Towards unravelling the Rosette agent enigma: Spread and emergence of the co-invasive host-pathogen complex, Pseudorasbora parva-Sphaerothecum destruens

The emergence of non-native fungal pathogens is a growing threat to global health, biodiversity, conservation biology, food security and the global economy. Moreover, a thorough understanding of the spread and emergence of pathogens among invasive and native host populations, as well as genetic analysis of the structure of co-invasive host populations, is crucial in terms of conservation biology and management strategies. Here we combined extensive catchment sampling, molecular detection tools and genomic signatures to i) assess the prevalence of the rosette agent Sphaerothecum destruens in invasive and native fish populations in contrasting french regions, and ii) characterize the genetic diversity and population structure of its co-invasive and asymptomatic carrier Pseudorasbora parva. Although S. destruens was not detected in all the fish collected its presence in contrasting freshwater ecosystems suggests that the disease may already be widespread in France. Furthermore, our results show that the detection of S. destruens DNA in its asymptomatic carrier P. parva is still limited. Finally, we found that P. parva populations show a homogeneous genetic and geographical structuring, which raises the possibility of the occurrence of successive introduction events in France from their native and invasive range.

Differential susceptibility of Yukon River and Salish Sea stocks of Chinook salmon Oncorhynchus tshawytscha to ichthyophoniasis

Preliminary evidence suggests that Chinook salmon Oncorhynchus tshawytscha from the Yukon River may be more susceptible to Ichthyophonus sp. infections than Chinook from stocks further south. To investigate this hypothesis in a controlled environment, we experimentally challenged juvenile Chinook from the Yukon River and from the Salish Sea with Ichthyophonus sp. and evaluated mortality, infection prevalence and infection load over time. We found that juvenile Chinook salmon from a Yukon River stock were more susceptible to ichthyophoniasis than were those from a Salish Sea stock. After feeding with tissues from infected Pacific herring Clupea pallasii, Chinook salmon from both stocks became infected. The infection was persistent and progressive in Yukon River stock fish, where infections sometimes progressed to mortality, and histological examinations revealed parasite dissemination and proliferation throughout the host tissues. In Salish Sea-origin fish, however, infections were largely transient; host mortalities were rare, and parasite stages were largely cleared from most tissues after 3-4 wk. Susceptibility differences were evidenced by greater cumulative mortality, infection prevalence, parasite density, proportion of fish demonstrating a cellular response, and intensity of the cellular response among fish from the Yukon River stock. These observed differences between Chinook salmon stocks were consistent when parasite exposures occurred in both freshwater and seawater. These results support the hypothesis that a longer-standing host-pathogen relationship, resulting in decreased disease susceptibility, exists among Salish Sea Chinook salmon than among Yukon River conspecifics.

A geographic hot spot of Ichthyophonus infection in the southern Salish Sea, USA

The prevalence of Ichthyophonus infection in Pacific herring Clupea pallasii was spatially heterogeneous in the southern Salish Sea, Washington State, USA. Over the course of 13 mo, 2232 Pacific herring were sampled from 38 midwater trawls throughout the region. Fork length was positively correlated with Ichthyophonus infection at all sites. After controlling for the positive relationship between host size and Ichthyophonus infection, the probability of infection was approximately 6-fold higher in North Hood Canal than in Puget Sound and the northern Straits (12 vs. 2% predicted probability for a 100 mm fish and 30 vs. 7% predicted probability for a 180 mm fish). Temporal changes in Ichthyophonus infection probability were explained by seasonal differences in fish length, owing to Pacific herring life history and movement patterns. Reasons for the spatial heterogeneity remain uncertain but may be associated with density-dependent factors inherent to the boom-bust cycles that commonly occur in clupeid populations.

Effect of Ichthyophonus on blood plasma chemistry of spawning Chinook salmon and their resulting offspring in a Yukon River tributary

