Introduced pathogens and native freshwater biodiversity: a case study of Sphaerothecum destruens

A recent threat to European fish diversity was attributed to the association between an intracellular parasite, Sphaerothecum destruens, and a healthy freshwater fish carrier, the invasive Pseudorasbora parva originating from China. The pathogen was found to be responsible for the decline and local extinction of the European endangered cyprinid Leucaspius delineatus and high mortalities in stocks of Chinook and Atlantic salmon in the USA. Here, we show that the emerging S. destruens is also a threat to a wider range of freshwater fish than originally suspected such as bream, common carp, and roach. This is a true generalist as an analysis of susceptible hosts shows that S. destruens is not limited to a phylogenetically narrow host spectrum. This disease agent is a threat to fish biodiversity as it can amplify within multiple hosts and cause high mortalities.     

Isolation and culture of Sphaerothecum destruens from Sunbleak (Leucaspius delineatus) in the UK and pathogenicity experiments in Atlantic salmon (Salmo salar)

The sunbleak (Leucaspius delineatus), a cyprinid fish native to continental Europe and now established in the UK, is experiencing population decline which appears to be linked to the spread of the invasive Asian cyprinid (Pseudorasbora parva). A population of sunbleak in the UK has previously been identified as infected with S. destruens at low prevalence. Because Sphaerothaecum destruens has, on occasion, caused severe disease in cultured and wild salmonids the aim of this work was to establish laboratory cultures of S. destruens from sunbleak in the UK and use these cultures in challenge experiments to determine if the UK isolate of S. destruens from cyprinid species is a potential threat to Atlantic salmon (Salmo salar). The first isolation and culture of S. destruens in the UK and from a cyprinid species is described. Cultured S. destruens spores from sunbleak are infective to EPC, CHSE and FHM cells, replicating most rapidly in FHM and EPC cells. Spores can be induced to zoosporulate in water forming motile, uni-flagellated zoospores. Challenge experiments indicated the spores are able to replicate and disperse in Atlantic salmon and are associated with increased mortality (up to 90%) when injected intraperitonealy.     

Development of PCR-based methods for detection of Sphaerothecum destruens in fish tissues

Single-round and nested polymerase chain reaction (PCR) tests were developed for amplification of a 434 bp fragment of the small subunit ribosomal RNA (18S rRNA) gene from Sphaerothecum destruens, previously known as the rosette agent, an intracellular parasite of salmonid fishes. Both tests have successfully amplified S. destruens-specific DNA from different isolates of S. destruens but not from related organisms. The limits of detection using the nested PCR test were 1 pg for purified S. destruens genomic DNA and 0.1 fg for plasmid DNA. We conducted 2 experimental transmission studies, consisting of injection or waterborne exposure of juvenile winter-run Chinook salmon Oncorhynchus tshawytscha to spore stages of the parasite. In the injection study, parasite DNA was detected in 100% of kidney samples from exposed fish (n = 83) at 1 and 3 mo post-exposure using nested PCR, versus 98% using microscopic analysis of Gram-stained impression smears made from the kidney. Following waterborne exposure, fish were sampled over the course of a year. From each fish, samples of gill, liver, posterior intestine and kidney were analyzed. S. destruens-specific DNA was detected most often in gill and kidney over the course of the experiment, and 71% (64/90) of the exposed fish were identified as positive for S. destruens using the nested PCR test, versus 16% (14/90) using microscopic analysis of Gram-stained kidney smears. Natural infections in captive broodstock of adult winter-run Chinook salmon, originally diagnosed by examination of Gram-stained kidney smears, were confirmed using the nested PCR test in all fish examined (15/15). Further, the nested test amplified parasite-specific DNA from other tissues in these fish with varying frequencies. This report introduces the first DNA-based detection method for S. destruens, to be used alone as a diagnostic tool or in conjunction with histologic tests for confirmatory identification of the parasite.     

