Temperature-related parasite infection dynamics: the case of proliferative kidney disease of brown trout

The Escalating threat of climate change-driven diseases in fish: Evidence from a global perspective - A literature review

Climate change has brought significant alterations to the aquatic environment, leading to the rapid spread of infectious fish diseases with increasing water temperatures. It is crucial to understand how aquatic pathogens will impact fish in the context of climate change. This study aimed to assess the effects of climate change on fish diseases globally. Data from 104 papers published between 2003 and 2022 were analyzed to identify recent trends in the field. The majority of the studies (54%) focused on parasites, particularly proliferative kidney disease, while 22% examined bacteria. The United States accounted for 19% of the studies, followed by Canada at 14%, covering a wide range of fish species. More research was published on farmed fish (54%) than wild fish (30%), with a higher emphasis on freshwater species (62%) compared to marine species (34%). Most published studies (64%) focused on the local environment rather than the farm level (7%). The findings highlight temperature as a significant threat to global aquaculture and fisheries, impacting the progression of fish diseases. These impacts could be exacerbated by factors such as pH, salinity, and ocean acidification, posing challenges to fish health. Therefore, there is a pressing need for enhanced research and management strategies to address these issues effectively in the future.

When does a parasite become a disease? eDNA unravels complex host-pathogen dynamics across environmental stress gradients in wild salmonid populations

Infectious diseases stem from disrupted interactions among hosts, parasites, and the environment. Both abiotic and biotic factors can influence infection outcomes by shaping the abundance of a parasite's infective stages, as well as the host's ability to fight infection. However, disentangling these mechanisms within natural ecosystems remains challenging. Here, combining environmental DNA analysis and niche modelling at a regional scale, we uncovered the biotic and abiotic drivers of an infectious disease of salmonid fish, triggered by the parasite Tetracapsuloides bryosalmonae. We found that the occurrence and abundance of the parasite in the water-i.e., the propagule pressure- were mainly correlated to the abundances of its two primary hosts, the bryozoan Fredericella sultana and the fish Salmo trutta, but poorly to local abiotic environmental stressors. In contrast, the occurrence and abundance of parasites within fish hosts-i.e., proxies for disease emergence-were closely linked to environmental stressors (water temperature, agricultural activities, dams), and to a lesser extent to parasite propagule pressure. These results suggest that pathogen distribution alone cannot predict the risk of disease in wildlife, and that local anthropogenic stressors may play a pivotal role in disease emergence among wild host populations, likely by modulating the hosts' immune response. Our study sheds light on the intricate interplay between biotic and abiotic factors in shaping pathogen distribution and raises concerns about the effects of global change on pathogen emergence.

Thermotactic behaviour in lacustrine and riverine forms of Salmo trutta and its relevance to an emerging parasitic disease (PKD) in the wake of climate change

The thermotactic response of brown trout (Salmo trutta) was examined with the goal to investigate potential effects of the emerging temperature-dependent fatal trout disease PKD (proliferative kidney disease). First the differences in cold-water preferences of two forms of brown trout, lacustrine (migratory) and riverine, were determined. Second, it was studied whether this preference was changed in fish infected with PKD. The experiment involved a one-week habituation period at 14 °C in a two-chamber runway followed by a week of 3 °C temperature difference between the two runways. The fish could freely move between lanes via an opening at the end where food was provided. The temperature manipulation was repeated twice, and there were 3 trials per experimental group. All fish developed a clear spatial preference in the test. Lacustrine trout demonstrated a preference for warmer water, while riverine trout preferred cooler water. This may increase the risk to PKD in the lacustrine form. Most strikingly, riverine trout experimentally exposed to Tetracapsuloides bryosalmonae, the parasite that causes PKD, demonstrated stronger cold-seeking behaviour than control fish. Cold seeking behaviour suggests the occurrence of a disease-induced behavioural chill response, which may play an important role in disease recovery. This demonstrates the significance of protecting river connectivity and cold-water sanctuaries as management strategies for preserving salmonid populations in a warming climate.

How does parasite environmental transmission stage concentration change before, during, and after disease outbreaks?

Outbreaks of environmentally transmitted parasites require that susceptible hosts encounter transmission stages in the environment and become infected, but we also know that transmission stages can be in the environment without triggering disease outbreaks. One challenge in understanding the relationship between environmental transmission stages and disease outbreaks is that the distribution and abundance of transmission stages outside of their hosts have been difficult to quantify. Thus, we have limited data about how changes in transmission stage abundance influence disease dynamics; moreover, we do not know whether the relationship between transmission stages and outbreaks differs among parasite species. We used digital PCR to quantify the environmental transmission stages of five parasites in six lakes in southeastern Michigan every 2 weeks from June to November 2021. At the same time, we quantified infection prevalence in hosts and host density. Our study focused on eight zooplankton host species (Daphnia spp. and Ceriodaphnia dubia) and five of their parasites from diverse taxonomic groups (bacteria, yeast, microsporidia, and oomycete) with different infection mechanisms. We found that parasite transmission stage concentration increased prior to disease outbreaks for all parasites. However, parasites differed significantly in the relative timing of peaks in transmission stage concentration and infection outbreaks. The "continuous shedder" parasites had transmission stage peaks at the same time as or slightly after the outbreak peaks. In contrast, parasites relying on host death for transmission ("obligate killers") had transmission stage peaks before outbreak peaks. For most parasites, lakes with outbreaks had higher spore concentrations than those without outbreaks, especially once an outbreak began; the exception was for a parasite, Pasteuria ramosa, with very strong genotypic specificity of infection. Overall, our results show that disease outbreaks are tightly linked to transmission stage concentration; outbreaks were preceded by increases in transmission stage concentration in the environment and then were fueled by the production of more transmission stages during the outbreak itself, with concentrations decreasing to pre-outbreak levels as outbreaks waned. Thus, tracking transmission stages in the environment improves our understanding of the drivers of disease outbreaks and reveals how parasite traits may affect these dynamics.

