الفهرس 
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Abstract
This study was planned to investigate the effect of ochratoxin and other factors (immunostimulants) on the expression and activity of the antimicrobial polypeptides an important innate defense in channel catfish fish; with Evidence for synergism of piscidin 2 with antiparasitic and anti-Oomycete drugs. First experiment was conducted to investigate the effect of ochratoxin on antimicrobial polypeptide expression and resistance to water mold infection in Channel catfish, Fish were fed semi-purified diets having one of four concentrations of ochratoxin A (0, 2.0, 4.0, or 8.0 mg OA/kg feed). The results revealed neither differences in the total antibacterial activity, HLP-1 level nor anti-E.ictaluri activity among any treatment groups at any of the sampling times. Ochratoxin A significantly reduced weight gain and feed conversion ratio (FCR) in a dose-dependent manner. Water mold infection was first grossly visible on Day 4 post-challenge, where in the 8 ppm OA treatment, had one fish was dead and all others were heavily infected. The 8 ppm OA group had a significantly shorter survival time than all three other treatments. Second experiment was conducted to investigate the effect of adjuvant and microbial challenge on the expression of antimicrobial polypeptide in Channel catfish (Ictalurus punctatus); that intraperitoneal injection of either Freund’s complete adjuvant (FCA) induced significant upregulation of total antibacterial activity, anti-Edwardsiella activity (the fraction of antibacterial activity active against E. ictaluri), and HLP-1 (the major AMPP in channel catfish skin). Tetrahymena induced a similar upregulation, except that HLP-1 was not significantly induced and the response appeaered to be more transient than FCA immunostimulation. AMPP levels were increased up to five-fold higher than resting levels and levels expressed were well within concentrations known to be inhibitory to many important pathogens in vitro. Third experiment was conducted to examine the evidence for synergism of piscidin 2 with antiparasitic and anti-oomycete drugs; results revealed that piscidin 2 was rapidly lethal in the presence of either copper or malachite green, two drugs commonly used to treat water molds, there was evidence for partial synergism with piscidin. There was also evidence for partial synergism after exposure of the ciliate parasite Tetrahymena pyriformis to piscidin 2 plus copper. The minimum oomyceticidal concentration (MOC100) of piscidin 2 against zoospores of three pathogenic isolates of Saprolegnia ranged from 12.5-25 µg/ml. This data provide further evidence that piscidins may be an important host defense against skin and gill pathogens and that the piscidin levels in host tissue might influence the success of drug treatments.