Comparison of formalin and chloramine-T for control of a mixed gill infection (bacterial gill disease and ichthyobodiasis) in rainbow trout

Juvenile rainbow trout Oncorhynchus mykiss suffering from a mixed gill infection caused by Flavobacterium branchiophilum and Ichthyobodo necatrix [Ichthyobodo necator] were treated with a daily 1-h formalin [formaldehyde] (0.167 mL/L) or chloramine-T (10mg/L) bath on each of three consecutive days. Neither formalin nor chloramine-T completely removed both I. necatrix and F. branchiophilum from infected gills. Analysis of variance indicated that formalin treatment was most effective for I. necatrix, which correlated with an improvement in the clinical course of the disease, but more formalin-treated fish died during the first few days than did chloramine-T-treated fish following treatment. Both formalin and chloramine-T significantly reduced cumulative mortality, compared to mortality of control (untreated) fish. Cumulative mortality was lowest among the chloramine-T-treated fish, but this difference was not significant. After the third treatment, only formalin significantly reduced the number of I. necatrix on the gills (compared to controls), whereas fewer but similar numbers of F. branchiophilum cells remained after treatment with either chloramine-T or formalin..