Hydrogen peroxide as pre-treatment stressor in experimental immersion challenge of rainbow trout (Oncorhynchus mykiss) fry with Flavobacterium psychrophilum

Flavobacterium psychrophilum is the causative agent of rainbow trout fry syndrome (RTFS), also called bacterial cold water disease (BCWD) in USA. Since being discovered in USA in the 1940s, the disease has spread to aquacultures worldwide [1]. Mortality rates as high as 90% have been reported in fry, although the mortality rate in Danish outbreaks average around 30–40%. Rainbow trout (Oncorhynchus mykiss) is the major species in Danish aquaculture and various rainbow trout related products, including eyed eggs and roe for export, amount to 40 % of the Danish fishery sector. This is equivalent to about 900 million DKK, making rainbow trout fry health an important issue [2]. Presently no commercial vaccines are available and followingly RTFS is controlled with antibiotics. Because the F. psychrophilum is most serious in fry, an immersion vaccine and a model for testing it is very desirable. Many approaches to experimental challenges have been carried out. Although injection-based experimental challenges are reproducible this approach is unsuitable for some types of investigations, since the mechanical barriers of the fish are bypassed in the delivery of the pathogen. Furthermore, the many difficulties producing mortality in immersion and cohabitation challenge models have highlighted the limited pathogenicity of F. psychrophilum without application of stress or scarification [3]. In addition, experiments, which were successful in producing mortality, have been hard to reproduce [4]. Various chemicals are used in aquaculture, including formalin, which has been used as a pretreatment stressor ahead of immersion challenge resulting in a mortality rate over 50% [5]. Formalin is to be phased out of Danish aquaculture by 2014 due to human health risks and hydrogen peroxide has been proposed as a possible substitute and has already been used for several purposes in Denmark for a number of years. In two studies regarding Tenacibaculum maritimum and Flavobacterium columnare infections in respectively turbot (Scophthalmus maximus) and channel catfish (Ictalurus punctatus), hydrogen peroxide was shown to accelerate the diseases [6,7], making it an evident candidate for a stressor. Furthermore, several studies uncovering the adverse effects of hydrogen peroxide in rainbow trout have been published over the past 15 years [8–13]. Varying recommendations for the prophylactic use of hydrogen peroxide against external pathogens are given in the literature, since many aspects must be taken into consideration. These include the size of the fish, starting concentration of hydrogen peroxide, duration of treatment and the water temperature while treating. The aim of this study was to establish a reproducible method for immersion challenge of rainbow trout fry to be used in investigations concerning immune response and vaccine testing. First, various concentrations of H2O2 were tested before being combined with immersion exposure to the virulent F. psychrophilum isolate 950106-1/1, which was used for all infections. In conclusion, pre-treatment with H2O2 elevated the cumulative mortality followingimmersion challenge with F. psychrophilum, although variation between replicates was veryhigh. The results from the experiments are still being processed and are consequently notincluded. References[1] Nematollahi A., Decostere A., Pasmans F., Haesebrouck F., 2003. Flavobacteriumpsychrophilum infections in salmonid fish. Journal of Fish Diseases, 26:563–574[2] Jokumsen A., Svendsen L. M., 2010. Farming of freshwater rainbow trout inDenmark. National Institute of Aquatic Resources, Technical University of Denmark,Charlottenlund, 2010. [3] Decostere A., Lammens M., Haesebrouck F., 2000. Difficulties in experimentalinfection studies with Flavobacterium psychrophilum in rainbow trout (Oncorhynchusmykiss) using immersion, oral and anal challenges. Research in Veterinary Science,69:165–169 [4] Aoki M., Kondo M., Kawai K., Oshima S., 2005. Experimental bath infection withFlavobacterium psychrophilum, inducing typical signs of rainbow troutOncorhynchus mykiss fry syndrome. Diseases of Aquatic Organisms, 67:73–79 [5] Madsen L., Dalsgaard I., 1999. Reproducible methods for experimental infection withFlavobacterium psychrophilum in rainbow trout Oncorhynchus mykiss. Diseases ofAquatic Organisms, 36:169–176 [6] Avendaño-Herrera R., Magariños B., Irgang R., Toranzo A.E., 2006. Use of hydrogenperoxide against the fish pathogen Tenacibaculum maritimum and its effect oninfected turbot (Scophthalmus maximus). Aquaculture, 257:104–110 [7] Thomas-Jinu S., Goodwin A.E., 2004. 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