Environmental DNA/RNA for pathogen and parasite detection, surveillance, and ecology

Detection of pathogens, parasites, and other symbionts in environmental samples via eDNA/eRNA (collectively eNA) is an increasingly important source of information about their occurrence and activity. There is great potential for using such detections as a proxy for infection of host organisms in connected habitats, for pathogen monitoring and surveillance, and for early warning systems for disease. However, many factors require consideration, and appropriate methods developed and verified, in order that eNA detections can be reliably interpreted and adopted for surveillance and assessment of disease risk, and potentially inclusion in international standards, such as the World Organisation for Animal Health guidelines. Disease manifestation results from host-symbiont-environment interactions between hosts, demanding a multifactorial approach to interpretation of eNA signals.

Ontogenetic and spatial variability in parasite communities of white shrimp Penaeus setiferus (Decapoda: Penaeidae)

Understanding the combined effects of multi-parasite infections on their hosts is necessary for documenting parasite impacts and is particularly important for developing effective management strategies for economically important organisms. The white shrimp Penaeus setiferus supports important recreational and commercial fisheries along the southeastern and Gulf coasts of the United States and occupies an important ecological niche in estuarine and offshore habitats throughout these regions. The goal of this study was to identify and assess ontogenetic and spatial variation in white shrimp parasite communities and their relation to shrimp health. We used a series of trawl surveys in tidal creek and open water habitats of an estuary in the southeastern USA to collect and identify parasites of white shrimp using morphological and DNA sequencing techniques. Parasite communities in white shrimp were composed of organisms belonging to 6 classes: Conoidasida (gregarines), Oligohymenophorea (apostome and sessilid ciliates), Microsporea (meiodihaplophasids), Chromadorea (rhabditids), Cestoda (cyclophyllideans, lecanocephalideans and trypanorhynchs) and Trematoda (plagiorchiids). Parasite communities differed significantly among white shrimp life stages and localities. Furthermore, the health condition known as black gill occurred in some shrimp and was significantly related to parasite community structure. Infection metrics for the apostome ciliate Hyalophysa lynni, the trypanorhynch larvae Prochristianella sp. and the rhabditid larvae Hysterothylacium sp. were significantly different between shrimp exhibiting and not exhibiting black gill. These results highlight the importance of understanding parasite communities and the potential interactive effects of multiple parasite infections on shrimp health.

Distribution and prevalence of the myxozoan parasite Tetracapsuloides bryosalmonae in northernmost Europe: analysis of three salmonid species

Global climate change is altering the abundance and spread of many aquatic parasites and pathogens. Proliferative kidney disease (PKD) of salmonids caused by the myxozoan Tetracapsuloides bryosalmonae is one such emerging disorder, and its impact is expected to increase with rising water temperature. Yet, the distribution and prevalence of T. bryosalmonae in Northern Europe remain poorly characterized. Here, we studied 43 locations in 27 rivers in northernmost Norway and Finland to describe T. bryosalmonae infection frequency and patterns in 1389 juvenile salmonids. T. bryosalmonae was discovered in 12 out of 27 rivers (44%) and prevalence ranged from 4.2 to 55.5% in Atlantic salmon and from 5.8 to 75% in brown trout among infected rivers. In sympatric populations, brown trout was more frequently infected with T. bryosalmonae than was salmon. Age-specific parasite prevalence patterns revealed that in contrast to lower latitudes, the infection of juvenile fish predominantly occurs during the second summer or later. Temperature monitoring over 2 yr indicated that the mean water temperature in June was 2.1 to 3.2°C higher in rivers containing T. bryosalmonae compared to parasite-free rivers, confirming the important role of temperature in parasite occurrence. Temporal comparison in T. bryosalmonae prevalence over a 10 yr period in 11 rivers did not reveal any signs of contemporary parasite spread to previously uninfected rivers. However, the wide distribution of T. bryosalmonae in rivers flowing to the Barents Sea indicates that climate change and heat waves may cause new disease outbreaks in northern regions.

Reliable Field Assessment of Proliferative Kidney Disease in Wild Brown Trout, Salmo trutta, Populations: When Is the Optimal Sampling Period?