Infection Characteristics of Clonorchis sinensis Metacercariae in Fish from Republic of Korea

The infection nature of Clonorchis sinensis metacercariae (CsMc) in freshwater fish hosts is closely related to the transmission of human clonorchiasis. This article reviewed the infection characteristics of CsMc in freshwater fish in the Republic of Korea (Korea). The status of CsMc infection was examined in a total of 17,792 cyprinid fish of 49 species in 9 water systems, which included Hantan-/Imjin-gang, Han-gang, Geum-gang, Mangyeong-gang, Yeongsan-gang, Tamjin-gang, Seomjin-gang, Nakdong-gang, and streams in the east coastal areas from 2010 to 2020. The infection status of CsMc was examined according to cyprinid fish species and water systems, after which analyzed by endemicity and susceptibility index. The high endemicity was shown in the cyprinid fish from 3 regions (6.1%) in the upper reaches of Nakdong-gang, such as Banbyeon-cheon (stream), Yongjeon-cheon, and Wi-cheon. The moderate levels were observed in fishes from 8 regions (16.3%), and low endemicity was shown in fishes from 20 regions (40.8%). No CsMc were detected in fish from 18 regions (36.7%). The susceptibility of CsMc in index fish, Puntungia herzi, was found to be a reliable index without examination of other fish species. CsMc infection rates were closely related to subfamily groups in the cyprinid fish hosts in a highly endemic area. In Korea, a total of 58 fish species in 10 families has been listed as the second intermediate hosts for C. sinensis. This review provides several novel features of CsMc infection and clarifies the species of second intermediate freshwater fish host in Korea.     

Milky hemolymph syndrome (MHS) in spiny lobsters, penaeid shrimp and crabs

Black tiger shrimp Penaeus monodon, European shore crab Carcinus maenas and spiny lobster Panulirus spp. can be affected by milky hemolymph syndrome (MHS). Four rickettsia-like bacteria (RLB) isolates of MHS originating from 5 geographical areas have been identified to date. The histopathology of the disease was characterized and a multiplex PCR assay was developed for detection of the 4 bacterial isolates. The 16S rRNA gene and 16-23S rRNA intergenic spacer region (ISR) were used to examine the phylogeny of the MHS isolates. Although the pathology of this disease appears similar in the various different hosts, sequencing and examination of the phylogenetic relationships reveal 4 distinct RLB involved in the infection process.     

Lactococcus garvieae and Streptococcus iniae infections in rainbow trout Oncorhynchus mykiss: similar, but different diseases.

Clinical and macroscopic findings (anorexia, lethargy, loss of orientation and exophthalmia) indicate that Streptococcus iniae and Lactococcus garvieae infections of trout share some common features, but histopathology reveals notable differences between the 2 diseases. Meningitis and panophthalmitis are the main lesions among S. iniae infected trout, whereas L. garvieae infection results in a hyperacute systemic disease. Differences in the LD50s of the 2 pathogens and the sudden onset of signs and death correlate with the histopathological findings, indicating the severity of L. garvieae infection of trout.     

Studies on ulcerative dermal necrosis of salmonids

The gross pathology and histopathology of a series of 150 cases of UDN disease of salmon Safmo sulur L. from a number of Scottish rivers is described. The lesions appeared first as small grey areas on the heads of affected fish, which gradually enlarged in size, ulcerated and became infected with fungus of the Saprolegniu group. Histologically, the earliest lesion was an acantholysis of the suprabasal cells of the epidermis developing into transient bullae which rapidly sloughed to produce a raggcd irregular epiderm's. Once the base-ment membrane was breached secondary infection by fungi rapidly ensued.
The histopathology, in the early stages, was consistent with possible viral infection, or anoxic atrophy but it was considered that the most similar lesion from mammalian experi-ence, was that of the Pemphigus group of dermatoses.     

Is topmouth gudgeon Pseudorasbora parva responsible for the decline in sunbleak Leucaspius delineatus populations?

