Identification of the Etiologic Agent for Ulcerative
Disease in Koi (Cyprinus carpio)
Brendan B. Anders, Victoria V. Burnley, Branson Ritchie,
Steven E. Poet
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.
Key Words: Koi, Erythrodermatitis, Aeromonas
salmonicida, Ulceration
Case Report
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.
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| Fig. 1. Koi carp with
an extensive ulcer on the lateral body wall. |
Fig. 2. Koi carp with
ulcer beneath the dorsal fin. |
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| Fig. 3. Closer view of
the ulcer beneath the dorsal fin. |
Fig. 4. Koi carp with
dorsal or saddle ulcer. |
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| 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.
This Page Last Updated November 15, 1999 |