Long-term investigation of the ‘soft flesh’ condition in Northeast Atlantic mackerel induced by the myxosporean parasite Kudoa thyrsites (Cnidaria, Myxozoa). Temporal trends and new molecular epidemiological observations

Northeast Atlantic (NEA) mackerel (Scomber scombrus, Scombridae) represents an economically important target for the Norwegian pelagic fishing industry. Despite the commercial significance of NEA mackerel, little is known about the infections with the myxosporean parasite Kudoa thyrsites (Kudoidae). The parasite may cause post-mortem myoliquefaction of the fish skeletal muscle and therefore reduce the quality of the fish product. In this study, we examined 'soft flesh' occurrence in commercial size groups of NEA 'autumn mackerel' caught between 2007 and 2020, and investigated the prevalence and density of K. thyrsites (qPCR) and how they related to the occurrence of 'soft flesh'. The present study is the first long-term investigation of the occurrence of K. thyrsites-induced 'soft flesh' in NEA mackerel. After appearing stable for over a decade, the 'soft flesh' occurrence increased three-to six-fold in 2019 and 2020. This increase, together with the findings that 'soft flesh' seems primarily to affect the commercially most valuable mackerel size group (>400 g), may have important implications for the fishing industry and the fishery management. Molecular analysis (qPCR) suggests that the prevalence of K. thyrsites is substantially higher than 'soft flesh' occurrence. The majority (87.4%, n = 76/87) of infected mackerel did not develop 'soft flesh' and only individuals with high parasite density in the musculature (12.6%, n = 11/87) showed the condition. Therefore, qPCR analyses should be used for estimating the prevalence of K. thyrsites in fish. The method may also be used to assess the risk of NEA mackerel to develop post-mortem 'soft flesh'

Multilocus sequence typing (MLST) and Random Polymorphic DNA (RAPD) comparisons of geographic isolates of Neoparamoeba perurans, the causative agent of Amoebic Gill Disease

Neoparamoba perurans, is the aetiological agent of amoebic gill disease (AGD), a disease that affects farmed Atlantic salmon worldwide. Multilocus sequence typing (MLST) and Random Amplified Polymorphic DNA (RAPD) are PCR-based typing methods that allow for the highly reproducible genetic analysis of population structure within microbial species. To the best of our knowledge, this study represents the first use of these typing methods applied to N. perurans with the objective of distinguishing geographical isolates. These analyses were applied to a total of 16 isolates from Australia, Canada, Ireland, Scotland, Norway, and the USA. All the samples from Australia came from farm sites on the island state of Tasmania. Genetic polymorphism among isolates was more evident from the RAPD analysis compared to the MLST that used conserved housekeeping genes. Both techniques consistently identified that isolates of N. perurans from Tasmania, Australia were more similar to each other than to the isolates from other countries. While genetic differences were identified between geographical isolates, a BURST analysis provided no evidence of a founder genotype. This suggests that emerging outbreaks of AGD are not due to rapid translocation of this important salmonid pathogen from the same area

Ichthyobodo salmonis sp. n. (Ichthyobodonidae, Kinetoplastida), an euryhaline ectoparasite infecting Atlantic salmon (Salmo salar L.)

Phylogenetic analyses of SSU rDNA sequences have previously revealed the existence of 2 Ichthyobodo species able to infect Atlantic salmon (Salmo salar L.). Ichthyobodo necator sensu stricto (s.s.) is assumed to be a freshwater parasite, while a genetically distinct but undescribed species, Ichthyobodo sp. II sensu Todal et al. (2004) have been detected on Atlantic salmon in both fresh- and seawater. In the present study a morphological description of Ichthyobodo sp. II from the gills of salmon reared in fresh-, brackish- and seawater is presented, using both light- and electron microscopy. Comparative morphometry show that Ichthyobodo sp. II from both freshwater and seawater displays a different cell shape, and is significantly smaller than I. necator s.s. Also, ultrastructural characteristics distinguish these two species, notably differences in the attachment region and the presence of spine-like surface projections in Ichthyobodo sp. II. Based on both unique SSU rDNA sequences and morphological characteristics, we conclude that Ichthyobodo sp. II. represents a novel species for which we propose the name Ichthyobodo salmonis sp. n

The evolving story of catadromy in the European eel (Anguilla anguilla)

Anguillid eels were once considered to be the classic example of catadromy. However, alternative life cycles have been reported, including skipping the freshwater phase and habitat shifting between fresh, brackish, and saltwater throughout the growth phase. There is a lack of knowledge regarding these alternate life strategies, for example, the proportion of individuals in the population that adopt them compared to classic catadromy. We provide a description of these alternate life cycle strategies in temperate anguillids, their possible drivers, and the methods available to investigate them. These methods (lethal and non-lethal), include otolith microchemistry, fatty acid and stable isotope analyses, parasite identification, blood transcriptomics, and electronic tags. We argue that since the current management framework for the European eel and other temperate eels is based mainly on the freshwater component of the population, it ignores eels growing in saline waters. Many of the factors that are thought to be responsible for the precipitous decline of the eel population are more prevalent in freshwater systems. Therefore, the contribution of saline eels may be more important than currently estimated. The habitat-shifting ability of eels may be all the more crucial for the persistence and recovery of those species that are endangered

Phylogenetic position of the freshwater fish trypanosome, Trypanosoma ophiocephali (Kinetoplastida) inferred from the complete small subunit ribosomal RNA gene sequence

The complete small subunit rRNA (SSrRNA) gene sequence (2,142 nucleotides) of the freshwater fish trypanosome Trypanosoma ophiocephali Chen (1964) was determined. The phylogenetic analysis deduced using neighbor-joining, maximum parsimony, and Bayesian methods demonstrated the existence of an “aquatic clade”. T. ophiocephali was revealed to be a member of the freshwater fish trypanosomes and form the sister species with Trypanosoma siniperca and Trypanosoma sp. Carpio with high bootstrap values (98% MP, 100% NJ, 100% Bay). The high similarity of SSrRNA gene sequences and morphometric characters showed that T. ophiocephali, T. siniperca and T. sp. Carpio probably were the same species. The phylogenetic trees further suggested that Chinese freshwater fish trypanosome might be paraphyletic, and fish trypanosomes should have low host specificity

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

Source at https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2019/infeksjoner-med-parasitten-nucleospora-cyclopteri-microsporidia-i-rognkjeks-cyclopterus-lumpus.Nucleospora cyclopteri (Microsporidia) is one of many parasites infecting lumpfish, Cyclopterus lumpus, 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, Lepeophtherius salmonis, off the salmon. Knowledge on the different disease agents of lumpfish is therefore of utmost importance. 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 N. cyclopteri 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 N. cyclopteri 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. Unfortunately, we were not able to obtain a group of lumpfish fry infected with N. cyclopteri that we intended to follow through the land phase. The study of pathogenesis, infection dynamics, or whether an infection with N. cyclopteri pre-disposes for secondary infections, was therefore abandoned. We studied the presence of co-infections, methods for optimal sampling and tissue tropism in wild caught lumpfish in this project. Nucleospora cyclopteri 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 Kudoa islandica (Myxozoa) in the muscle tissue and coccidians in the intestine, were frequently found. Nucleospora cyclopteri 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 N. cyclopteri 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. We also show that N. cyclopteri 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 N. cyclopteri in lumpfish. Amongst these, the most promising non-lethal samples for detection were gill biopsies and leukocyte fractions from blood samples. 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. 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 N. cyclopteri to avoid using positive individuals for the production of eggs and fry. Given that N. cyclopteri 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