title: Reservoir hosts for Gyrodactylus salaris may play a more significant role in epidemics than previously thought.
abstract: BACKGROUND
  Gyrodactylus salaris Malmberg, 1957 has had a devastating impact on wild Norwegian stocks of Atlantic salmon Salmo salar L., and it is the only Office International des Epizooties (OIE) listed parasitic pathogen of fish. The UK is presently recognised as G. salaris-free, and management plans for its containment and control are currently based on Scandinavian studies. The current study investigates the susceptibility of British salmonids to G. salaris, and determines whether, given the host isolation since the last glaciation and potential genetic differences, the populations under test would exhibit different levels of susceptibility, as illustrated by the parasite infection trajectory over time, from their Scandinavian counterparts.
  
  
  METHODS
  Populations of S. salar, brown trout Salmo trutta L., and grayling Thymallus thymallus (L.), raised from wild stock in UK government hatcheries, were flown to Norway and experimentally challenged with a known pathogenic strain of G. salaris. Each fish was lightly anaesthetised and marked with a unique tattoo for individual parasite counting. A single Norwegian population of S. salar from the River Lærdalselva was used as a control. Parasite numbers were assessed every seven days until day 48 and then every 14 days.
  
  
  RESULTS
  Gyrodactylus salaris regularly leads to high mortalities on infected juveniles S. salar. The number of G. salaris on British S. salar rose exponentially until the experiment was terminated at 33 days due to fish welfare concerns. The numbers of parasites on S. trutta and T. thymallus increased sharply, reaching a peak of infection on days 12 and 19 post-infection respectively, before declining to a constant low level of infection until the termination of the experiment at 110 days.
  
  
  CONCLUSIONS
  The ability of S. trutta and T. thymallus to carry an infection for long periods increases the window of exposure for these two hosts and the potential transfer of G. salaris to other susceptible hosts. This study demonstrates that G. salaris can persist on S. trutta for longer periods than previously thought, and that the role that S. trutta could play in disseminating G. salaris needs to be considered carefully and factored into management plans and epidemics across Europe.
authors.affiliation: Institute of Aquaculture, School of Natural Sciences, University of Stirling, Stirling, FK9 4LA, Scotland, UK. paladini2000@yahoo.it.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Paladini
authors.affiliation: Norwegian Veterinary Institute, Section for Parasitology, P.O. Box 750, Sentrum, NO-0106, Oslo, Norway. haakon.hansen@vetinst.no.
authors.forename: Haakon
authors.initials: H
authors.lastname: Hansen
authors.affiliation: Environment Agency, National Fisheries Laboratory, Bromholme Lane, Brampton, PE28 4NE, UK. chris.williams@environment-agency.gov.uk.
authors.forename: Chris F
authors.initials: CF
authors.lastname: Williams
authors.affiliation: Centre for Environment, Fisheries & Aquaculture (Cefas), Weymouth Laboratory, Barrack Road, Weymouth, DT4 8UB, UK. nick.taylor@cefas.co.uk.
authors.forename: Nick G H
authors.initials: NG
authors.lastname: Taylor
authors.affiliation: Institute of Aquaculture, School of Natural Sciences, University of Stirling, Stirling, FK9 4LA, Scotland, UK. rubiomo@hotmail.com.
authors.forename: Olga L
authors.initials: OL
authors.lastname: Rubio-Mejía
authors.affiliation: Animal Breeding and Genomics, Animal & Veterinary Sciences, SRUC, Roslin Institute Building, Easter Bush, Midlothian, EH25 9RG, UK. scott.denholm@sruc.ac.uk.
authors.forename: Scott J
authors.initials: SJ
authors.lastname: Denholm
authors.affiliation: Institute of Aquaculture, School of Natural Sciences, University of Stirling, Stirling, FK9 4LA, Scotland, UK. sigurd.hytterod@vetinst.no.
authors.forename: Sigurd
authors.initials: S
authors.lastname: Hytterød
authors.affiliation: Institute of Aquaculture, School of Natural Sciences, University of Stirling, Stirling, FK9 4LA, Scotland, UK. jeb1@stir.ac.uk.
authors.forename: James E
authors.initials: JE
authors.lastname: Bron
authors.affiliation: Institute of Aquaculture, School of Natural Sciences, University of Stirling, Stirling, FK9 4LA, Scotland, UK. andy.shinn@fishvetgroup.com.|Fish Vet Group Asia Ltd., 99/386, Chaengwattana Building, Chaengwattana Rd., Kwaeng Toongsonghong, Khet Laksi, Bangkok, 10210, Thailand. andy.shinn@fishvetgroup.com.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Shinn
mesh_terms: D000818:Animals; D002590:Cestode Infections / Q000469:parasitology / Q000473:pathology / Q000662:veterinary*; D004198:Disease Susceptibility; D005060:Europe; D005393:Fish Diseases / Q000469:parasitology* / Q000473:pathology; D010986:Platyhelminths / Q000502:physiology*; D020317:Salmo salar / Q000469:parasitology*; D006113:United Kingdom