Infeksjoner med parasitten Nucleospora cyclopteri (Microsporidia) i rognkjeks, Cyclopterus lumpus

Abstract

<p><i>Nucleospora cyclopteri</i> (Microsporidia) is one of many parasites infecting lumpfish, <i>Cyclopterus lumpus</i>, and has been shown to cause disease and mortality in lumpfish. Infections in fish are often multifactorial and the impact of one agent on the development of disease can be difficult to elucidate. In addition to mortality, infections in lumpfish can lead to diseases with subsequently lowered appetite. This is of particular importance since lumpfish are used as a biological control agent, eating salmon lice, <i>Lepeophtherius salmonis</i>, off the salmon. Knowledge on the different disease agents of lumpfish is therefore of utmost importance. <p>The main aim of this project was to identify how to obtain an infection-free lumpfish in land-based hatcheries and to study the impact that <i>N. cyclopteri</i> has on the health of the lumpfish and thereby its effect as a biological control agent. The project therefore aimed to map the presence of <i>N. cyclopteri</i> and other disease agents in wild caught lumpfish and in eggs/sperm, in fry and in farmed lumpfish stocked in the sea. In addition, we wanted to study the transmission pathways and clinical significance of the parasite. <p>Unfortunately, we were not able to obtain a group of lumpfish fry infected with <i>N. cyclopteri</i> that we intended to follow through the land phase. The study of pathogenesis, infection dynamics, or whether an infection with <i>N. cyclopteri</i> pre-disposes for secondary infections, was therefore abandoned. <p>We studied the presence of co-infections, methods for optimal sampling and tissue tropism in wild caught lumpfish in this project. <i>Nucleospora cyclopteri</i> was present in 60% of the sampled individuals from the waters around Averøy, in county Møre og Romsdal. The fish were analysed with regard to a range of infectious agents (viruses, bacteria and parasites) commonly found in other fish species, or previously recorded in lumpfish. No viral agents or other important pathogens were detected, but supposedly nonpathogenic microparasites, like <i>Kudoa islandica</i> (<i>Myxozoa</i>) in the muscle tissue and coccidians in the intestine, were frequently found. <p><i>Nucleospora cyclopteri</i> was detected in all tissues examined: anterior, mid and posterior kidney, spleen, heart, gills, brain, muscle liver and blood, thus indicating that the infection is systemic. The density of <i>N. cyclopteri</i> was highest in the anterior kidney, followed by mid and posterior kidney, spleen and gills, while the prevalence was highest in the ventricle of the heart. Observations from this study indicate that the parasite is released through urine and bile. <p>We also show that <i>N. cyclopteri</i> can be detected using swabs from the skin, gill and vent, and by blood samples and gill biopsies, thus demonstrating the possibility of non-lethal detection of <i>N. cyclopteri</i> in lumpfish. Amongst these, the most promising non-lethal samples for detection were gill biopsies and leukocyte fractions from blood samples. <p>Images normal histology and pathological agents from this project is included in an openly available online image database. This image database can be accessed by diagnosticians and researchers and used when evaluating pathological findings in lumpfish. <p>While vertical transmission cannot be excluded, the results from this project indicate that this is not the dominant route. It is in any case advisable to routinely screen broodfish for <i>N. cyclopteri</i> to avoid using positive individuals for the production of eggs and fry. <p>Given that <i>N. cyclopteri</i> undoubtedly destroys leukocytes in high numbers and over large areas of tissue, it is reasonable to assume that the parasite has an effect on the immune competence of the fish.