Laboratory Elucidation of Myxozoan Life Cycles Involving the Benthic Oligochaete Dero digitata, Including Three Newly Described Myxobolid Species From Freshwater Fish

1,2Ethan T. Woodyard, 2T. Graham Rosser, 1Stephen R. Reichley, 2Larry Hanson, 1,3Lester H. Khoo, 2Beth Peterman, 2Will G. McInnis, 2Jonah A. Nguyen, 2Madison R. Rawdon, 1,3Matt J. Griffin

1Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS; 2Department of Comparative Biomedical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS; 3Thad Cochran National Warmwater Aquaculture Center, Delta Research and Extension Center, Mississippi State University, Stoneville, MS.

ABSTRACT

Myxozoan parasitism is a significant burden to the US farm-raised catfish industry.  Multiple myxozoan species have been reported from channel, blue and channel × blue catfish hybrids, most notably Henneguya ictaluri, which causes proliferative gill disease in channel and hybrid catfish.  Myxospores of Henneguya spp. were isolated from channel catfish Ictalurus punctatus submitted for diagnostic assessment to the Aquatic Diagnostic Laboratory at Mississippi State University’s College of Veterinary Medicine (MSU CVM) in Starkville, MS, and the MSU CVM Aquatic Research and Diagnostic Laboratory of the National Warmwater Aquaculture Center in Stoneville, MS.  Isolated myxospores were characterized by morphological and molecular features and identified as H. exilis, H. ictaluri, H. mississippiensis, and a previously undescribed Henneguya species. Subsamples of myxospores for each Henneguya spp. were used to infect laboratory reared colonies of the benthic oligochaete Dero digitata. While previous efforts to infect D. digitata in the lab have been unsuccessful, the maintenance of D. digitata cultures supplemented with commercially available algal wafers has significantly increased population fitness, facilitating the establishment of several Henneguya spp. life cycles which have previously been elusive. In addition to these trials, myxospores of two undocumented Myxobolus spp. were recovered from the gills of bluntnose minnows Pimephales notatus ancillary to a parasitology workshop for aspiring veterinary students as part of the MSU CVM’s Summer Veterinary Camp. The two novel Myxobolus spp. were characterized and used for experimental infections in laboratory reared D. digitata colonies as above. Actinospores of H. exilis, H. ictaluri, H. mississippiensis, Henneguya n. sp., Myxobolus n. sp. 1, and Myxobolus n. sp. 2 were observed from experimentally infected D. digitata beginning at 6, 7, 6, 11, 5 and 5 weeks post exposure, respectively. These experimental infections constitute the laboratory replication of three known myxozoan life cycles and the elucidation of three previously unknown life cycles.  Previous research investigating catfish associated myxozoans has been hindered by the lack of a functional experimental model and these works significantly advance our understanding of the myxozoan pests endemic to commercial operations.  Moving forward, the experimental model developed here will be integral in the elucidation of myxozoan developmental timelines within the oligochaete host, provide a more reliable source for infectivity trials in fish and facilitate more thorough descriptions of novel myxozoan species which use D. digitata as a definitive host.