Comparison of the Efficacy of Iodine, Formalin, Salt, and Hydrogen Peroxide for Control of External Bacteria on Rainbow Trout Eggs

Abstract

Two experiments were conducted in vivo with eggs of rainbow trout Oncorhynchus mykiss to compare the bactericidal ability of four common disinfectants. A third test compared bacterial abundance estimation methods for fish eggs (use of a vortex mixer for agitating an egg versus rolling the egg across a petri dish). In the first test, the number of colony forming units (CFU) counted on enriched Ordahl's agar with tobramycin (EOT) or trypticase soy agar (TSA) was compared among eggs treated with various doses of iodine, hydrogen peroxide, formalin, or rock salt. A treatment of 1,667 mg of formalin/L of water and all iodine, salt, and hydrogen peroxide treatments had significantly fewer bacteria on EOT than did controls, but CFU counts for a formalin treatment of 500 or 1,000 mg/L did not. All chemical treatments significantly reduced CFU counts on TSA relative to controls except salt at 0.030 mg/L and formalin at 500 mg/L. The least growth was observed on iodine-treated eggs. In the second experiment, we evaluated the effect of keeping the eggs suspended (i.e., constantly tumbling) during disinfection to increase chemical contact. Treatments were (1) static application of iodine at 100 mg/L, (2) suspension in iodine at 100 mg/L, (3) static application of iodine at 500 mg/L, (4) suspension in iodine at 500 mg/L, (5) suspension in formalin at 2,000 mg/L, (6) suspension in hydrogen peroxide at 2,000 mg/L, (7) static control, and (8) suspended control. Bacterial abundance was significantly reduced in suspended eggs in some cases but not others. Formalin and hydrogen peroxide reduced bacterial abundance but were inferior to iodine in some cases. Each chemical treatment resulted in the survival of bacteria despite attempts to attain better chemical contact by egg suspension. Comparison of the methods used to estimate total CFU per egg indicated that agitation recovered 71–100% of the bacteria on the outside of the egg. Hatchery managers should be aware that not all bacteria are killed by chemical treatment of eggs, and therefore a significant number of pathogens could still enter a hatchery via the importation of treated eggs.