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.

Herpesvirus-Associated Proliferative Skin Disease in Frogs and Toads: Proposed Pathogenesis.

A comparative study was carried out on common and agile frogs (Rana temporaria and R. dalmatina) naturally infected with ranid herpesvirus 3 (RaHV3) and common toads (Bufo bufo) naturally infected with bufonid herpesvirus 1 (BfHV1) to investigate common pathogenetic pathways and molecular mechanisms based on macroscopic, microscopic, and ultrastructural pathology as well as evaluation of gene expression. Careful examination of the tissue changes, supported by in situ hybridization, at different stages of development in 6 frogs and 14 toads revealed that the skin lesions are likely transient, and part of a tissue cycle necessary for viral replication in the infected hosts. Transcriptomic analysis, carried out on 2 naturally infected and 2 naïve common frogs (Rana temporaria) and 2 naturally infected and 2 naïve common toads (Bufo bufo), revealed altered expression of genes involved in signaling and cell remodeling in diseased animals. Finally, virus transcriptomics revealed that both RaHV3 and BfHV1 had relatively high expression of a putative immunomodulating gene predicted to encode a decoy receptor for tumor necrosis factor in the skin of the infected hosts. Thus, the comparable lesions in infected frogs and toads appear to reflect a concerted epidermal and viral cycle, with presumptive involvement of signaling and gene remodeling host and immunomodulatory viral genes.

Integrated field, laboratory, and theoretical study of PKD spread in a Swiss prealpine river.

Proliferative kidney disease (PKD) is a major threat to wild and farmed salmonid populations because of its lethal effect at high water temperatures. Its causative agent, the myxozoan Tetracapsuloides bryosalmonae, has a complex lifecycle exploiting freshwater bryozoans as primary hosts and salmonids as secondary hosts. We carried out an integrated study of PKD in a prealpine Swiss river (the Wigger). During a 3-year period, data on fish abundance, disease prevalence, concentration of primary hosts' DNA in environmental samples [environmental DNA (eDNA)], hydrological variables, and water temperatures gathered at various locations within the catchment were integrated into a newly developed metacommunity model, which includes ecological and epidemiological dynamics of fish and bryozoans, connectivity effects, and hydrothermal drivers. Infection dynamics were captured well by the epidemiological model, especially with regard to the spatial prevalence patterns. PKD prevalence in the sampled sites for both young-of-the-year (YOY) and adult brown trout attained 100% at the end of summer, while seasonal population decay was higher in YOY than in adults. We introduce a method based on decay distance of eDNA signal predicting local species' density, accounting for variation in environmental drivers (such as morphology and geology). The model provides a whole-network overview of the disease prevalence. In this study, we show how spatial and environmental characteristics of river networks can be used to study epidemiology and disease dynamics of waterborne diseases.

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

Climate change, in particular rising temperature, is suspected to be a major driver for the emergence of many wildlife diseases. Proliferative kidney disease of salmonids, caused by the myxozoan Tetracapsuloides bryosalmonae, was used to evaluate how temperature dependence of host-parasite interactions modulates disease emergence. Brown trout (Salmo trutta fario) kept at 12 and 15 °C, were experimentally infected with T. bryosalmonae. Parasite development in the fish host and release of spores were quantified simultaneously to unravel parasite transmission potential from the vertebrate to the invertebrate host. A change to a stable plateau in infection intensity of the kidney coincided with a threshold at which spore shedding commenced. This onset of parasite release was delayed at the low temperature in accordance with reaching this infection intensity threshold, but the amount of spores released was irrespective of temperature. The production of parasite transmission stages declined with time. In conclusion, elevated temperature modifies the parasite transmission opportunities by increasing the duration of transmission stage production, which may affect the spread and establishment of the parasite in a wider range of rivers.

Multilocus sequence typing reveals clonality in Saprolegnia parasitica outbreaks.

The molecular epidemiology of fish pathogen Saprolegnia parasitica is still largely unknown. We developed a multilocus sequence typing scheme based on seven housekeeping genes to characterize 77 S. parasitica strains isolated from different fish host species at different times and from different geographic areas in Switzerland between 2015 and 2017. Ten different diploid sequence types (DSTs) were identified. The majority (52%) of outbreaks in Switzerland seemed to be caused by one genotype, namely DST3, which was recovered from farm-raised and wild-caught fish in all the geographic areas and river basins included in the study. DST3 was also recovered from the rivers Bienne (eastern France) and Doubs, where the episodes of massive mortality due to saprolegniosis started in 2009. Another genotype (DST7) showed, to a lesser extent, a distribution across different river basins, while eight DSTs were unique to a defined geographic area or river basin. The occurrence of sporadic DSTs indicates a certain degree of diversity within S. parasitica in the environment. The wide distribution of DST3 suggests that a clonal population may have spread in eastern France and Switzerland across geographic barriers, with strong implications for the management of both captive and wild fish populations.

