Therapeutic and prophylactic measures for winter saprolegniosis in channel catfish

Winter saprolegniosis in channel catfish Ictalurus punctatus is associated with low temperature (similar to 12 degrees C) induced immunosuppression and invasion by a ubiquitous, opportunistic water mold, identified as a Saprolegnia sp. In this study, attempts were made to identify antimicrobials/chemicals which may (1) be effective for the therapeutic treatment of winter saprolegniosis, and (2) prophylactically control water concentrations of Saprolegnia and hence prevent the onset of disease under laboratory challenge conditions. The antimicrobials used were amphotericin B and the cecropins A, B, and P1. While amphotericin B halted Saprolegnia growth, the cecropins were ineffective. The chemicals and herbicides approved for use by the U.S. Food and Drug Administration and considered in this study for the prevention of Saprolegnia zoospore production or cyst germination were used at concentrations less than or equal to those recommended for use in commercial catfish ponds for other purposes. Compared with malachite green (a positive control only), sodium chloride and potassium permanganate were ineffective, while copper sulphate and formalin inhibited both zoospore production and cyst germination. However, at the lowest inhibitory dosage, copper sulphate was toxic to catfish challenged under laboratory conditions; this was likely due to the low alkalinity of our lank water when compared to that of catfish ponds. Several herbicides were tested including diquat, simazine, hydrothol 191, and aquathol K. Only diquat had an inhibitory effect on Saprolegnia growth. In subsequent laboratory challenge experiments, both formalin and diquat were efficacious in preventing the onset of winter saprolegniosis in channel catfish.