Proliferative kidney disease (PKD), caused by the myxozoan parasite Tetracapsuloides bryosalmonae, is suspected to contribute to the decline of wild brown trout Salmo trutta populations. Different factors need to be taken into consideration for PKD outbreaks. Among them, water temperature appears as a main driver of the disease. To understand the epidemiology and impact of the disease on wild fish populations, reliable sampling approaches to detect the presence of T. bryosalmonae-infected fish are needed. This study aimed to characterize the seasonal variation of the prevalence of T. bryosalmonae-infected fish in brown trout populations in two small streams with differing temperature regimes between upstream and downstream sites. As water temperature is known to influence PKD manifestation in brown trout, we hypothesized that the number of T. bryosalmonae-positive fish, as well as their seasonal distribution, will vary between upper and downstream parts of the two streams. Since, in field studies, results can strongly vary across years, we extended the study over a 3-year-period. The number of infected fish and the intensity of infection were assessed by histology. The results confirmed the hypothesis of pronounced temporal- and site-related differences in the percentage of PKD-positive fish and the intensity of the infection. Comparison of water temperatures (total degree days as well as the number of days with a daily mean temperature ≥15 °C) with PKD data indicated that temperature was the driving factor for the temporal development and the intensity of the infection. A mean of 1500 degree days or 30 days with a daily mean temperature ≥15 °C was required before the infection could be detected histologically. From our findings, recommendations are derived for a water temperature-driven sampling strategy campaigns that enables the detection of PKD infection and prevalence in wild brown trout populations.

Mitigating human impacts including climate change on proliferative kidney disease in salmonids of running waters

Over the last two decades, an increasing number of reports have identified a decline in salmonid populations, possibly linked to infection with the parasite Tetracapsuloides bryosalmonae and the corresponding disease, that is, proliferative kidney disease (PKD). The life cycle of this myxozoan parasite includes sessile bryozoan species as invertebrate host, which facilitates the distribution of the parasite in running waters. As the disease outcome is temperature dependent, the impact of the disease on salmonid populations is increasing with global warming due to climate change. The goal of this review is to provide a detailed overview of measures to mitigate the effects of PKD on salmonid populations. It first summarizes the parasite life cycle, temperature-driven disease dynamics and new immunological and molecular research into disease resistance and, based on this, discusses management possibilities. Sophisticated management actions focusing on local adaptation of salmonid populations, restoration of the riverine ecosystem and keeping water temperatures cool are necessary to reduce the negative effects of PKD. Such actions include temporary stocking with PKD-resistant salmonids, as this may assist in conserving current populations that fail to reproduce.

Proteases as Therapeutic Targets Against the Parasitic Cnidarian Ceratonova shasta: Characterization of Molecules Key to Parasite Virulence In Salmonid Hosts

Proteases and their inhibitors play critical roles in host-parasite interactions and in the outcomes of infections. Ceratonova shasta is a myxozoan pathogen that causes enteronecrosis in economically important salmonids from the Pacific Northwest of North America. This cnidarian parasite has host-specific genotypes with varying virulence, making it a powerful system to decipher virulence mechanisms in myxozoans. Using C. shasta genome and transcriptome, we identified four proteases of different catalytic types: cathepsin D (aspartic), cathepsin L and Z-like (cysteine) and aminopeptidase-N (metallo); and a stefin (cysteine protease inhibitor), which implied involvement in virulence and hence represent target molecules for the development of therapeutic strategies. We characterized, annotated and modelled their 3D protein structure using bioinformatics and computational tools. We quantified their expression in C. shasta genotype 0 (low virulence, no mortality) and IIR (high virulence and mortality) in rainbow trout Oncorhynchus mykiss, to demonstrate that there are major differences between the genotypes during infection and parasite development. High proliferation of genotype IIR was associated with high expression of the cathepsin D and the stefin, likely correlated with high nutrient demands and to regulate cell metabolism, with upregulation preceding massive proliferation and systemic dispersion. In contrast, upregulation of the cathepsin L and Z-like cysteine proteases may have roles in host immune evasion in genotype 0 infections, which are associated with low proliferation, low inflammation and non-destructive development. In contrast to the other proteases, C. shasta aminopeptidase-N appears to have a prominent role in nematocyst formation in both genotypes, but only during sporogenesis. Homology searches of C. shasta proteases against other myxozoan transcriptomes revealed a high abundance of cathepsin L and aminopeptidase homologs suggesting common gene requirements across species. Our study identified molecules of potential therapeutic significance for aquaculture and serves as a baseline for future research aimed at functional characterisation of these targets.

