Pathology and Molecular Identification of Developing Henneguya spp. Actinospores within the Intestinal Epithelium of Dero digitata during an Outbreak of Proliferative Gill Disease in a Commercial Catfish Pond

Dero digitata, a freshwater oligochaete worm found worldwide, serves as the definitive host for the myxozoan Henneguya ictaluri, the cause of proliferative gill disease (PGD) in ictalurid catfish. The study objectives were to describe pathologic changes associated with developing actinospores within D. digitata from a commercial catfish pond during an active PGD outbreak. Specific in situ hybridization probes identified H. ictaluri, Henneguya exilis, and Raabeia-type TGR 2014 actinospore development in pure infections associated with intestinal epithelial hypertrophy, hyperplasia, and vacuolation. Results support findings from other myxozoans demonstrating clonal expansion of myxozoan populations occurs through asexual reproduction of the oligochaete host.