Ali M. Marchant, Lorelei M. Ford, Ann E. (Beth) Peterman, Lora Petrie-Hanson, Larry A. Hanson
Mississippi State University, College of Veterinary Medicine, Department of Comparative Biomedical Sciences, Starkville, MS
ABSTRACT
Cyanobacterial blooms have become increasingly common in natural water bodies and aquaculture systems. These blooms can release several toxins that are health risks for many species including humans, domesticated animals, wildlife, and fish. The most studied environmentally stable toxin produced by cyanobacteria is the hepatotoxin, Microcystin (MC-LR). Channel catfish (Ictalurus punctatus) ponds often have blooms that express MC-LR, but losses due to the toxin are rare. However, because the liver is a critical component of the immune system, we believe that exposure to the toxin may be a predisposing factor for bacterial diseases in catfish. In our studies, we investigated the effects of a known dosage of MC-LR on the channel catfish liver, and its effects on channel catfish susceptibility to the summer bacterial pathogens Edwardsiella piscicida and Aeromonas hydrophila. Both pathogens are ubiquitous in freshwater aquaculture environments, which makes controlling outbreaks through pathogen avoidance very difficult. Identifying predisposing factors for these outbreaks could lead to future management practices. To observe the effects of MC-LR on channel catfish liver, treatment fish were intraperitoneally injected with a 500 ng/g bw dose and were sampled alongside control fish over 6 days. MC-LR treated fish were not visibly affected but completely stopped eating. When sampled, all treated fish through day four had no ingesta and had full gall bladders, while most control fish demonstrated ingesta in the gut and lighter colored typical gall bladders. Serum aspartate aminotransferase and alanine aminotransferase levels were significantly elevated from 6 hours through 96 hours post-exposure indicating hepatotoxicity. Alkaline phosphatase and bilirubin levels were not substantially affected. Histology confirmed substantial hepatic injury among the treated fish. Later trials that contained both MC-LR and the bacterial pathogens resulted in significantly greater mortality in the treatment group containing both MC-LR and the bacteria in comparison to bacteria-only treatments, and no losses within only MC-LR. Our study demonstrated exposure to subclinical doses of MC-LR, can compromise the function of the liver and digestive system, these are critical organs of the innate immune defenses which may have resulted in the increased mortality when encountering bacterial pathogens.