title: Disease resistance in Atlantic salmon (Salmo salar): coinfection of the intracellular bacterial pathogen Piscirickettsia salmonis and the sea louse Caligus rogercresseyi.
abstract: BACKGROUND
  Naturally occurring coinfections of pathogens have been reported in salmonids, but their consequences on disease resistance are unclear. We hypothesized that 1) coinfection of Caligus rogercresseyi reduces the resistance of Atlantic salmon to Piscirickettsia salmonis; and 2) coinfection resistance is a heritable trait that does not correlate with resistance to a single infection.
  
  
  METHODS
  In total, 1,634 pedigreed Atlantic salmon were exposed to a single infection (SI) of P. salmonis (primary pathogen) or coinfection with C. rogercresseyi (secondary pathogen). Low and high level of coinfection were evaluated (LC = 44 copepodites per fish; HC = 88 copepodites per fish). Survival and quantitative genetic analyses were performed to determine the resistance to the single infection and coinfections.
  
  
  RESULTS
  C. rogercresseyi significantly increased the mortality in fish infected with P. salmonis (SI mortality = 251/545; LC mortality = 544/544 and HC mortality = 545/545). Heritability estimates for resistance to P. salmonis were similar and of medium magnitude in all treatments (h2SI = 0.23 ± 0.07; h2LC = 0.17 ± 0.08; h2HC = 0.24 ± 0.07). A large and significant genetic correlation with regard to resistance was observed between coinfection treatments (rg LC-HC = 0.99 ± 0.01) but not between the single and coinfection treatments (rg SI-LC = -0.14 ± 0.33; rg SI-HC = 0.32 ± 0.34).
  
  
  CONCLUSIONS
  C. rogercresseyi, as a secondary pathogen, reduces the resistance of Atlantic salmon to the pathogen P. salmonis. Resistance to coinfection of Piscirickettsia salmonis and Caligus rogercresseyi in Atlantic salmon is a heritable trait. The absence of a genetic correlation between resistance to a single infection and resistance to coinfection indicates that different genes control these processes. Coinfection of different pathogens and resistance to coinfection needs to be considered in future research on salmon farming, selective breeding and conservation.
authors.affiliation: Aquainnovo S.A, Puerto Montt, Chile.
authors.forename: Jean Paul
authors.initials: JP
authors.lastname: Lhorente
authors.affiliation: Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile.
authors.forename: José A
authors.initials: JA
authors.lastname: Gallardo
authors.affiliation: Departamento de Mejora Genética Animal, INIA, Madrid, Spain.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Villanueva
authors.affiliation: Departamento de Mejora Genética Animal, INIA, Madrid, Spain.
authors.forename: María J
authors.initials: MJ
authors.lastname: Carabaño
authors.affiliation: Aquainnovo S.A, Puerto Montt, Chile; Departamento de Producción Animal, Facultad de Ciencias Agronómicas, Universidad de Chile, Santiago, Chile.
authors.forename: Roberto
authors.initials: R
authors.lastname: Neira
mesh_terms: D000818:Animals; D060085:Coinfection / Q000235:genetics / Q000276:immunology*; D033342:Copepoda / Q000502:physiology*; D060467:Disease Resistance; D005393:Fish Diseases / Q000235:genetics / Q000276:immunology*; D042541:Intracellular Space / Q000382:microbiology / Q000469:parasitology; D053522:Piscirickettsia / Q000502:physiology*; D020317:Salmo salar / Q000235:genetics / Q000276:immunology / Q000382:microbiology* / Q000469:parasitology*; D013312:Stress, Physiological / Q000276:immunology
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