Ulcerative Mycosis Caused by Aphanomyces invadans in Channel Catfish, Black Bullhead, and Bluegill from Southeastern Louisiana

Eight cases of chronic ulcerative mycosis affecting populations of channel catfish Ictalurus punctatus, black bullhead Ameiurus melas, and bluegill Lepomis macrochirus cultured in recreational fishing ponds were submitted to the Louisiana Aquatic Diagnostic Laboratory between April 2000 and January 2002. Diseased clinical specimens presented with multiple foci of skin ulceration, typically overlying more extensive areas of granulomatous myositis that extended to the vertebral column in advanced cases. Lesions were predominated by fields of plump macrophages and multinucleated giant cells surrounding nonseptate, thick-walled hyphae with nonparallel cell walls suggestive of an oomycete. Because deep ulcerative mycoses in other fish species in the western Atlantic (USA), Australia, and Southeast Asia have been attributed to Aphanomyces spp., diagnostic methods were employed to isolate and identify this oomycete along with other potential pathogens. The organism was isolated from sites deep in the musculature with modified peptone-yeast-glucose medium containing 200 mug/mL streptomycin and 100 mug/mL ampicillin. Morphological features were consistent with A. invadans; the internal transcriber spacer (ITS) and 5.8S subunit regions of the rRNA gene were amplified, sequenced, and found to be identical with the ITS I sequences from five isolates of A. invadans deposited in the GenBank database, indicating complete homology. Koch's postulates were fulfilled in juvenile (40-50-g) channel catfish. This report of ulcerative mycosis caused by A. invadans represents new host records for channel catfish and black bullheads from freshwater ponds in southeastern Louisiana.