Physiological response of channel (Ictalurus punctatus) and hybrid (I. punctatus x I. furcatus) catfish following Bolbophorus damnificus infection

Abstract

The trematode Bolbophorus damnificus is associated with reduced production in farm-raised catfish in the southeastern United States. Previous research demonstrated that even mild outbreaks, which may go unnoticed by producers, can result in >60% reduction in net return due to decreased feeding and trematode-induced mortality. While infectivity rates in channel (Ictalurus punctatus) and hybrid catfish (I. punctatus x I. furcatus) are similar, hybrid catfish experience lower mortality when exposed to comparable numbers of Bolbophorus cercariae than channel catfish cohorts. This study compared physiological responses in channel (21.4 ± 0.93 g) and hybrid (27.5 ± 2.63 g) catfish fingerlings exposed to 450 cercariae/L. Fish were sampled over the next 49 days for hematocrit, hemoglobin, red blood cell (RBC) concentration, plasma glucose, plasma lactate, and plasma osmolality. Exposed channel and hybrid catfish experienced a dramatic drop in hematocrit, hemoglobin, and RBC concentration during the peak mortality window associated with metacercarial development, but recovered afterwards, suggesting an anemic response to the development of the parasite and subsequent recovery once metacercarial encapsulation is complete (∼15 days post-exposure). There were no differences over time in any of the plasma parameters measured, but there was an overall decreased osmolality in both exposed groups, suggesting a prolonged low-level stress response to infection. There was high mortality in the exposed channel catfish groups from days 9 to 15 post-infection, consistent with previous studies. Comparably, hybrids appeared healthy and active throughout the trial with only negligible mortality. In all trials, 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 catfish types, suggesting the physiological response to B. damnificus infection may not differ between channel and hybrid catfish and the significant differences in mortality may be attributed to survivor bias, hybrids greater tolerance of the anemic state or other unidentified factors. The biological and economic implications of these findings are unclear, but the behavioral differences observed between the two fish groups in reponse to B. damnificus, coupled with reduced mortality in hybrids, supports previous work indicating hybrids are more tolerant of B. damnificus infection and outbreaks in hybrid catfish may not yield the same deleterious effects as channel catfish.