The impact of helminth infection in fish farms on wild trout populations: host immune regulation of disease risk

Intensive rearing of farmed fish may risk disease spillover into free-living populations. This study concerns the blood-feeding gill monogenean of salmonids, <i>Discocotyle sagitatta</i>, on the Isle of Man, UK. Heavy infections in 2 fish farms have led to severe disease with periodic mass mortality. Infection levels in rainbow trout (<i>Oncorhynchus mykiss</i>, overall <i>n</i> = 556) increased with age (i.e. years exposed): by year 3, prevalence was 100%, mean intensity <i>c.</i> 100 (maximum 1150) worms/host. Output from farms of many millions of parasite eggs/day has the potential for transmission to downstream populations of free-living trout. Infections of <i>Discocotyle sagittata</i> were recorded in 132 brown trout and 49 sea trout (<i>Salmo trutta</i>) at 9 sites in rivers associated with or independent of the farms. Its occurrence in all 5 rivers studied confirmed that it is endemic on the Isle of Man irrespective of the farms. Wild brown and sea trout in rivers local to the farms (Rivers Corrany and Neb) had similar burdens to fish from independent drainage systems (Rivers Laxey, Santon and Sulby), and all burdens were within the range reported for other free-living populations in the distribution of <i>D. sagittata</i>. Low worm burdens in brown trout persisted even where these occurred in farm ponds contiguous with heavily infected rainbow trout. It had seemed predictable that high worm burdens in the farms would increase infection in downstream wild fish, but no elevation was detectable. Instead, this and other studies indicate that brown trout develop protective immunity despite intensive re-infection from rainbow trout, preventing pathogenic disease.