Formalin treatment to reduce mortality associated with Saprolegnia parasitica in rainbow trout, Oncorhynchus mykiss

Formalin has been used to treat fish egg fungal infections, but its ability to reduce mortality in fish with fungal infections has not been clearly demonstrated. An experimental infection was induced with abrasion, exposure, and temperature stress to evaluate the ability of formalin to reduce mortalities in rainbow trout (Oncorhynchus mykiss) infected with Saprolegnia parasitica (ATCC 22284). The trout were anesthetized and abraded with a controlled abrasion technique. Temperature stress was performed concurrently with the fungal exposure by moving the trout from an acclimation temperature (15 +/- 2 degrees C) to a higher exposure temperature (22 +/- 2 degrees C) as they were immersed in an exposure tank inoculated with S. parasitica for 4 h. After the exposure, the fish were randomly distributed into 16 experimental tanks (3 fish/tank). Four treatment doses (0, 50, 100, and 150 ppm formalin solution) were evaluated. Fish were given three treatments of 1 h duration: on day 1, 1 h after the fungal exposure, and then the morning of days 3 and 5. The infection rate was 100% for all of the four trials in the study. The average percent mortality from the four trials was 67 (0 ppm), 35 (50 ppm), 29 (100 ppm), and 40 (150 ppm). Statistical analysis of the percent mortality at days 5 and 19 showed that the 50-, 100-, and 150-ppm doses were all significantly different from the 0-ppm dose on both days. Published by Elsevier B.V.