Patterns of Tetracapsuloides bryosalmonae infection of three salmonid species in large, deep Norwegian lakes

Proliferative kidney disease (PKD), caused by the myxozoan endoparasite Tetracapsuloides bryosalmonae, is of serious ecological and economical concern to wild and farmed salmonids. Wild salmonid populations have declined due to PKD, primarily in rivers, in Europe and North America. Deep lakes are also important habitats for salmonids, and this work aimed to investigate parasite presence in five deep Norwegian lakes. Kidney samples from three salmonid species from deep lakes were collected and tested using real-time PCR to detect PKD parasite presence. We present the first detection of T. bryosalmonae in European whitefish in Norway for the first time, as well as the first published documentation of the parasite in kidneys of Arctic charr, brown trout and whitefish in four lakes. The observed prevalence of the parasite was higher in populations of brown trout than of Arctic charr and whitefish. The parasite was detected in farmed, but not in wild, charr in one lake. This suggests a possible link with a depth of fish habitat and fewer T. bryosalmonae-infected and PKD-affected fish. Towards a warmer climate, cold hypolimnion in deep lakes may act as a refuge for wild salmonids, while cold deep water may be used to control PKD in farmed salmonids.

Effect of water temperature on the morbidity of Tetracapsuloides bryosalmonae (Myxozoa) to brown trout (Salmo trutta) under laboratory conditions

Proliferative kidney disease (PKD) is a disease found in salmonid fish that is widespread in Europe and North America. The dependency of the clinical signs on the water temperature is extensively reported in rainbow trout, but detailed information on brown trout is lacking. In this study, juvenile brown trout were exposed to the spores of Tetracapsuloides bryosalmonae and then kept at different ambient water temperatures (16°C, 19°C and 22°C) for 10 weeks along with recording of morbidity throughout the experiment. At 6, 8 and 10 weeks post-exposure, fish from each temperature group were sampled and underwent pathoanatomical examination to survey disease progression. At 16°C, brown trout showed a significantly higher survival probability compared to those kept in 19°C and 22°C water. Additionally, the parasitic burden (MSQ) was higher and the clinical signs were more pronounced in the brown trout kept at 19°C and 22°C compared with the ones kept at 16°C. This study highlights the correlation of PKD outbreaks and water temperature increases related to global climate change, which will impact the future distribution of brown trout in natural waters.

It's a hard knock life for some: Heterogeneity in infection life history of salmonids influences parasite disease outcomes

Heterogeneity in immunity occurs across numerous disease systems with individuals from the same population having diverse disease outcomes. Proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae, is a persistent parasitic disease negatively impacting both wild and farmed salmonids. Little is known of how PKD is spread or maintained within wild susceptible populations. We investigated an aspect of fish disease that has been largely overlooked, that is, the role of the host phenotypic heterogeneity in disease outcome. We examined how host susceptibility to T. bryosalmonae infection, and the disease PKD, varied across different infection life-history stages and how it differs between naïve, re-infected and persistently infected hosts. We investigated the response to parasite exposure in host phenotypes with (a) different ages and (b) heterogeneous infection life histories. Among (a) the age phenotypes were young-of-the-year (YOY) fish and juvenile 1+ fish (fish older than one) and, for (b) juvenile 1+ infection survivors were either re-exposed or not re- exposed to the parasite and response phenotypes were assigned post-hoc dependant on infection status. In fish not re-exposed this included fish that cleared infection (CI) or had a persistent infection (PI). In fish re-exposed these included fish that were re-infected (RI), or re-exposed and uninfected (RCI). We assessed both parasite-centric (infection prevalence, parasite burden, malacospore transmission) and host-centric parameters (growth rates, disease severity, infection tolerance and the immune response). In (a), YOY fish, parasite success and disease severity were greater and differences in the immune response occurred, demonstrating an ontogenetic decline of susceptibility in older fish. In (b), in PI and RI fish, parasite success and disease severity were comparable. However, expression of several adaptive immunity markers was greater in RI fish, indicating concomitant immunity, as re-exposure did not intensify infection. We demonstrate the relevance of heterogeneity in infection life history on disease outcome and describe several distinctive features of immune ontogeny and protective immunity in this model not previously reported. The relevance of such themes on a population level requires greater research in many aquatic disease systems to generate clearer framework for understanding the spread and maintenance of aquatic pathogens.

The role of migration barriers for dispersion of Proliferative Kidney Disease-Balance between disease emergence and habitat connectivity

Natural and uninterrupted water courses are important for biodiversity and fish population stability. Nowadays, many streams and rivers are obstructed by artificial migration barriers, often preventing the migration of fish. On the other hand, distribution of pathogens by migrating fishes is still a point of concern. Pathogen transport and transmission is a driving force in the dynamics of many infectious diseases. The aim of the study was to investigate the possible consequences of the removal of an artificial migration barrier for the upstream transport of Tetracapsuloides bryosalmonae, the causative agent of Proliferative Kidney Disease (PKD) in brown trout, by migrating fish. To test this question, a river system was selected with a migration barrier separating a PKD positive river from a PKD negative tributary. After removal of the barrier, PKD prevalence and pathology was examined during five years after elimination of the barrier. In the tributary, no PKD was recorded at any time of the survey. By means of unidirectional PIT (passive integrated transponder)-tagging, we confirmed upstream migration of adult brown trout into the tributary during the cold season, presumably for spawning. By eDNA, we confirmed presence of T. bryoalmonae and Fredericella sp., the definitive host, DNA in water from the PKD positive river stretch, but not in the PKD negative tributary. Our study illustrates the importance of the connectivity of streams for habitat maintenance. Although migration of brown trout from a PKD-positive river into a PKD-negative tributary, mainly for spawning, was confirmed, upstream spreading of PKD was not observed.

