Vaccination is considered crucial for disease prevention and fish health in the global salmon farming industry. Nevertheless, some aspects, such as the efficacy of vaccines, can be largely circumvented during natural coinfections. Sea lice are ectoparasitic copepods that can occur with a high prevalence in the field, are frequently found in co-infection with other pathogens, and are highly detrimental to fish health. The aim of this case-control study was to evaluate the interaction between the detrimental effects of coinfection and the protective effects of vaccination in fish. We used the interaction between the sea louse Caligus rogercresseyi, the bacterial pathogen Piscirickettsia salmonis, and their host, the Atlantic salmon Salmo salar, as a study model. Our results showed that coinfection decreased the accumulated survival (AS) and specific growth rate (SGR) of vaccinated fish (AS = 5.2 ± 0.6%; SGR = −0.05 ± 0.39%) compared to a single infection of P. salmonis (AS = 42.7 ± 1.3%; SGR = 0.21 ± 0.22%). Concomitantly, the bacterial load and clinical signs of disease were significantly increased in coinfected fish. Coinfection may explain the reduced efficacy of vaccines in sea cages and highlights the need to test fish vaccines in more diverse conditions rather than with a single infection.

No abstract

The genes of the major histocompatibility complex (MHC) are amongst the most variable in vertebrates and represent some of the best candidates to study processes of adaptive evolution. However, despite the number of studies available, most of the information on the structure and function of these genes come from studies in mammals and birds in which the MHC class I and II genes are tightly linked and class II alpha exhibits low variability in many cases. Teleost fishes are among the most primitive vertebrates with MHC and represent good organisms for the study of MHC evolution because their class I and class II loci are not physically linked, allowing for independent evolution of both classes of genes. We have compared the diversity and molecular mechanisms of evolution of classical MH class II alpha and class II beta loci in farm populations of three salmonid species: Oncorhynchus kisutch, Oncorhynchus mykiss and Salmo salar. We found single classical class II loci and high polymorphism at both class II alpha and beta genes in the three species. Mechanisms of evolution were common for both class II genes, with recombination and point mutation involved in generating diversity and positive selection acting on the peptide-binding residues. These results suggest that the maintenance of variability at the class IIalpha gene could be a mechanism to increase diversity in the MHC class II in salmonids in order to compensate for the expression of one single classical locus and to respond to a wider array of parasites.

Vaccination is a widely used control strategy to prevent Piscirickettsia salmonis causing disease in salmon farming. However, it is not known why all the currently available commercial vaccines generally fail to protect against this pathogenic bacteria. Here, we report, from two different populations, that between-family variation is a strong intrinsic factor that determines vaccine protection for this disease. While in some full-sib families, the protection added by vaccination increased the survival time in 13 days in comparison with their unvaccinated siblings; in other families, there was no added protection by vaccination or even it was slightly negative. Resistance to P. salmonis, measured as days to death, was higher in vaccinated than unvaccinated fish, but only a moderate positive genetic correlation was obtained between these traits. This disputes a previous hypothesis, that stated that both traits were fully controlled by the same genes, and challenges the use of unvaccinated fish as gold standard for evaluating and selecting fish resistant to P. salmonis, particularly if the offspring will be vaccinated. More studies are necessary to evaluate if variation in the host immune response to vaccination could explain the between-family differences in resistance observed in vaccinated fish.

ABSTRACT. Infectious pancreatic necrosis virus (IPNV) is the causal agent of a highly prevalent disease that affects salmonid fish, mostly during their fresh water life period. Like many other viruses, IPNV produces highly heterogeneous populations. Therefore, diagnostic methods need to be checked constantly so that no variants of the virus escape detection. The IPNV genome is composed of two double-stranded RNA segments: A and B, polymerase chain reaction (PCR) methods normally use segment A as a target. In order to develop an optimized protocol to diagnose IPNV, we present a real-time RT-PCR (reverse transcription) technique, using primers designed to recognize segment B of the virus. To validate the ubiquity of the primers used, the IPNV isolates tested were sequenced and compared with previously published cladograms, which include a wide spectrum of genogroups. These primers made it possible to detect viral isolates belonging to genogroups 1 and 5, which were obtained from different locations linked to fish farming. As expected, we were able to detect the virus from distant Aquabirnavirus genogroups. Keywords: infectious pancreatic necrosis virus (IPNV), real-time RT-PCR assay, VP1 gene, phylogenetic tree.Detección y cuantificación de cepas chilenas del virus de la necrosis pancreática infecciosa por medio de la técnica de RT-PCR en tiempo real usando el segmento B como objetivo RESUMEN. El virus de la necrosis pancreática infecciosa (IPNV) es el agente causal de una enfermedad altamente prevalente que afecta a peces salmónidos, principalmente durante su período de vida en agua dulce. IPNV, así como muchos otros virus, produce poblaciones altamente heterogéneas. Por lo tanto los métodos de diagnóstico necesitan ser constantemente revisados para evitar que ciertas variantes del virus escapen de la detección. El genoma del IPNV está compuesto por dos segmentos de ARN de doble hebra, A y B, los métodos de PCR (reacción en cadena de la polimerasa) normalmente usan el fragmento A como blanco. Con el propósito de generar un protocolo optimizado para el diagnóstico del IPNV, se presenta una técnica de RT-PCR (transcripción reversa-) en tiempo real, usando partidores diseñados para reconocer el segmento B del virus. Para validar la universalidad de los partidores utilizados, los aislados del IPNV probados fueron secuenciados y comparados con cladogramas previamente publicados, los cuales incluyen un amplio rango de genogrupos. Con estos partidores fue posible detectar aislados virales pertenecientes a los genogrupos 1 y 5, provenientes de distintas localidades relacionadas con el cultivo de peces. Como se esperaba, se logró detectar virus pertenecientes a genogrupos distantes de Aquabirnavirus. Palabras clave: virus de la necrosis pancreática infecciosa (IPNV), ensayo en tiempo real RT-PCR, gen VP1, árbol filogenético.