Ichthyophonus is a protozoan parasite of Alaska Chinook salmon Oncorhynchus tshawytscha. In this study, we determined whether spawning Chinook salmon in the Yukon River drainage exhibited a measurable stress response (i.e. elevated plasma cortisol concentrations) and detectable changes in selected blood plasma chemistry parameters when infected with Ichthyophonus. The resulting alevin were also analyzed for any differences in blood plasma chemistry caused by parental infection with Ichthyophonus. In 2010, 2011, and 2012, spawning adult Chinook salmon were collected from the Salcha River, Alaska, USA, and the prevalence of Ichthyophonus in these fish was 7.8, 6.3, and 8.3%, respectively. Fish with no clinical signs of Ichthyophonus and Ichthyophonus-positive parents were cross-fertilized to investigate potential second-generation effects as a result of Ichthyophonus infection. We found no significant difference in cortisol concentrations in blood plasma between Ichthyophonus-positive and -negative adults or between alevin from Ichthyophonus-positive and -negative parents. There were no significant differences in blood plasma parameters (e.g. alanine aminotransferase, creatine kinase, glucose) of Ichthyophonus-negative and -positive adults, with the exception of aspartate aminotransferase, which was significantly higher in plasma of Ichthyophonus-negative adults. All clinical chemistry parameters for alevin resulting from both Ichthyophonus-negative and -positive parents were not significantly different. Based on this study, which has a limited sample size and low prevalence of Ichthyophonus, offspring of Chinook salmon appear to suffer no disadvantage as a result of Ichthyophonus infection in their parents on the Salcha River.

Infecting Pacific Herring with Ichthyophonus sp. in the Laboratory

The protistan parasite Ichthyophonus sp. occurs in coastal populations of Pacific Herring Clupea pallasii throughout the northeast Pacific region, but the route(s) by which these planktivorous fish become infected is unknown. Several methods for establishing Ichthyophonus infections in laboratory challenges were examined. Infections were most effectively established after intraperitoneal (IP) injections with suspended parasite isolates from culture or after repeated feedings with infected fish tissues. Among groups that were offered the infected tissues, infection prevalence was greater after multiple feedings (65%) than after a single feeding (5%). Additionally, among groups that were exposed to parasite suspensions prepared from culture isolates, infection prevalence was greater after exposure by IP injection (74%) than after exposure via gastric intubation (12%); the flushing of parasite suspensions over the gills did not lead to infections in any of the experimental fish. Although the consumption of infected fish tissues is unlikely to be the primary route of Ichthyophonus sp. transmission in wild populations of Pacific Herring, this route may contribute to abnormally high infection prevalence in areas where juveniles have access to infected offal.

Ketoconazole modulates the infectivity of Ichthyophonus sp. (Mesomycetozoa) in vivo in experimentally injected European sea bass

In vitro studies have confirmed the inhibitory effect of the azol-derivative ketoconazole (KZ) on the growth of Ichthyophonus, an important pathogen causing epizootics in wild and cultured fish. We evaluated the effect of KZ in vivo in European sea bass Dicentrarchus labrax experimentally infected with the same Ichthyophonus isolate. Liposomes were used to vehiculate different doses of KZ to increase the effect on Ichthyophonus and lower the toxicity of the drug, and KZ toxicity was assessed in cultured sea bass juveniles. We also studied the effect of liposome-vehiculated KZ included in medicated food on ichthyophoniasis. KZ causes clear toxic effects in D. labrax juveniles at doses >80 mg kg-1, apparent in the reduced survival of fish and histological alterations to livers, kidneys and spleens. Fish injected with Ichthyophonus and treated with KZ dosages of ≤80 mg kg-1 d-1 presented lower ichthyophoniasis prevalence, fewer organs infected per fish, and fewer spores in the affected organs than the untreated fish. KZ seems to delay the onset of infection, but cannot stop further progression once established. However, this behaviour is not clearly reflected in the biometric and haematological data collected from these fish. We hypothesise that KZ's delaying effect would increase, if lower infective doses (more similar to natural situations) were used. The drug administration vehicle (liposomes vs. emulsions) did not affect the results. Our data confirm the potential utility of KZ in treating ichthyophoniasis and reveal its low toxicity for sea bass. Nevertheless, the optimal dose and appropriate application protocol remain to be determined.

Sphaerothecum destruens pathology in cyprinids

Sphaerothecum destruens is a significant intracellular parasite of fish which has caused disease and mortalities in cultured north American Chinook salmon Oncorhynchus tshawytscha and Atlantic salmon Salmo salar. Several hosts for S. destruens have been identified within the Salmonidae family, and the histopathology of the infection can differ between hosts. Recently, S. destruens has been associated with the most invasive cyprinid species in Europe, topmouth gudgeon Pseudorasbora parva. Accurate disease identification based on thorough descriptions of clinical signs and histopathology in this new range of hosts is thus paramount to support further epizootiological studies. In this study, the associated histopathology of S. destruens infection is described along with its pathogenesis in the endangered cyprinid sunbleak Leucaspius delineatus. Histological examination of 100 L. delineatus in a wild population in the south of England revealed the presence of S. destruens infections, with a prevalence of 5% with S. destruens, suggesting an over-dispersed distribution within the L. delineatus sample. Clinical signs of the infection were absent, but histological examination revealed the presence of both disseminated and nodular lesions in several organs.

