Unpredicted transmission strategy of Gyrodactylus salaris (Monogenea: Gyrodactylidae) : survival and infectivity of parasites on dead hosts

SUMMARY The viviparous monogenean Gyrodactylus salaris continues to devastate Norwegian Atlantic salmon populations despitethe extreme measures taken to control this pathogen. Increased understanding of parasite biology is needed to developalternativecontrolandmanagementstrategiesofwildAtlanticsalmon.Wehaveexaminedtemperature-dependentsurvivalof G. salaris, both on and off the host. At 18 xC, survival off the host was 1 day, but at 3 xC parasites survived for 4 days.However, in contrast to assumptions made by earlier authors, many parasites remained with their host following itsdeath. Ultrastructural evidence indicated that G. salaris individuals can feed on a dead host, and laboratory testsdemonstrated that worms on their hosts more than double their life-span compared with individuals maintained offthe host. Experimental infections also demonstrated that establishment and subsequent population growth of parasitespreviously maintained on dead hosts for 3 days, was similar to that of parasites transferred directly between living hosts.Hence, for G. salaris, dead infected hosts may increase the chances of successful transmission and be a potential importantinfection source in rivers and hatcheries.Key words: Atlantic salmon, Salmo salar, parasite control, risk assessment, disease management.