Who needs the hotspot? The effect of temperature on the fish host immune response to Tetracapsuloides bryosalmonae the causative agent of proliferative kidney disease.

Proliferative kidney disease (PKD) of salmonids, caused by Tetracapsuloides bryosalmonae may lead to high mortalities at elevated water temperatures. However, it has not yet been investigated how temperature affects the fish host immune response to T. bryosalmonae. We exposed YOY (young of the year) rainbow trout (Oncorhynchus mykiss) to T. bryosalmonae at two temperatures (12 °C and 15 °C) that reflect a realistic environmental scenario and could occur in the natural habitat of salmonids. We followed the development of the parasite, host pathology and immune response over seven weeks. We evaluated the composition and kinetics of the leukocytes and their major subgroups in the anterior and posterior kidney. We measured immune gene expression profiles associated with cell lineages and functional pathways in the anterior and posterior kidney. At 12 °C, both infection prevalence and pathogen load were markedly lower. While the immune response was characterized by subtle changes, mainly an increased amount of lymphocytes present in the kidney, elevated expression of Th1-like signature cytokines and strong upregulation of the natural killer cell enhancement factor, NKEF at week 6 P.E. At 15 °C the infection prevalence and pathogen burden were ominously greater. While the immune response as the disease progressed was associated with a Th2-like switch at week 6 P.E and a prominent B cell response, evidenced at the tissue, cell and transcript level. Our results highlight how a subtle, environmentally relevant difference in temperature resulted in diverse outcomes in terms of the immune response strategy, altering the type of interaction between a host and a parasite.

Histopathological alterations of the heart in fish: proposal for a standardized assessment.

Histopathological alterations in the heart are often reported in fish as a result of exposure to a variety of chemical compounds. However, researchers presently lack a standardized method for the evaluation of histopathological alterations in the cardiovascular system of fish and the calculation of an 'organ index'. Therefore, we designed a method for a standardized assessment and evaluation of histopathological alterations in the heart of fish. As a model species, we used rainbow trout Oncorhynchus mykiss, but the protocol was also successfully applied to other fish species belonging to different taxonomic orders. To test the protocol, we re-evaluated sections of atenolol-exposed and unexposed rainbow trout obtained in a previous study. The results were in accordance with those previously published, demonstrating the applicability of the protocol. The protocol provides a universal method for the comparative evaluation of histopathological changes in the heart of fish.

First isolation of a rhabdovirus from perch Perca fluviatilis in Switzerland.

Perca fluviatilis is a fish species of increasing interest to the Swiss fish farming industry. In recent years, recirculation systems have been specifically set up to increase production. In one of these farms, abnormal spiral swimming associated with elevated mortalities occurred in repeated batches of imported perch shortly after stocking on several occasions. No bacterial or parasitic etiology was detected, but a virus grown in bluegill fry (BF-2) cells was identified as perch rhabdovirus. Subsequent investigations of other samples suggested a viral tropism for the central nervous system (CNS). Phylogenetic analysis of the partial N and entire G gene sequences positioned this isolate in genogroup C of the species Perch rhabdovirus, with high nucleotide and amino acid (aa) sequence identities with the DK5533 strain isolated in Denmark in 1989. Comparative studies using other closely related isolates allowed the distinction of 2 serological patterns among perch rhabdoviruses and the identification of a proline substitution by a serine in position 147 of the glycoprotein potentially involved in antigenic differentiation. Even if perch imported onto the farm tested negative by virus isolation prior to transport, they may have been the origin of this outbreak since CNS tissue was not included in the samples that were analyzed. Another possibility might be a sub-clinical infection with a viral load in resident fish too low to be detected. This study reports the first isolation of a perch rhabdovirus in Switzerland, and emphasizes the necessity of optimizing diagnostic tools that facilitate better control of the risks associated with fish translocation.

Effect of temperature and diet on wound healing in Atlantic salmon (Salmo salar L.).