In England, a severe decline of introduced sunbleak Leucaspius delineatus populations has been attributed to the introduction of the invasive topmouth gudgeon Pseudorasbora parva . In France, however, after 4 years of P. parva colonization in a large natural lake, no demonstrated impacts on the native L. delineatus populations have been observed. This suggests that the original impacts observed in England, such as spawning inhibition and high mortality, were the result of an emerging pathogen, the rosette-like agent, hosted by L. delineatus rather than P. parva .     

Serum agglutinating titres against Renibacterium salmoninarum the causative agent of bacterial kidney disease, in rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L.

Serum agglutinating titres against Renibacterium salmoninarum were determined from clinically infected, sub-clinically infected and non-infected rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L. Titres ≥16 occurred in clinically infected rainbow trout, but declined after a few months when disease signs were absent. Non-infected rainbow trout had titres ≤ 16. No correlation between the agglutinating titre and the level of infection was found in farmed Atlantic salmon. Variable titres were found in wild salmon broodstock although there was no evidence of BKD during 4 years of testing.     

Life history and virulence are linked in the ectoparasitic salmon louse Lepeophtheirus salmonis

Models of virulence evolution for horizontally transmitted parasites often assume that transmission rate (the probability that an infected host infects a susceptible host) and virulence (the increase in host mortality due to infection) are positively correlated, because higher rates of production of propagules may cause more damages to the host. However, empirical support for this assumption is scant and limited to microparasites. To fill this gap, we explored the relationships between parasite life history and virulence in the salmon louse, Lepeophtheirus salmonis, a horizontally transmitted copepod ectoparasite on Atlantic salmon Salmo salar. In the laboratory, we infected juvenile salmon hosts with equal doses of infective L. salmonis larvae and monitored parasite age at first reproduction, parasite fecundity, area of damage caused on the skin of the host, and host weight and length gain. We found that earlier onset of parasite reproduction was associated with higher parasite fecundity. Moreover, higher parasite fecundity (a proxy for transmission rate, as infection probability increases with higher numbers of parasite larvae released to the water) was associated with lower host weight gain (correlated with lower survival in juvenile salmon), supporting the presence of a virulence–transmission trade‐off. Our results are relevant in the context of increasing intensive farming, where frequent anti‐parasite drug use and increased host density may have selected for faster production of parasite transmission stages, via earlier reproduction and increased early fecundity. Our study highlights that salmon lice, therefore, are a good model for studying how human activity may affect the evolution of parasite virulence.     

Examining Landscape Determinants of Opisthorchis viverrini Transmission

Liver fluke (Opisthorchis viverrini, O.v.) infection, along with its associated cholangiocarcinoma, is a major public health problem in Southeast Asia. Despite the vast amount of epidemiological research, human O.v. prevalence remains high and varies greatly across the region. This paper examines the landscape determinants that influence O.v. transmission in relation to the three hosts of its life cycle and identifies areas that require further research so as to advance the understanding of the spatial variation in disease risk. A critical agent functionally connects all sequential life cycle stages of O.v. is water. Seasonality and water quality appear to affect the habitats and population dynamics of the two intermediate hosts, Bithynia snails and cyprinid fish. Land use practice through the construction of irrigation ditches increases the connections between the hosts, thereby functionally facilitating the disease transmission. Multi-season sampling data of host infections and habitat characteristics are needed for integration with analyses of landscape connectivity and human behavior to allow better understanding of the interactions among the landscape determinants on the spatial-temporal dynamics of disease transmission.     

A potential role for invasive sunbleak in the further dissemination of a non‐native parasite

Since its first introduction to English waters in the mid 1980s, sunbleak Leucaspius delineatus has spread widely and it is shown in the present study that this non‐native cyprinid is a new host for Neoergasilus japonicus . This copepod parasite from South East Asia has a limited host range and distribution in the U.K. Rapid dispersal of sunbleak in the U.K. raises concerns for further dispersal of this introduced parasite.     

