Gunn

Comparative Mortality and Parasite Induced Anemia in Channel Ictalurus punctatus And Hybrid I. punctatus x I. furcatus Catfish Exposed to Bolbophorus damnificus Cercariae

Mackenzie A. Gunn1, Matt J. Griffin2,3, Brian Ott4, T. Graham Rosser5, David J. Wise1,3, and P. J. Allen1

1Department of Wildlife, Fisheries and Aquaculture, Mississippi State University, Mississippi State, MS 39762; 2Delta Research and Extension Center, Thad Cochran National Warmwater Aquaculture Center, Mississippi State University, Stoneville, MS 38776; 3Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Stoneville, MS 38776; 4U.S. Department of Agriculture, Agricultural Research Service, Warmwater Aquaculture Research Unit, Thad Cochran National Warmwater Aquaculture Center, Mississippi State University, Stoneville, Mississippi 38776;5Department of Basic Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, MS 39762.

ABSTRACT

The trematode Bolbophorus damnificus has deleterious effects on production efficiency in farm raised channel (Ictalurus punctatus) and hybrid catfish (I. punctatus x I. furcatus) in the southeastern United States. Previous work has demonstrated that even mild outbreaks, which may go unnoticed by producers, can result in >60% reduction in net return. While infectivity rates in channel and hybrid catfish are similar, preliminary data suggest hybrid catfish experience lower mortality when exposed to comparable numbers of cercariae than channel catfish cohorts. Mortality in juvenile channel and hybrid catfish was observed in two challnges (100 and 250 cercariae/L) for 15 and 60 days following exposure to B. damnificus. Cumulative mortality in hybrid catfish was significantly reduced in both challenges. Preliminary data also identified a parasite-induced anemia associated with the encapsulation of B. damnificus metacercariae occurring 7-15 days post exposure. A second experiment compared physiological response in channel and hybrid catfish fingerlings exposed to 450 cercariae/L. Fish were sampled over 49 days post-challenge for hematocrit, hemoglobin, red blood cell (RBC) concentration, plasma glucose, plasma lactate, and plasma osmolality. Both exposed groups experienced a dramatic drop in hematocrit, hemoglobin, and RBC concentration during the peak mortality window, but recovered afterwards, suggesting an anemic response to the development of the parasite and subsequent recovery once encapsulation is complete. There were no differences over time in any of the plasma parameters measured, but there was an overall decreased osmolality in the exposed groups, suggesting a long term low-level stress response to infection. Throughout this study, there was high mortality in the channel catfish exposure groups, while mortality in hybrids was negligible. Similarly, in non-sampling tanks used to estimate the mortality rate of the administered dose, mortality in channel catfish was significantly higher. In all studies, infected fish presented clinical signs consistent with B. damnificus infection and all exposed fish had visible metacercariae below the skin. There was a substantial anemic response in both species, suggesting the response to B. damnificus infection may not differ between species, even with a significant difference in mortality. The biological and economic implications of these findings on catfish production are unknown, but suggest outbreaks of B. damnificus in hybrids may not result in the same reductions in productivity as observed in channel catfish.