Use of Penicillin and Streptomycin to Reduce Spread of Bacterial Coldwater Disease II: Efficacy of Using Antibiotics in Diluents and During Water Hardening.

Bacterial coldwater disease, caused by Flavobacterium psychrophilum, has lead to the loss of significant numbers of hatchery-reared salmonids. The bacteria can be spread from parent to progeny within contaminated sperm and ovarian fluid and can enter the egg during fertilization. The addition of antibiotics to diluents and water-hardening solutions could prevent the spread of the disease. In separate trials, a mixture of 0.197 mg/mL penicillin plus 0.313 mg/mL streptomycin was added to both a 0.5% sodium chloride fertilization diluent and hatchery well water during hardening. Tests showed that the addition of the antibiotics to the diluent and during up to 60 min of water hardening had no effect on the eye-up, hatch and deformity rates of Rainbow Trout Oncorhynchus mykiss eggs compared with the nonantibiotic-treated controls. Also, significant reductions in the prevalence of F. psychrophilum on the surface and inside eggs were observed when compared with controls. These results indicate that the addition of penicillin and streptomycin to diluents and during water hardening can prevent the vertical transmission of bacterial coldwater disease.