The role of mechanistic physiology in investigating impacts of global warming on fishes

Warming of aquatic environments as a result of climate change is already having measurable impacts on fishes, manifested as changes in phenology, range shifts and reductions in body size. Understanding the physiological mechanisms underlying these seemingly universal patterns is crucial if we are to reliably predict the fate of fish populations with future warming. This includes an understanding of mechanisms for acute thermal tolerance, as extreme heatwaves may be a major driver of observed effects. The hypothesis of gill oxygen limitation (GOL) is claimed to explain asymptotic fish growth, and why some fish species are decreasing in size with warming; but its underlying assumptions conflict with established knowledge and direct mechanistic evidence is lacking. The hypothesis of oxygen- and capacity-limited thermal tolerance (OCLTT) has stimulated a wave of research into the role of oxygen supply capacity and thermal performance curves for aerobic scope, but results vary greatly between species, indicating that it is unlikely to be a universal mechanism. As thermal performance curves remain important for incorporating physiological tolerance into models, we discuss potentially fruitful alternatives to aerobic scope, notably specific dynamic action and growth rate. We consider the limitations of estimating acute thermal tolerance by a single rapid measure whose mechanism of action is not known. We emphasise the continued importance of experimental physiology, particularly in advancing our understanding of underlying mechanisms, but also the challenge of making this knowledge relevant to the more complex reality.

Prevalence, intensity and histopathology of Zeylanicobdella arugamensis infestation on groupers reared on different aquaculture systems

The marine leech, Zeylanicobdella arugamensis, is a major threat to aquaculture in grouper-producing countries including Indonesia. This study aimed at investigating prevalence, intensity and histopathology of the ectoparasite in humpback and hybrid groupers cultured in different rearing systems. A total of 260 groupers (60 humpback groupers and 200 hybrid groupers) were used for samples. The marine leech was observed on skin, fins, gills and mouth, followed by histopathological assay on the skin tissue. The results showed that prevalence of the leech in both groupers was higher when they were cultured in the floating net cages compared with the hatchery, p < .05. Furthermore, humpback grouper had a higher prevalence than hybrid grouper when they were cultured in a similar system, p < .05. Meanwhile, there was no significant difference in intensity between the two groupers, p > .05. Within the hybrid groupers, the highest prevalence was obtained from hybrid groupers reared in the earthen ponds. Histopathological studies showed that the infected groupers exhibited inflammation, congestion and erosion of the epidermis layer. Hybrid grouper had more severe histopathological lesions in the skin tissues. These results suggested that species and type of aquaculture system had significantly determined the prevalence, intensity and severity of lesion in Z. arugamensis infestation.

Warming can alter host behavior in a similar manner to infection with behavior-manipulating parasites

Parasites are ecologically ubiquitous and, by modifying the physiology and behavior of their host organisms, act as key regulators of the dynamics and stability of ecosystems. It is, however, as yet unclear how parasitic relationships will act to moderate or accelerate the ecological impacts of global climate change. Here, we explore experimentally how the effects of parasites on both the physiology and behavior of their hosts can be moderated by warming, utilising a well-established aquatic host-parasite model system-the ecologically important amphipod Gammarus duebeni and its acanthocephalan parasite Polymorphus minutus. We show that, while only warming affected measured components of host physiology, parasite infection and warming both supressed predator-avoidance behavior of the host independently, yet in a similar manner. Six degrees of warming altered geotactic behaviors to the same extent as infection with behavior-manipulating parasites. These results indicate a novel mechanism by which parasites impact their ecosystems that could be critical to predicting the ecological impacts of warming. Our findings highlight the need for holistic knowledge of interaction networks, incorporating multiple interaction types and behaviors, to predict the effects of both warming and parasitism on the dynamics and stability of ecosystems.

Long-term exposure to low 17α-ethinylestradiol (EE2) concentrations disrupts both the reproductive and the immune system of juvenile rainbow trout, Oncorhynchus mykiss

Estrogenic endocrine disrupting compounds (EEDCs), such as ethinylestradiol (EE2), are well studied for their impact on the reproductive system of fish. EEDCs may also impact the immune system and, as a consequence, the disease susceptibility of fish. It is currently not yet known whether the low concentrations of EEDCs that are able to disrupt the reproductive system of trout are effective in disrupting the immune system and the fish host resistance towards pathogens, too, or whether such immunodisruptive effects would occur only at higher EEDC concentrations. Therefore, in the present study we compare the effect thresholds of low 17α-ethinylestradiol concentrations (1.5 and 5.5 EE2 ng/L) on the reproductive system, the immune system, the energy expenditures and the resistance of juvenile rainbow trout (Oncorhynchus mykiss) against the parasite Tetracapsuloides bryosalmonae - the etiological agent of proliferative kidney disease (PKD) of salmonids. The parasite infection was conducted without injection and under low pathogen exposure concentrations. The disease development was followed over 130 days post infection - in the presence or absence of EE2 exposure. The results show that the long-term EE2 exposure affected, at both concentrations, reproductive parameters like the mRNA levels of hepatic vitellogenin and estrogen receptors. At the same concentrations, EE2 exposure modulated the immune parameters: mRNA levels of several immune genes were altered and the parasite intensity as well as the disease severity (histopathology) were significantly reduced in EE2-exposed fish compared to infected control fish. The combination of EE2 exposure and parasite infection was energetically costly, as indicated by the decreased values of the swim tunnel respirometry. Although further substantiation is needed, our findings suggest that EE2 exerts endocrine disruptive and immunomodulating activities at comparable effect thresholds, since reproductive and immune parameters were affected by the same, low EE2 concentrations.

