Reservoir hosts for Gyrodactylus salaris may play a more significant role in epidemics than previously thought

Abstract

BackgroundGyrodactylus 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.MethodsPopulations 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.ResultsGyrodactylus 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.ConclusionsThe 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.

Predicting the Potential for Natural Recovery of Atlantic Salmon (Salmo salar L.) Populations following the Introduction of Gyrodactylus salaris Malmberg, 1957 (Monogenea)

“…Higher levels of virulence, common in G salaris [15,23,35,36], encourages the evolution of a stronger immune response.…”

“…salaris infections [10,28–30,37]. As highlighted earlier, Atlantic salmon populations in the UK are believed to be just as susceptible as those found in Norway [15,35], hence, if G . salaris was introduced a similar environmental impact to that of Norway can be expected.…”

“…The parasite is believed to be native to the waters of northern Russia, western Sweden and northern Finland [2], but is now known to be widely distributed throughout Europe [3–10] and recently confirmed in Romania [11]. In Norway, the parasite has caused catastrophic damage to wild populations of Atlantic salmon parr since it was first observed the mid-1970s after a period of mass salmon mortality [12–15]. Moreover, this parasite is known to have been introduced to Norway on at least three separate occasions [16] and can reduce salmon stock in rivers by approximately 85% on average [10].…”

“…salaris is not present in the UK but G . thymalli is, it has been demonstrated that UK salmon populations have similar levels of susceptibility to infection as those in Norway [15,23,35,36]. Due to this, G .…”

Are hybrids between Atlantic salmon and brown trout suitable long‐term hosts of Gyrodactylus salaris during winter?

“…; Paladini et al . ). Both multiannual and seasonal studies have shown that winter is the most critical period for G. salaris with regard to maintaining a metapopulation in a reservoir host such as Arctic charr, as prolonged periods with low temperatures challenge the transmission and/or reproduction rates of the parasite (Jansen & Bakke ; Winger et al .…”