Abstract

An outbreak of dermal ulceration in koi (Cyprinus carpio), similar to erythrodermatitis in carp, is having a substantial economic impact on the ornamental fish industry. Affected fish develop ulcers primarily along the body wall and head. Secondary bacterial infection and death have also been observed. Those fish that survive are commonly stunted and permanently scarred. At present there is no report that identifies the etiologic agent for this disease in koi. Research in food carp and goldfish has identified an atypical strain of Aeromonas salmonicida as an etiologic agent. The hypothesis for this project states that the primary etiologic agent for ulcerative disease in koi is an atypical Aeromonas salmonicida. The objective of this study, therefore, was to establish Koch's postulates for an organism isolated from a koi with erythrodermatitis.

In May of 1998 an adult male Lemon Ogon with multiple dermal ulcers on its body and head was presented. The fish was donated by a retail facility that has experienced periodic outbreaks of ulcerative disease for the past two years. Bacteriological cultures where taken from the periphery of active ulcers on the head and body wall and inoculated onto Blood, Rimler-Shotts and Tryptone Yeast Gelatin agars. From these cultures, a non-motile, small gram-negative rod was isolated. The colonies grow with a gray-greenish tint on Blood agar and are moveable when pushed with an inoculating loop. This organism, suspected to be the etiologic agent, was submitted to the National Fish Health Research Laboratory for characterization.

The bacterium was grown in luria broth for 48 hours at 30 degrees Celsius and then injected into healthy juvenile koi at a concentration of approximately 7.5x10 ⁷ Colony forming units (CFU). At this dose the fish died within one to three days and showed no signs of ulceration. Subsequently, additional koi were scraped along the body wall below the dorsal fin with a #21 scalpel blade and immersed for 30 minutes in 100 mL of tank water containing approximately 3.7x10⁸ CFU of bacteria. At this dilution all fish developed ulcers and showed signs of bacterial sepsis within seven days. From these ulcers the original organism was reisolated with regularity. For comparative analysis a pair of juvenile fish were exposed to alternate bacterial isolates. One was immersed in a dilution containing a motile small gram-negative rod, which appeared orange on blood agar and was isolated from an experimental fish. The second fish was infected with a non-motile small gram-negative rod that produced yellow colonies on blood agar and was isolated from an ulcer on a fish within the University's collection. No ulcers or signs of illness developed in either fish.

An organism, believed to be an atypical Aeromonas salmonicida, was isolated from lesions associated with koi erythrodermatitis. Following immersion in water containing the purified isolate, koi developed ulcers resembling lesions seen in naturally infected individuals. The reisolation of the original organism from ulcers on experimentally infected fish fulfils Koch's postulates. In addition to further characterization of the organism, future studies should be directed towards vaccine development and treatment.