Effects of host migration, diversity and aquaculture on sea lice threats to Pacific salmon populations

Animal migrations can affect disease dynamics. One consequence of migration common to marine fish and invertebrates is migratory allopatry-a period of spatial separation between adult and juvenile hosts, which is caused by host migration and which prevents parasite transmission from adult to juvenile hosts. We studied this characteristic for sea lice (Lepeophtheirus salmonis and Caligus clemensi) and pink salmon (Oncorhynchus gorbuscha) from one of the Canada's largest salmon stocks. Migratory allopatry protects juvenile salmon from L. salmonis for two to three months of early marine life (2-3% prevalence). In contrast, host diversity facilitates access for C. clemensi to juvenile salmon (8-20% prevalence) but infections appear ephemeral. Aquaculture can augment host abundance and diversity and increase parasite exposure of wild juvenile fish. An empirically parametrized model shows high sensitivity of salmon populations to increased L. salmonis exposure, predicting population collapse at one to five motile L. salmonis per juvenile pink salmon. These results characterize parasite threats of salmon aquaculture to wild salmon populations and show how host migration and diversity are important factors affecting parasite transmission in the oceans.     

Cerebrospinal nematodiasis caused by Parelaphostrongylus tenuis in Angora goats in Texas.

A natural infection of meningeal worm (Parelaphostrongylus tenuis) accounted for the death of 11 of 17 (65%) Angora goats in a study in South Texas during 1975. Clinical signs, gross pathology and histopathology in Angoras were similar to other abnormal hosts.     

Comparative susceptibility of Atlantic salmon, lake trout and rainbow trout to Myxobolus cerebralis in controlled laboratory exposures

The susceptibility of lake trout Salvelinus namaycush, rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar to Myxobolus cerebralis, the causative agent of whirling disease, was compared in controlled laboratory exposures. A total of 450 (225 for each dose) fry for each species were exposed to a low (200 spores per fish) or high (2000 spores per fish) dose of the infective triactinomyxon. At 22 wk post-exposure, 60 fish from each group, as well as controls for each species, were examined for clinical signs (whirling behavior, blacktail, deformed heads and skeletal deformities), microscopic lesions, and presence of spores. Rainbow trout were highly susceptible to infection, with 100% being positive for spores and with microscopic pathological changes in both exposure groups. Rainbow trout were the only species to show whirling behavior and blacktail. Atlantic salmon were less susceptible, with only 44 and 61% being positive for spores, respectively, in the low and high dose groups, while 68 and 75%, respectively, had microscopic pathology associated with cartilage damage. Rainbow trout heads contained mean spore concentrations of 2.2 (low dose) or 4.0 (high dose) x 10(6) spores g tissue(-1). The means for positive Atlantic salmon (not including zero values) were 1.7 (low) and 7.4 (high) x 10(4) spores g tissue(-1). Lake trout showed no clinical signs of infection, were negative for spores in both groups and showed no histopathological signs of M. cerebralis infection.     

Salmonella persistence and transmission strategies

Host-adapted strains of Salmonella enterica cause systemic infections and have the ability to persist systemically for long periods of time and pose significant public-health problems. Multidrug-resistant Salmonella enteric serovar Typhi (S. Typhi) and non-Typhoidal Salmonella (NTS) are on the increase, and are often associated with HIV infection. Chronically infected hosts are often asymptomatic and transmit disease to na?ve hosts via fecal shedding of bacteria, thereby serving as a critical reservoir for disease. Salmonella utilizes multiple strategies to evade and modulate host innate and adaptive immune responses in order to persist in the presence of a robust immune response. In addition, the intestinal microbiota plays a critical role in controlling Salmonella infection, disease, and transmissibility.     