Compromised skin integrity of farmed Atlantic salmon, commonly occurring under low temperature and stressful conditions, has major impacts on animal welfare and economic productivity. Even fish with minimal scale loss and minor wounds can suffer from secondary infections, causing downgrading and mortalities. Wound healing is a complex process, where water temperature and nutrition play key roles. In this study, Atlantic salmon (260 g) were held at different water temperatures (4 or 12 °C) and fed three different diets for 10 weeks, before artificial wounds were inflicted and the wound healing process monitored for 2 weeks. The fish were fed either a control diet, a diet supplemented with zinc (Zn) or a diet containing a combination of functional ingredients in addition to Zn. The effect of diet was assessed through subjective and quantitative skin histology and the transcription of skin-associated chemokines. Histology confirmed that wound healing was faster at 12 °C. The epidermis was more organised, and image analyses of digitised skin slides showed that fish fed diets with added Zn had a significantly larger area of the epidermis covered by mucous cells in the deeper layers after 2 weeks, representing more advanced healing progression. Constitutive levels of the newly described chemokines, herein named CK 11A, B and C, confirmed their preferential expression in skin compared to other tissues. Contrasting modulation profiles at 4 and 12 °C were seen for all three chemokines during the wound healing time course, while the Zn-supplemented diets significantly increased the expression of CK 11A and B during the first 24 h of the healing phase.

Detection and quantification of Flavobacterium psychrophilum in water and fish tissue samples by quantitative real time PCR.

BACKGROUND: Flavobacterium psychrophilum is the agent of Bacterial Cold Water Disease and Rainbow Trout Fry Syndrome, two diseases leading to high mortality. Pathogen detection is mainly carried out using cultures and more rapid and sensitive methods are needed. RESULTS: We describe a qPCR technique based on the single copy gene ß' DNA-dependent RNA polymerase (rpoC). Its detection limit was 20 gene copies and the quantification limit 103 gene copies per reaction. Tests on spiked spleens with known concentrations of F. psychrophilum (106 to 101 cells per reaction) showed no cross-reactions between the spleen tissue and the primers and probe. Screening of water samples and spleens from symptomless and infected fishes indicated that the pathogen was already present before the outbreaks, but F. psychrophilum was only quantifiable in spleens from diseased fishes. CONCLUSIONS: This qPCR can be used as a highly sensitive and specific method to detect F. psychrophilum in different sample types without the need for culturing. qPCR allows a reliable detection and quantification of F. psychrophilum in samples with low pathogen densities. Quantitative data on F. psychrophilum abundance could be useful to investigate risk factors linked to infections and also as early warning system prior to potential devastating outbreak.

Molecular epidemiology of Flavobacterium psychrophilum from Swiss fish farms.

Bacterial cold water disease (BCWD) and rainbow trout fry syndrome (RTFS) caused by Flavobacterium psychrophilum are 2 of the major diseases causing high fish mortality in salmonid fish farms. The molecular epidemiology of F. psychrophilum is still largely unknown. Multilocus sequence typing (MLST) has been previously used for this pathogen and underscored a correlation between clonal complexes and host fish species. Here we used MLST to study the relationships among 112 F. psychrophilum isolates from rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta fario and S. t. lacustris in Swiss fish farms between 1993 and 2012. The isolates belonged to 27 different sequence types (STs). Most of the Swiss outbreaks were associated with strains belonging to clonal complexes CC-ST2 and CC-ST90, found in both rainbow trout and brown trout and represented by several STs. Eight ST singletons could not be connected to any known clonal complex. Already reported from other parts of Europe and North America, CC-ST2 was the most frequent clonal complex observed, and it caused the majority of outbreaks in Switzerland, with CC-ST90 being the second most important type. In the tightly interconnected Swiss fish farms, no association between clonal complex and host fish was detected, but a temporal evolution of the frequency of some STs was observed. The occurrence of sporadic STs suggests high F. psychrophilum diversity and may reflect the presence of different sequence types in the environment.

Kidney pathology and parasite intensity in rainbow trout Oncorhynchus mykiss surviving proliferative kidney disease: time course and influence of temperature.

Proliferative kidney disease (PKD) is an endoparasitic disease of salmonids caused by the myxozoan parasite Tetracapsuloides bryosalmonae. We recently described the development of the disease from initial infection until manifestation of clinical disease signs in rainbow trout held at 2 water temperatures, 12 and 18°C. The aim of the present study is to investigate whether (1) infected fish surviving the clinical phase would recover from renal pathological changes, (2) whether they would be able to reduce the parasite load in the kidneys, and (3) whether water temperatures would influence renal recovery and parasite clearance. At 18°C, fish showed a gradual recovery of normal kidney morphology which was associated with a decline in parasite numbers and infection prevalence. Fish kept at 12°C initially showed an enhancement of kidney lesions before recovery of normal kidney morphology took place. The decrease in renal parasite load was retarded compared to 18°C. The results from the present study provide evidence that rainbow trout surviving the clinical phase of PKD are able to (1) fully restore renal structure, and (2) significantly reduce renal parasite loads, although 100% clearance was not achieved within the experimental period of this study. Water temperature influences the rate but not the outcome of the recovery process.

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.

MyFishCheck: A Model to Assess Fish Welfare in Aquaculture.

