Ichthyobodo infections in Norwegian fish farms

Ectoparasitic flagellates of the genus Ichthyobodo (”Costia”) have been associated with disease (ichthyobodosis) and mortalities in fish farms worldwide. These flagellates reproduce by cell division and occur in two forms; one free-swimming infective form and one attached parasitic form. Heavy infections on skin or gills may cause severe damage to the epithelial layer and increase host susceptibility to secondary infections (e.g. bacteria, fungi). Ichthyobodosis is regarded as one of the most economically important parasitic diseases in Norwegian salmon farms. Ichthyobodo infections are also described as a problem to Norwegian farmed marine Atlantic species like cod, halibut and wolf-fish in hatcheries and net pens, often associated with gill inflammation. There is an increasing production of salmonids and marine fish in Norway, which require more effective prophylactic strategies and diagnostic methods to better prevent and control diseases. Comparative studies of molecular and morphological characters of the different Ichthyobodo species are taxonomically important but will also contribute to more accurate diagnoses. Knowledge of life cycle and the host range (natural reservoir hosts) are needed for the development of prophylactic strategies against the parasites. Recent molecular studies have revealed that the genus Ichthyobodo comprise several species, isolated from different fish hosts in fresh- and seawater. Two species have so far been described, namely Ichthyobodo necator sensu stricto isolated from salmonids and sticklebacks in freshwater and I. hippoglossi from farmed juvenile halibut. Different Ichthyobodo species have wrongly been identified as I. necator during time due to the old concept of one cosmopolitan Ichthyobodo species, and the epizootiology of I. necator sensu lato as described in literature is therefore confused. Several Ichthyobodo spp. have been identified that infect salmonids. For this reason, we studied the infection patterns of Ichthyobodo parasites in a salmon hatchery (W Norway) during an 11 month period, from eyed-eggs to smoltification. The results are in agreement with similar studies from salmonid farms in Europe, showing peaks of infection during first feeding in spring and a second peak among fingerlings and pre-smolt during summer and autumn. Wild fish, brown trout and sticklebacks, in the lakes were also found infected with Ichthyobodo; hence these could act as natural reservoir hosts of these parasites infecting salmon in the hatchery. PCR assays that distinguish Ichthyobodo necator sensu stricto from other known Ichthyobodo species were designed and used to confirm that all the infections on farmed and wild fish in this present study represent Ichthyobodo necator sensu stricto.