On the potential role of Mergus merganser as transport hosts for Tetracapsuloides bryosalmonae

Transmission paths in the distribution of proliferative kidney disease (PKD) of salmonids are still largely unknown. In this study, the role of goosander (Mergus merganser) as possible transport host for Tetracapsuloides bryosalmonae through faeces was examined. Goosander fledglings were fed exclusively with diseased brown trout (Salmo trutta fario). In all trout used for feeding, intratubular sporogonic stage of the parasite was confirmed histologically. Between one to 10 hours post-feeding, the goosander faeces were sampled and tested for T. bryosalmonae DNA. In qPCR, only DNA fragments were found, and in conventional PCR, no amplification was confirmed. Therefore, we hypothesize that the role of goosander as transport hosts for T. bryosalmonae via their faeces can be neglected.

Back From the Brink: Alterations in B and T Cell Responses Modulate Recovery of Rainbow Trout From Chronic Immunopathological Tetracapsuloides bryosalmonae Infection

Proliferative kidney disease (PKD) caused by the myxozoan parasite Tetracapsuloides bryosalmonae is one of the most serious infectious diseases negatively impacting farmed and wild salmonids throughout Europe and North America. PKD pathogenesis results in a massive B cell proliferation and dysregulation with aberrant immunoglobulin production and plasma cell differentiation along with a decrease in myeloid cells and inhibition of innate pathways. Despite the huge immunopathological reaction in the kidney during infection, under specific conditions, fish can survive and return to full fitness. Fish are unique in this ability to recover renal structure and functionality from extensive tissue damage in contrast to mammals. However, only limited knowledge exists regarding the host immune response coinciding with PKD recovery. Moreover, almost no studies of the immune response during disease recovery exist in fish. We utilized the rainbow trout-T. bryosalmonae system as an immunological model of disease recovery. Our results demonstrated that recovery is preceded by an intense immune response at the transcript level, decreasing parasite burden, and an increased degree of kidney inflammation. Later in the recovery phase, the immune response transpired with a significant decrease in lymphocytes and an increase in myeloid cells. These lymphocytes populations contained lower levels of B cells comparative to the control in the anterior and posterior kidney. Additionally, there was downregulation of several transcripts used as markers for plasma cells (blimp1, igt sec, igm sec, igd sec, and cd38) and T cell subsets (cd4, cd8α, cd8β, and tcrβ). The decrease in these T cell transcripts significantly correlated with decreasing parasite intensity. Alternatively, there was strong upregulation of pax-5 and igt mem. This suggests a change in B cell processes during the recovery phase relative to clinical PKD may be necessary for the host to re-establish homeostasis in terms of an arrest in the dominant antibody like response transitioning to a transcriptional profile associated with resting B cells. The knowledge generated here in combination with earlier studies illuminates the full power of analyzing the entire trajectory of disease from the normal healthy state to recovery enabling the measurement of an immune response to pinpoint a specific disease stage.

Proliferative kidney disease in Alaskan salmonids with evidence that pathogenic myxozoans may be emerging north

Proliferative kidney disease (PKD) of salmonids, a chronic immunopathology caused by the myxozoan parasite Tetracapsuloides bryosalmonae, is exacerbated by increased water temperatures. PKD causes economic concerns to trout farmers and contributes to the decline of wild salmonid populations in North America and Europe. The parasite occurs as far north as Norway and Iceland in Europe and was confirmed from California to southern British Columbia in the American continent. In mid-September 2011 adult chum salmon (Oncorhynchus keta) were sampled from Kantishna River, a tributary to Yukon River in Alaska. Clinical PKD was diagnosed based on the macroscopic appearance of mottled kidneys that were uniformly swollen and by the detection of tumultuous histozoic extrasporogonic and coelozoic sporogonic stages of T. bryosalmonae in renal tissue by histopathology. Archived samples provided the molecular confirmation and local strain identification, representing the first confirmed case of PKD in wild adult chum salmon, also co-infected with Parvicapsula minibicornis that represents another novel myxozoan detection in Alaska. Our investigation was extended to another case from August/September 1997, with mortality following furunculosis and ectoparasite co-infections, in sockeye salmon (Oncorhynchus nerka) pre-smolts net-pen reared in English Bay Lakes, Alaska. Immunohistochemistry on archived histological preparations confirmed T. bryosalmonae sporogonic and extrasporogonic stages, indicating a severe to resolving PKD, with concomitant Chloromyxum spp. infection. Those cases provide the first documentation that this parasite is present in Alaska and causes PKD in wild and cultured salmonids in the region. The known geographic range of T. bryosalmonae can be extended to ~267 km south of the Arctic Circle, representing the northernmost detection in America. Given the vast size of Alaska and small resident population, it is likely that T. bryosalmonae remained undetected, but more recently became evident due to the clinical manifestation of PKD, possibly linked to increasing water temperatures reported at the sample locations.