Welfare in animal husbandry includes considerations of biology, ethics, ecology, law and economics. These diverse aspects must be translated into common quantifiable parameters and applicable methods to objectively assess welfare in animals. To assist this process in the field of aquaculture, where such methods are largely missing, we developed a model to assess fish welfare. A network of information was created to link needs, i.e., fundamental requirements for welfare, with parameters, i.e., quantifiable aspects of welfare. From this ontology, 80 parameters that are relevant for welfare, have practicable assessment methods and deliver reliable results were selected and incorporated into a model. The model, named MyFishCheck, allows the evaluation of welfare in five distinct modules: farm management, water quality, fish group behaviour, fish external and fish internal appearance, thereby yielding five individual grades categorising welfare ranging from critical, to poor, to acceptable, and good. To facilitate the use of the model, a software application was written. With its adaptability to different fish species, farming systems, regulations and purposes as well as its user-friendly digital version, MyFishCheck is a next step towards improved fish welfare assessment and provides a basis for ongoing positive developments for the industry, the farmers and the fish.

Evolution of infectious hematopoietic necrosis virus (IHNV), a fish rhabdovirus, in Europe over 20 years: implications for control.

The fish pathogenic rhabdovirus infectious hematopoietic necrosis virus (IHNV) causes substantial losses in European aquaculture. IHNV was first detected in Europe in 1987 and has since undergone considerable spread. Phylogenetic analyses of the full G-gene sequences of 73 isolates obtained from 4 countries in Europe (France, n = 18; Italy, 9; Switzerland, 4; Germany, 42) enable determination of the evolution of the virus in Europe since the first detection, and identification of characteristic changes within the G-genes of European strains. Further, the database allows us to analyse the pathways of distribution in Europe over time. The results suggest that in most of the recent cases, spread of IHNV was related to trade of infected fish. The data further demonstrate that knowledge of the sequence is required to determine the source of infections in farms.

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, cd8ß, and tcrß). 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.

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 in rainbow trout: time- and temperature-related renal pathology and parasite distribution.

Proliferative kidney disease is a parasitic infection of salmonid fishes caused by Tetracapsuloides bryosalmonae. The main target organ of the parasite in the fish is the kidney. To investigate the influence of water temperature on the disease in fish, rainbow trout Oncorhynchus mykiss infected with T bryosalmonae were kept at 12 degrees C and 18 degrees C. The number of parasites, the type and degree of lesions in the kidney and the mortality rate was evaluated from infection until full development of disease. While mortality stayed low at 12 degrees C, it reached 77% at 18 degrees C. At 12 degrees C, pathological lesions were dominated by a multifocal proliferative and granulomatous interstitial nephritis. This was accompanied by low numbers of T. bryosalmonae, mainly located in the interstitial lesions. With progression of the disease, small numbers of parasites appeared in the excretory tubuli, and parasite DNA was detected in the urine. Parasite degeneration in the interstitium was observed at late stages of the disease. At 18 degrees C, pathological lesions in kidneys were more severe and more widely distributed, and accompanied by significantly higher parasite numbers. Distribution of parasites in the renal compartments, onset of parasite degeneration and time course of appearance of parasite DNA in urine were not clearly different from the 12 degrees C group. These findings indicate that higher mortality at 18 degrees C compared to 12 degrees C is associated with an enhanced severity of renal pathology and increased parasite numbers.

Exploring the immune response, tolerance and resistance in proliferative kidney disease of salmonids.

Proliferative kidney disease (PKD) of salmonids is a disease of economic and environmental concern caused by the myxozoan parasite Tetracapsuloides bryosalmonae. Finer details of the immune repertoire during T. bryosalmonae infection have been elucidated in rainbow trout (Oncorhynchus mykiss). In contrast, there remain many unanswered questions regarding the immune response of the wild fish host in Europe, the brown trout (Salmo trutta) to this parasite. The first aim of this study is to examine the brown trout immune response to T. bryosalmonae and compare it with the published information on rainbow trout as two species that have undergone a different coevolution with the parasite. According to ecoimmunology terminology, infected organisms may manage infection by reducing the damage caused by parasites (tolerance) or by limiting parasite burden (resistance). The second aim of this study is to investigate tolerance/resistance patterns of these species during PKD infection. Our results suggest subtle differences in sequential aspects of the immune response and of immune genes that correlate with parasite intensity for the brown trout, in contrast to rainbow trout, in terms of the B cell response and Th-like interplay that may be linked to PKD pathogenesis. These differences in the immune response also correlate with species-specific differences in tolerance/resistance patterns, in that brown trout had increased tolerance but rainbow trout had greater resistance to infection. The variance in tolerance/resistance investment resulted in a different evolutionary outcome for each host-parasite interaction. A greater exploration of these concepts and an association of immune mechanisms could open an additional gateway for interpreting fish host-parasite interactions.

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.