Transmission dynamics of Discocotyle sagittata (Monogenea) in farmed rainbow trout interpreted from parasite population age structure

Water temperature in the Isle of Man, Great Britain, is generally below 10 °C for half the year, from November to April. During 3 consecutive years, samples of rainbow trout, Oncorhynchus mykiss were taken at 2 fish farms in May (following 6 months < 10 °C) and in November (after a semester > 10 °C), to study the populations of the gill fluke Discocotyle sagittata. Four distinct types of parasite population structure were found, 2 in May, 2 in November. In the first May scenario, the majority of parasites found were adults, and almost no developing worms occurred: this indicates that no major transmission takes place during the cold season. In the second May scenario, large numbers of freshly-invaded larvae appeared alongside the established mature worms, indicating that intensive transmission can take place when permissive temperatures allow the mass hatching of eggs laid in winter/spring. The first pattern shown by sampling in November was characterised by the co-occurrence of all parasite developmental stages reflecting continuous transmission over several months. A second pattern of infection evident from November samples may indicate that despite recent, intense transmission, some hosts carrying relatively low burdens of adult parasites experienced little or no successful recruitment during preceding periods favourable for transmission. This may provide evidence of differences in susceptibility between hosts. Overall, the 4 contrasting patterns document the effects of temperature as a major factor shaping the population age structure of D. sagittata.