Toxoplasmosis in a Pet Peach-Faced Lovebird (Agapornis roseicollis)

Toxoplasma gondii atypical type II genotype was diagnosed in a pet peach-faced lovebird (Agapornis roseicollis) based on histopathology, immunohistochemistry, and multilocus DNA typing. The bird presented with severe neurological signs, and hematology was suggestive of chronic granulomatous disease. Gross post-mortem examination revealed cerebral hemorrhage, splenomegaly, hepatitis, and thickening of the right ventricular free wall. Histologic sections of the most significant lesions in the brain revealed intralesional protozoan organisms associated with malacia, spongiform changes, and a mild histiocytic response, indicative of diffuse, non-suppurative encephalitis. Immunohistochemistry confirmed the causative organisms to be T. gondii. DNA isolated from the brain was used to confirm the presence of T. gondii DNA. Multilocus genotyping based on SAG1, altSAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1, and Apico markers demonstrated the presence of ToxoDB PCR-RFLP genotype #3 and B1 gene as atypical T. gondii type II. The atypical type II strain has been previously documented in Australian wildlife, indicating an environmental transmission route.     

Phylogenetic Evidence of Long Distance Dispersal and Transmission of Piscine Reovirus (PRV) between Farmed and Wild Atlantic Salmon

The extent and effect of disease interaction and pathogen exchange between wild and farmed fish populations is an ongoing debate and an area of research that is difficult to explore. The objective of this study was to investigate pathogen transmission between farmed and wild Atlantic salmon (Salmo salar L.) populations in Norway by means of molecular epidemiology. Piscine reovirus (PRV) was selected as the model organism as it is widely distributed in both farmed and wild Atlantic salmon in Norway, and because infection not necessarily will lead to mortality through development of disease. A matrix comprised of PRV protein coding sequences S1, S2 and S4 from wild, hatchery-reared and farmed Atlantic salmon in addition to one sea-trout (Salmo trutta L.) was examined. Phylogenetic analyses based on maximum likelihood and Bayesian inference indicate long distance transport of PRV and exchange of virus between populations. The results are discussed in the context of Atlantic salmon ecology and the structure of the Norwegian salmon industry. We conclude that the lack of a geographical pattern in the phylogenetic trees is caused by extensive exchange of PRV. In addition, the detailed topography of the trees indicates long distance transportation of PRV. Through its size, structure and infection status, the Atlantic salmon farming industry has the capacity to play a central role in both long distance transportation and transmission of pathogens. Despite extensive migration, wild salmon probably play a minor role as they are fewer in numbers, appear at lower densities and are less likely to be infected. An open question is the relationship between the PRV sequences found in marine fish and those originating from salmon.     

Aquaculture-induced changes to dynamics of a migratory host and specialist parasite: a case study of pink salmon and sea lice

Exchange of diseases between domesticated and wild animals is a rising concern for conservation. In the ocean, many species display life histories that separate juveniles from adults. For pink salmon (Oncorhynchus gorbuscha) and parasitic sea lice (Lepeophtheirus salmonis), infection of juvenile salmon in early marine life occurs near salmon sea-cage aquaculture sites and is associated with declining abundance of wild salmon. Here, we develop a theoretical model for the pink salmon/sea lice host–parasite system and use it to explore the effects of aquaculture hosts, acting as reservoirs, on dynamics. Because pink salmon have a 2-year lifespan, even- and odd-year lineages breed in alternate years in a given river. These lineages can have consistently different relative abundances, a phenomenon termed “line dominance”. These dominance relationships between host lineages serve as a useful probe for the dynamical effects of introducing aquaculture hosts into this host–parasite system. We demonstrate how parasite spillover (farm-to-wild transfer) and spillback (wild-to-farm transfer) with aquaculture hosts can either increase or decrease the line dominance in an affected wild population. The direction of the effect depends on the response of farms to wild-origin infection. If aquaculture parasites are managed to a constant abundance, independent of the intensity of infections from wild to farm, then line dominance increases. On the other hand, if wild-origin parasites on aquaculture hosts are proportionally controlled to their abundance then line dominance decreases.