Effects of parasite concentrations on infection dynamics and proliferative kidney disease pathogenesis in brown trout (Salmo trutta)

Proliferative kidney disease (PKD) is an emerging disease of salmonids, which is exacerbating with increasing water temperature. Its causative agent, the myxozoan parasite Tetracapsuloides bryosalmonae, exploits freshwater bryozoans as primary hosts and salmonids as intermediate hosts. Our experiments showed that the manipulation of exposure concentrations of infective malacospores had relatively minor impacts for the disease outcomes in the fish host. In this study, brown trout (Salmo trutta) were exposed to three different exposure concentrations of T. bryosalmonae malacospores: (a) a single low parasite concentration (LC), (b) a single high parasite concentration (HC) and (c) three times a low concentration (repeat exposure, RE). Parasite dynamics in the fish host and release of fish malacospores were quantified and fish kidney histopathology was evaluated to determine PKD pathogenesis. Infection prevalence was always lower in the LC group than in the other groups over the course of the study. While the parasite proliferation phase was slower in the LC group, the maximum parasite burden did not differ significantly amongst treatments. The onset of fish malacospore release (day 45 post-exposure), indicated by detection of T. bryosalmonae DNA in the tank water, occurred at the same time point for all groups. Reduced intensity of kidney pathological development was observed in the LC treatment indicating lower disease severity. While the LC treatment resulted in reduced outcomes across several infection parameters (infection prevalence, parasite proliferation, total fish malacospores released), the overall differences were small. The RE and HC treatment outcomes were for most parameters comparable. Our results suggest that repeated exposure, as is likely to occur in the wild during the summer months, might play a more important role in the dynamics of PKD as an emerging infectious disease than the actual concentration of spores.

Proliferative Kidney Disease and Proliferative Darkening Syndrome are Linked with Brown Trout (Salmo trutta fario) Mortalities in the Pre-Alpine Isar River

For many years, brown trout (Salmo trutta fario) mortalities within the pre-alpine Isar River in Germany were reported by the Bavarian Fisheries Association (Landesfischereiverband Bayern e.V.) and local recreational anglers during August and September. Moribund fish seemed to be affected by proliferative darkening syndrome (PDS). In addition, proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae was discussed. To investigate this phenomenon, the present field study monitored brown trout mortalities by daily river inspection in 2017 and 2018. Moribund brown trout (n = 31) were collected and examined using histology, immunohistochemistry, qPCR, and quantitative stereology. Our investigations identified 29 (93.5%) brown trout affected by PKD. Four brown trout (12.9%) displayed combined hepatic and splenic lesions fitting the pathology of PDS. The piscine orthoreovirus 3, suspected as causative agent of PDS, was not detectable in any of the samples. Quantitative stereological analysis of the kidneys revealed a significant increase of the renal tissue volumes with interstitial inflammation and hematopoietic hyperplasia in PKD-affected fish as compared to healthy brown trout. The identified T. bryosalmonae strain was classified as part of the North American clade by phylogenetical analysis. This study highlights PKD and PDS as contributing factors to recurrent autumnal brown trout mortalities.

Keeping an Eye on Wild Brown Trout (Salmo trutta) Populations: Correlation Between Temperature, Environmental Parameters, and Proliferative Kidney Disease

Proliferative kidney disease (PKD) is an emerging disease of salmonids caused by the myxozoan parasite Tetracapsuloides bryosalmonae, which plays a major role in the decrease of wild brown trout (Salmo trutta) populations in Switzerland. Strong evidence demonstrated that water temperature modulates parasite infection. However, less knowledge exists on how seasonal water temperature fluctuations influence PKD manifestation under field conditions, how further environmental factors such as water quality may modulate the disease, and whether these factors coalesce with temperatures role possibly giving rise to cumulative effects on PKD. The aims of this study were to (1) determine the correlation between seasonal course of water temperature and PKD prevalence and intensity in wild brown trout populations, (2) assess if other factors such as water quality or ecomorphology correlate with the infection, and (3) quantitatively predict the implication of these factors on PKD prevalence with a statistical model. Young-of-the-year brown trout were sampled in 45 sites through the Canton of Vaud (Switzerland). For each site, longitudinal time series of water temperature, water quality (macroinvertebrate community index, presence of wastewater treatment plant effluent) and ecomorphological data were collected and correlated with PKD prevalence and intensity. 251 T. bryosalmonae-infected trout of 1,118 were found (overall prevalence 22.5%) at 19 of 45 study sites (42.2%). Relation between PKD infection and seasonal water temperature underlined that the mean water temperature for June and the number of days with mean temperature ≥15°C were the most significantly correlated parameters with parasite prevalence and intensity. The presence of a wastewater treatment plant effluent was significantly correlated with the prevalence and infection intensity. In contrast, macroinvertebrate diversity and river ecomorphology were shown to have little impact on disease parameters. Linear and logistic regressions highlighted quantitatively the prediction of PKD prevalence depending on environmental parameters at a given site and its possible increase due to rising temperatures. The model developed within this study could serve as a useful tool for identifying and predicting disease hot spots. These results support the importance of temperature for PKD in salmonids and provides evidence for a modulating influence of additional environmental stress factors.

