The protective effects of autogenous and commercial ERM immersion vaccines (bacterins based on Yersinia ruckeri, serotype O1, biotypes 1 and 2) for rainbow trout (Oncorhynchus mykiss) were compared in order to evaluate whether the use of local pathogen strains for immunization can improve protection. In addition, the effect of the bacterin concentration was established for the commercial product. Following sublethal challenge of vaccinated and non‐vaccinated control fish with live bacteria, we followed the bacterial count in the fish (gills, liver and spleen). The expression of genes encoding immune factors (IL‐1β, IL‐6, IL‐8, IL‐10, IFN‐γ, MHCI, MHCII, CD4, CD8, TCRβ, IgM, IgT, IgD, cathelicidins 1 and 2, SAA and C3) and densities of immune cells in organs were recorded. Both vaccines conferred protection as judged from the reduced bacterial load in exposed fish. Innate immune genes were upregulated in all groups following bacterial challenge but significantly more in non‐vaccinated naive fish in which densities of SAA‐positive immune cells increased. Immunoglobulin genes were upregulated on day 5 post‐challenge, and fish vaccinated with the high commercial bacterin dosage showed increased IgM levels by ELISA on day 14 post‐challenge, reflecting that the vaccine dosage was correlated to protection. In conclusion, both vaccine types offered protection to rainbow trout when exposed to live Y. ruckeri and no significant difference between commercial and autogenous vaccines was established.

The parasitic ciliate Ichthyophthirius multifiliis has a low host specificity eliciting white spot disease (WSD) in a wide range of freshwater fishes worldwide. The parasite multiplies rapidly whereby the infection may reach problematic levels in a host population within a few days. The parasite targets both wild and cultured fish but the huge economic impact of the protozoan is associated with mortality, morbidity and treatment in aquacultural enterprises. We have investigated the potential for genetic selection of WSD‐resistant strains of rainbow trout. Applying the DNA typing system Affymetrix^® and characterizing the genome of the individual fish by use of 57,501 single nucleotide polymorphisms (SNP) and their location on the rainbow trout chromosomes, we have genetically characterized rainbow trout with different levels of natural resistance towards WSD. Quantitative trait loci (QTL) used for the selection of breeders with specific markers for resistance are reported. We found a significant association between resistance towards I. multifiliis infection and SNP markers located on the two specific rainbow trout chromosomes Omy 16 and Omy 17. Comparing the expression of immune‐related genes in fish—with and without clinical signs—we recorded no significant difference. However, trout surviving the infection showed high expression levels of genes encoding IgT, T‐cell receptor TCRβ, C3, cathelicidins 1 and 2 and SAA, suggesting these genes to be associated with protection.

Control of the skin parasitic ciliate Ichthyophthirius multifiliis is currently based on laborious chemical and mechanical approaches advocating for alternative control measures. We here show that the early development of trophonts (feeding stage residing in the epidermis of the fish) in rainbow trout Oncorhynchus mykiss epidermis is inhibited 5 days post‐infection (dpi) following the administering of feed additives including garlic (Allium sativum), oregano (Origanum onites), thyme (Thymus vulgaris), coriander (Coriandrum sativum) and astaxanthin (a carotenoid derived from the algae Haematococcus pluvalis). However, no inhibition was observed 8 dpi. We investigated if the early inhibition was caused by 1) direct effect on the parasite of the feed additives and/or 2) stimulation of trout immune responses. Garlic exhibited the highest in vitro killing capacity towards theronts (the infective stage of the parasite) followed by oregano, thyme and astaxanthin whereas coriander had no in vitro parasiticidal effect. Immune reactions were measured by plasma lysozyme activity in trout following feeding and by recording immune gene expression in trout leukocytes exposed to feed additives. Oregano fed fish showed a significantly (P = 0.0045, P = 0.0405) elevated plasma lysozyme activity. Especially oregano and thyme and to a lower degree garlic, astaxanthin and coriander, induced a significant regulation of innate and adaptive immune genes in vitro. The results suggest that the investigated additives may inhibit early parasite development both directly and indirectly by elevating the host immune response.

Vibriosis is a bacterial disease in fish caused by the Gram negative bacterium Vibrio anguillarum with severe impact on rainbow trout (Oncorhynchus mykiss) farming. Sustainable control methods should be developed and we here show that marker assisted selective breeding of fish naturally resistant to the disease is feasible. We have validated the use of a single nucleotide polymorphism (SNP) marker SNP AX-89,945,921 (QTL on chromosome 21). The QTL was previously found associated with resistance to vibriosis and described following a genome wide association analysis (GWAS) of trout exposed to the bacterium. For this validation spawners were genotyped by use of the 57 K Axiom®Trout Microarray (Affymetrix) and homozygous male fish carrying the allele with the SNP AX-89,945,921 were then selected and used to fertilize eggs from outbred female trout resulting in fish all carrying the SNP (QTL-fish). Control fish (non-QTL fish) were produced by fertilizing the same batch of eggs by use of male parents negative for the SNP. The fish were exposed in freshwater to V. anguillarum (water bath infection) at 19 C°. A total of 900 fish were challenged in a common garden set-up in triplicate. A bacterial solution of V. anguillarum (serotype O1) was added to each of three freshwater fish tanks, each with 150 QTL and 150 non-QTL fish. Fish were tagged by tail fin cut (upper/lower) to discern the two groups, whereafter fish were monitored around the clock to detect disease signs and remove moribund fish. Clinical vibriosis developed within two days in non-QTL-fish (overall morbidity of 70%). QTL fish developed clinical signs later and the morbidity was significantly lower and did not reach 50%. Rainbow trout farming may benefit from using the QTL associated with higher resistance towards vibriosis. The effect may be optimized in the future by use of both male and female parents homozygous for the marker allele.