Ichthyobodo infections on farmed and wild fish. Methods for detection and identification of Ichthyobodo spp.

Abstract

Ichthyobodosis is an important parasitic disease that has caused severe loss among ornamental and farmed fish world wide for more than a century. The disease is caused by heavy infections on skin and gills by parasitic flagellates belonging to the genus Ichthyobodo. In the past, infections worldwide have commonly been identified as due to a single variable species, I. necator. However, recent molecular studies have revealed that the genus Ichthyobodo consist of several different species and is far more complex than previously believed. The overall aim of this PhD project has been to identify and characterise Ichthyobodo species with emphasis on those that are known from Norwegian aquaculture. Therefore, effective and sensitive molecular methods (PCR techniques) for detection and identification of Ichthyobodo spp. have been developed and validated. With the aid of such methods several new Ichthyobodo genotypes have been detected from both farmed and wild fish. Working with molecularly identified Ichthyobodo genotypes has led to improved knowledge of character variations in the genus. Also, novel morphological characters have been identified, aiding species discrimination. Hence both molecular and morphological tools are provided that may facilitate the future description of other Ichthyobodo spp. Morphological descriptions of three Ichthyobodo species is presented, so far the only valid species within genus Ichthyobodo that have also been characterized through their DNA sequences. All presently valid Ichthyobodo species have been detected on both farmed and wild caught hosts in Norway: 1. Ichthyobodo necator; until recently the only species in the genus, is redescribed. It has been detected on Atlantic salmon, brown trout, rainbow trout and three-spined sticklebacks in freshwater. 2. Ichthyobodo salmonis; an euryhaline species able to infect Atlantic salmon in both freshwater and seawater. 3. Ichthyobodo hippoglossi; a marine species infecting Atlantic halibut.