Kidney anatomy, histology and histometric traits associated to renosomatic index in Gymnotus inaequilabiatus (Gymnotiformes: Gymnotidae)

ABSTRACT The Gymnotus inaequilabiatus is a Neotropical fish widely distributed in marginal areas of bays. The aim of this study was to describe the main histological and histopathological traits in the head and exocrine kidney. Here, histometric and structural density techniques were associated with renosomatic index (RSI). The kidney was processed for light microscopy. Lipofuscin and hemosiderin content were visually estimated in the melonomacrophages centers (MMCs). All the biometric body variables were correlated with RSI, especially the kidney weight and gross lesions count. The general architecture of head and exocrine kidney was similar to that described for other teleost species. MMCs were prevalent in both portions and correlated with RSI in the head and exocrine kidney. Granulomatous structures were often observed in both portions; however, they were associated only in the exocrine kidney with RSI. Of all the structures hystometrically estimated, only proximal tubular diameter and thickness, and distal tubular thickness were correlated to renosomatic index. The RSI is an useful biometric variable that represent some physiological and morphological characteristics of kidney in G. inaequilabiatus. These findings may be used in future studies to evaluate the effects of environmental stressors on the renal adaptative physiological process.

Trade-Offs Underwater: Physiological Plasticity of Rainbow Trout (Oncorhynchus mykiss) Confronted by Multiple Stressors

Organisms have evolved mechanisms to partition the available resources between fitness-relevant physiological functions. Organisms possess phenotypic plasticity to acclimate to changing environmental conditions. However, this comes at a cost that can cause negative correlations or “trade-offs”, whereby increasing investments in one function lead to decreased investments in another function. The aim of the present study was to investigate the prioritization of resource allocation between growth, pathogen defense, and contaminant response in juvenile rainbow trout (Oncorhynchus mykiss) exposed to changes of resource income or expenditure. We performed a multifactorial experiment with three resource-impacting stressors—limited food availability, a parasitic infection, exposure to a vitellogenesis-inducing contaminant—and combinations thereof. Treatment with the individual stressors evoked the expected responses in the respective physiological target systems—body growth, immune system, and hepatic vitellogenin transcription—but we found little evidence for significant negative relations (trade-offs) between the three systems. This also applied to fish exposed to combinations of the stressors. This high phenotypic flexibility of trout in their resource allocation suggests that linear resource allocations as mechanisms of phenotypic plasticity may be too simplistic, but it also may point to a greater capacity of ectothermic than endothermic vertebrates to maintain key physiological processes under competing resource needs due to lower maintenance costs.

Do fish get wasted? Assessing the influence of effluents on parasitic infection of wild fish

Many ecosystems are influenced simultaneously by multiple stressors. One important environmental stressor is aquatic pollution via wastewater treatment plant (WWTP) effluents. WWTP effluents may contribute to eutrophication or contain anthropogenic contaminants that directly and/or indirectly influence aquatic wildlife. Both eutrophication and exposure to anthropogenic contaminants may affect the dynamics of fish-parasite systems. With this in mind, we studied the impact of WWTP effluents on infection of brown trout by the parasite Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease (PKD). PKD is associated with the long-term decline of wild brown trout (Salmo trutta) populations in Switzerland. We investigated PKD infection of brown trout at two adjacent sites (≈400 m apart) of a Swiss river. The sites are similar in terms of ecology except that one site receives WWTP effluents. We evaluated the hypothesis that fish inhabiting the effluent site will show greater susceptibility to PKD in terms of prevalence and disease outcome. We assessed susceptibility by (i) infection prevalence, (ii) parasite intensity, (iii) host health in terms of pathology, and (iv) estimated apparent survival rate. At different time points during the study, significant differences between sites concerning all measured parameters were found, thus providing evidence of the influence of effluents on parasitic infection of fish in our study system. However, from these findings we cannot determine if the effluent has a direct influence on the fish host via altering its ability to manage the parasite, or indirectly on the parasite or the invertebrate host via increasing bryozoa (the invertebrate host) reproduction. On a final note, the WWTP adhered to all national guidelines and the effluent only resulted in a minor water quality reduction assessed via standardized methods in this study. Thus, we provide evidence that even a subtle decrease in water quality, resulting in small-scale pollution can have consequences for wildlife.