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

The viviparous monogenean Gyrodactylus salaris continues to devastate Norwegian Atlantic salmon populations despite the extreme measures taken to control this pathogen. Increased understanding of parasite biology is needed to develop alternative control and management strategies of wild Atlantic salmon. We have examined temperature-dependent survival of G. salaris, both on and off the host. At 18 degrees C, survival off the host was 1 day, but at 3 degrees C parasites survived for 4 days. However, in contrast to assumptions made by earlier authors, many parasites remained with their host following its death. Ultrastructural evidence indicated that G. salaris individuals can feed on a dead host, and laboratory tests demonstrated that worms on their hosts more than double their life-span compared with individuals maintained off the host. Experimental infections also demonstrated that establishment and subsequent population growth of parasites previously 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 important infection source in rivers and hatcheries.