be lethargic and anorexic. In contrast, those fish with only small ulcers eat very well and remained active. A ring of hemorrhagic tissue from which scales were easily removed commonly surrounded the ulcerative lesions. The loose epithelium associated with the scale pockets extended along the periphery of these lesions and was the sites from which bacterial cultures were taken. The ulcers were cultured onto Blood, Rimler Shotts, and Tryptone Yeast Gelatin agars for 12-36 hours at 30º C. Wet mounts of skin, fin and gill biopsies were also examined microscopically for parasites.

Fig. 1. Koi carp with an extensive ulcer on the lateral body wall.

Fig. 2. Koi carp with ulcer beneath the dorsal fin.

Fig. 3. Closer view of the ulcer beneath the dorsal fin.

Fig. 4. Koi carp with dorsal or saddle ulcer.

Fig. 5. Koi carp with slightly more advanced dorsal ulcer.

Fig. 6. Koi carp with a large pedal ulcer.

Research in food carp and goldfish, which identified an Aeromonas species as the cause of erythrodermatitis, prompted an investigation for a similar pathogen in koi. Based on gram stains, colony morphology and growth rates, bacteria suspected to be an Aeromonas sp. was isolated. The colony type in question was almost always present on blood agar culture. However, these slow growing, small colonies frequently were overtaken by more aggressive secondary invading bacteria if the cultures were not checked with regularity. An isolate obtained from one of the original Koi was tested for pathogenicity in previously uninfected (non-ulcerated) Koi and produced disease in the fish without fail. In contrast, two alternate bacterial isolates failed to produce disease in our experimental model.

This disease in fish is indicative of a widespread problem within the ornamental fish industry and has stimulated many debates as to what treatments are effective. For the fish in this study there seems to be no distinct advantage to intraperitoneal injection of antibiotics or antibiotic food medication, and in some instances the injection may have initiated an ulcer. Frequent water changes, daily in some cases, and adding 0.3% NaCl to the water was the most beneficial treatment. These treatments were done along with regular cleaning of the filtration system and maintenance of optimal water quality parameters including temperatures, pH, ammonia and nitrate levels. Fish also seemed to be responsive to daily cleansing of the ulcers with a 7.5% betadine solution. Current ongoing research includes further characterizing the bacteria and evaluating the efficacy of vaccines. Preliminary trials have also been completed in an attempt to develop treatment alternatives and diagnostic techniques.

Ulcerative disease of Koi has had a profound impact on the industry and will continue to do so without further research. This case provided an opportunity to investigate potential treatment alternatives and most importantly identify the etiologic agent. With continued work it is entirely possible that intraperitoneal or water bath vaccinations may become available. It is important to remember that although the agent genus and species have been identified there are numerous potential atypical strains of Aeromonas salmonicida that must be characterized.