psychrophilum by qPCR. Data seem thus to suggest a high prevalence of the pathogen in 2009, with a regression in 2010, but this is most likely a consequence of the different sampling strategies adopted in the two GSK458 seasons. In 2009, in fact, we screened LY411575 in vivo only fish farms in Ticino where outbreaks of F. psychrophilum occurred, whereas in 2010 all Swiss fish farms under investigation were screened independently of any outbreaks diagnosis. We also used only 15 ml water samples, whereas increasing the sample volume may also increase the probability to detect

F. psychrophilum in environmental water samples. In addition, this was only a preliminary study to test the technique and its limits in natural field conditions: the study was neither planned nor powered to allow drawing any conclusions or making any interpretations about the disease distribution. Unfortunately little is known about the pathogen in its environment and about its mode of transmission. We suggest that F. psychrophilum could be present and replicate in the tank (in both, fish and organic layer) and diffuse in the water [37], where favourable ecological conditions would allow colonization/infection of other fishes. F. psychrophilum detection by qPCR in the spleen JIB04 order of diseased and symptomless fishes suggests that the pathogen may have already been present in the spleen of

symptomless fish at densities below QL but above LOD. Marancik and Wiens [25] report similar results using their qPCR, which detected the presence of F. psychrophilum in few symptomless

carriers that had been infected with the pathogen. In contrast, no infection was recorded prior to sampling of healthy-looking fishes in our study. Thus, F. psychrophilum is apparently able to colonize and live asymptomatically in the spleen, where it is inactive until favorable environment conditions and a weakening of the fish immune system allow this opportunistic pathogen to multiply, spread in the fish and eventually in the whole fish population. During outbreaks, fish spleen harbored higher amounts of the pathogen, at concentrations markedly higher than the QL. Healthy, colonized fish may thus act Erastin supplier as reservoirs for infection: in our opinion, this is a valid assumption, because another study has demonstrated the presence of this pathogen in eggs and ovarian fluids [38]. Further investigations, however, are needed to assess the mode of transmission and ecology of this species. qPCR detected and quantified F. psychrophilum in all 4 F. psychrophilum outbreaks investigated in this study; 13 of 15 qPCR values were higher than LOD, and in 8 cases higher than the QL. FISH could also detect all outbreaks, while culture methods could detect only 3 outbreaks and one was incorrectly recorded as negative. Changes in water temperature (e.g. a temperature variation of 4°C), oxygen availability in water, pH and conductibility could lead to a disease outbreak.