Two skin parasites of Rana temporaria: Dermocystidium ranae Guyénot & Naville and Dermosporidium granulosum n.sp

In the skin of Rana temporaria from Czechoslovakia two unicellular parasites have been found: Dermocystidium ranae Guyénot & Naville and Dermosporidium granulosum n.sp.

Dermocystidium ranae. Two stages were found, one of which is new. An account is given of the morphology, development and occurrence of both stages, including details which have hitherto not been recorded.

Dermosporidium granulosum n.sp. This species is represented by cysts situated in the stratum spongiosum corii. They are divided by septa into small chambers containing groups of spherical spores with a central nucleus and with numerous small cytoplasmic inclusions. Within the cyst, the sporoblasts occupy the periphery, while the ripe spores are in the centre.

In vitro leukocyte-encapsulation model in rainbow trout (Oncorhynchus mykiss)

We developed an in vitro model to study the cellular and molecular mechanisms of granulomatous inflammation in response to invading pathogens. Ichthyophonus hoferi was used as a target for encapsulation by cultivated leukocytes from the kidney of the rainbow trout (Oncorhynchus mykiss). The encapsulation process was observed over 1 week. The leukocytes were identified as either macrophages in the inner layer, or neutrophils and lymphocytes in the outer layer. The encapsulation response was inhibited by treatment with heat, but not formalin or methanol. The recognition of heat-unstable molecules on the pathogen surface could induce encapsulation. Increased expression of pro-inflammatory cytokines, such as interleukin (IL)-1beta, IL-8 and tumor necrosis factor-alpha2, was observed during encapsulation. These cytokines might play crucial roles in the encapsulation process. In particular, IL-8, which was expressed at a late phase, might recruit specific cell populations, such as the lymphocytes comprising the outer cellular layer around the target.

Field evidence for leech-borne transmission of amphibian Ichthyophonus sp

Parasites have been implicated in mass mortality events and population declines of amphibians around the world. One pathogen associated with mortality events in North America is an Ichthyophonus sp.-like organism that affects red-spotted newts (Notophthalmus viridescens) and several frog species, yet little is known about the distribution of this pathogen in wild populations or the mechanism of transmission. In an effort to identify factors influencing the distribution and abundance of this pathogen, we measured Ichthyophonus sp. prevalence and a series of factors that could contribute to transmission in 16 newt populations during spring 2004. In contrast to our initial hypotheses of trophic transmission, several lines of evidence suggested a role for the amphibian leech (Placobdella picta) in Ichthyophonus sp. transmission. We propose the mechanistic hypothesis that a leech acquires Ichthyophonus sp. infection when inserting its proboscis into the muscles beneath the skin of infected newts and transmits the infection to other newts in subsequent feeding bouts. We also found effects of host sex, body mass, and breeding condition on Ichthyophonus sp. prevalence and the number of attached leeches. The number of leeches attached to newts was strongly related to the proportion of newt habitat containing emergent vegetation, suggesting that anthropogenic eutrophication might lead to more frequent or severe outbreaks of Ichthyophonus sp. infection in amphibians.

Differences in Ichthyophonus prevalence and infection severity between upper Yukon River and Tanana River chinook salmon, Oncorhynchus tshawytscha (Walbaum), stocks

Two genetically distinct populations of chinook salmon, Oncorhynchus tshawytscha (Walbaum), were simultaneously sampled at the confluence of the Yukon and Tanana rivers in 2003. Upper Yukon-Canadian fish had significantly higher infection prevalence as well as more severe infections (higher parasite density in heart tissue) than the lower Yukon-Tanana River fish. Both populations had migrated the same distance from the mouth of the Yukon River at the time of sampling but had significantly different distances remaining to swim before reaching their respective spawning grounds. Multiple working hypotheses are proposed to explain the differences between the two stocks: (1) the two genetically distinct populations have different inherent resistance to infection, (2) genetically influenced differences in feeding behaviour resulted in temporal and/or spatial differences in exposure, (3) physiological differences resulting from different degrees of sexual maturity influenced the course of disease, and (4) the most severely infected Tanana River fish either died en route or fatigued and were unable to complete their migration to the Tanana River, thus leaving a population of apparently healthier fish.