Columnaris disease is a bacterial infection of fish caused by the Gram-negative bacillus Flavobacterium columnare. An experimental model of this disease was developed to reproduce F. columnare-induced dermatitis in koi with high (80% to 100% ) mortality compared to uninfected controls (0% to 20% mortality). The protective mucus layer was wiped from the right side of the fish while leaving the left side intact. When fish were challenged with a virulent strain (strain 12/99) of F. columnare, cutaneous lesions consistently appeared on the right side of the body, but the left side of the body remained normal. This experimental model of columnaris disease differed from natural disease outbreaks in that cutaneous lesions were common while gill lesions were rare. The antibacterial properties of the surface mucus layer in preventing F. columnare infection was demonstrated by incubating cultures of log-phase growth bacteria with isolated mucus from healthy koi. F. columnare viability decreased as quantitated by manual counting of bacterial colonies on agar plates and by differentiation of viable and dead bacteria using propidium iodide staining and fluorescence microscopy. F. columnare in cytologic and histologic specimens appeared as long, thin, (5-12 mm x 0.5 mm) bacilli. Cytology was the best technique to tentatively diagnose columnaris disease. F. columnare was more readily visualized by Giemsa, as opposed to hematoxylin and eosin, staining in histologic sections. Both polymerase chain reaction (PCR) and DNA in-situ hybridization (ISH) diagnostic assays were developed to specifically detect F. columnare nucleic acid in fresh biological samples and in formalin fixed, paraffin embedded tissues. These techniques demonstrated that F. columnare infection in koi was restricted to the skin, fins, and, rarely, gills; systemic infection did not occur. Hematologic and biochemical reference intervals were established to evaluate subsequent changes during experimental disease. Hematologic changes in F. columnare infected koi included a nonregenerative anemia and mild leukopenia with lymphopenia, neutrophilia, and monocytosis. Biochemical changes included significantly decreased sodium and chloride concentrations. Decreases in total serum protein and albumin concentrations were present but minimal. A new proprietary solution (Tricide-Neo), was evaluated as a treatment for columnaris disease. Following experimental challenge with F. columnare, Tricide-Neo reduced mortality by 40% to 50% if treatment was performed before deep skin ulcers developed.
