Are all koi ulcer cases associated with infection by atypical Aeromonas salmonicida? Polymerase chain reaction assays of koi carp skin swabs submitted by hobbyists.

Infection by atypical Aeromonas salmonicida is regarded as the cause of ulcer disease (KUD) in koi carp Cyprinus carpio and goldfish Carassius auratus. However, other causes--including parasites, viral infection, and fungi--have been proposed. In our diagnostic work, we often fail to isolate A. salmonicida even when clear clinical signs of KUD are present. This failure may be because these fastidious and slow-growing bacteria are difficult to isolate in culture or because the bacteria are not actually present in the lesions. In this study, we used polymerase chain reaction (PCR) to detect A. salmonicida in DNA samples swabbed from koi carp ulcers. These alcohol-preserved samples were collected and submitted by hobbyists and included 40 separate cases from 12 different states. We identified atypical A. salmonicida by PCR in 52 of 62 samples submitted and in 33 of 40 unique cases. The negative findings for A. salmonicida by PCR could all be attributed to high water temperatures, prior antibiotic use, poor sample quality, or misdiagnosis of columnaris disease as KUD. Tests for Aphanomyces invadans by PCR were negative in every case. This work confirms that A. salmonicda is still the predominant cause of KUD and that our negative culture results were most likely due to technical failures rather than an absence of A. salmonicda in the ulcer lesions.