Proliferative Gill Disease in Catfish Aquaculture: Past and Present, With Special Focus on Mitigation Strategies

1,3Noor-ul-Huda; 2Bradley M. Richardson, 1Cyndi Ware, 4Al Camus, 2Caitlin E. Older, 5Fernando Yamamoto, 5Penny Goodman, 5Grant Riefers, 5David J. Wise,3Dave Marancik and 1Matt J. Griffin

1Thad Cochran National Warmwater Aquaculture Center, Aquatic Research and Diagnostic Laboratory, Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Stoneville, MS1; Thad Cochran National Warmwater Aquaculture Center, Warmwater Aquaculture Research Unit, Agricultural Research Service, United States Department of Agriculture, Stoneville, MS 39776; 3Department of Pathobiology, St. George’s University, School of Veterinary Medicine, True Blue, Grenada, West Indies; 4Department of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602; 5Thad Cochran National Warmwater Aquaculture Center, Mississippi Agriculture and Forestry Experiment Station, Delta Research and Extension Center, Mississippi State University, Stoneville, MS 38776

ABSTRACT

Over the past twenty years, Proliferative Gill Disease (PGD) has been the most diagnosed parasitic disease in catfish aquaculture in the southeastern United States. Colloquially known as “Hamburger gill”, the etiological agent is the myxozoan Henneguya ictaluri. Research suggests H. ictaluri is present in most, if not all, catfish ponds during the spring of the year, and to a lesser extent in the fall. The parasite exhibits a typical myxozoan life cycle, with a myxospore stage in the channel catfish Ictalurus punctatus and an actinospore stage released by the ubiquitous benthic oligochaete, Dero digitata. Predominantly a problem in fingerling production, outbreaks occur in all sized fish, with mortality approaching 100% in severe outbreaks. Biological and chemotherapeutic control methods have been largely unsuccessful. To avoid losing fish during springtime stocking, several producers in the Mississippi Delta submit pond water samples to the Thad Cochran National Warmwater Aquaculture Center (NWAC) for molecular testing. NWAC scientists use an H. ictaluri-specific qPCR to estimate H. ictaluri levels in ponds, which helps producers formulate their stocking strategies. In response to producer requests, the effects of fingerling size on PGD severity was investigated. To this end, two separate experimental trials were conducted using caged "sentinel" fish across a range of sizes. Briefly, channel catfish fingerlings reared indoors at the Thad Cochran National Warmwater Aquaculture Center were confined in nylon mesh net-pens which allow for the free exchange of water. The cages were placed in commercial catfish ponds for 7 days, after which PGD severity was assessed based on survival, gill clip wetmounts, histology, and real-time PCR. In the first experimental trial, fingerings of three different sizes (~40 lbs/1000; ~80 lbs/1000; ~120 lbs/1000) were placed in 8 different commercial catfish ponds with varying levels of H. ictaluri as determined by qPCR assessment of pond water.  In the second trial, to maximize the number of available net-pens, only two fish sizes (~40 lbs/1000; 120 lbs/1000) were assessed, placing them in 11 different ponds. Across the two trials, fish size had no appreciable effect on survival or gill damage based on wetmount assessments and real-time PCR. Effects of fish size on PGD burden were assessed using generalized linear models and estimated marginal means. Results indicate the risk of losing fish stocked into ponds deemed moderate to high risk cannot be substantially reduced by stocking larger fingerlings.