Inactivation of bacterial and viral fish pathogens by ozonation or UV irradiation in water of different salinity

Abstract

Bacterial and viral fish pathogens were exposed to ozone or ultraviolet (UV) irradiation in laboratory batch systems. Inactivation curves were made for Aeromonas salmonicida subsp. salmonicida, Vibrio anguillarum, Vibrio salmonicida, Yersinia ruckeri and the infectious pancreatic necrosis virus (IPNV) in ozonated lake, brackish and sea water at 9–12°C. The four bacteria tested were inactivated by 99·99% (4 log reductions in viable count) in all three waters within 180s at residual ozone concentrations of 0·15-0·20 mg/liter measured by the indigo colorimetric method. After establishing these residuals, the differences in water salinity did not cause any substantial differences in bactericidal activity of ozone, illustrating the usefulness of concentration measurements by the indigo colorimetric method to predict inactivation also in saline waters. The rate of bacterial inactivation was fast during the first 60 s in all three waters. After that point the slope of the curves levelled off. This observation was explained by loss of ozone and reduced bactericidal activity during the course of the experiments. IPNV was inactivated (99·99%) in all three waters within 60 s when exposed to 0·10-0·20 mg/liter residual ozone. V. anguillarum, V. salmonicida, Y. ruckeri and IPNV were UV irradiated in brackish water at room temperature. An UV dose of 2·7 m Ws/cm 2 resulted in 99·999% (5 log) reduction in viable count for all three bacteria. IPNV was much more resistant to irradiation than the bacteria. An average UV dose of 122 mWs/cm 2 was required for 99·9% (3 log) reduction in virus titer.