Marine anemia in farmed chinook salmon (Oncorhynchus tshawytscha): Development of a working case definition

Marine anemia (plasmacytoid leukemia) is a histologically defined syndrome affecting farmed chinook salmon (Oncorhynchus tshawytscha) in British Columbia, Canada. Although the disease has received recent attention and has been diagnosed in many fish farms, past criteria for diagnosis have limited the depth and breadth of epidemiological investigations. By identifying a repeatably diagnosable subset of signs and symptoms of affected fish, a new working case definition of marine anemia was developed. This definition relied exclusively on histological features to achieve the diagnosis. The intra-observer repeatability of diagnoses using the new definition was high ( κ = 0.84). An algorithm employing gross pathological signs was created for field investigation of the disease. The algorithm was capable of predicting histological diagnosis with a reasonable level of accuracy, and was judged to be a valuable tool for diagnostic decisions. We concluded that by using the case definition created in this study together with the algorithm, the comparability and accuracy of clinical and epidemiological research of marine anemia could be improved.     

Survey of Salmonid Pathogens in Ocean-Caught Fishes in British Columbia,Canada

Abstract

A survey of wild fishes captured around marine net-pen salmon farms and from open waters for certain salmonid pathogens was conducted in the coastal waters of British Columbia. Viral hemorrhagic septicemia virus was detected in Pacific herring Clupea pallasi, shiner perch Cymatogaster aggregata, and threespine sticklebacks Gasterosteus aculeatus. Infectious hematopoietic necrosis (IHN) virus was detected in one Pacific herring (collected well away from the farms) and in tube-snouts Aulorhynchus flavidus and shiner perch collected from a farm experiencing an IHN outbreak. Renibacterium salmoninarum was observed in moribund Pacific hakes Merluccius productus collected from within a net-pen and was also detected in several ocean-caught salmon. Aeromonas salmonicida subsp. salmonicida (typical strain) was isolated from a juvenile chinook salmon Oncorhynchus tshawytscha, whereas the atypical strain of this organism was isolated from a lingcod Ophiodon elongatus. Loma salmonae (Microsporea) was observed in chinook salmon, chum salmon Oncorhynchus keta, coho salmon O. kisutch, sockeye salmon O. nerka, and pink salmon O. gorbuscha, all of which were captured well away from net-pens. Loma spp. (Microsporea) were observed in the gills of shiner perch, lingcod, Pacific tomcod Microgadus proximus, Pacific cod Gadus macrocephalus, walleye pollock Theragra chalcogramma, and sablefish Anoplopoma fimbria; all but the first species represent new hosts for Loma. Epitheliocystis, caused by a chlamydia-like organism, was detected in the gills of chinook salmon, chum salmon, coho salmon, pink salmon, lingcod, Pacific cod, Pacific hakes, Pacific tomcod, walleye pollock, sablefish, shiner perch, Dover soles Microstomus pacificus, Pacific sanddabs Citharichthys sordidus, and various species of rockfish Sebastes spp., most of which represent new host records for this infection.     

A field evaluation of an indirect immunofluorescent antibody test developed to diagnose plasmacytoid leukemia in chinook salmon (Oncorhynchus tshawytscha).

An immunofluorescent antibody test (IFAT) developed for the diagnosis for plasmacytoid leukemia was evaluated against histology under field conditions. Previously published results from a laboratory evaluation indicated that the IFAT had a much higher sensitivity than did histology. One hundred seventy-seven moribund chinook salmon from 3 farms located in British Columbia were sampled. Sensitivity, specificity and their respective quality indices were estimated for the IFAT relative to histology. The IFAT was shown to be unreliable, particularly with respect to sensitivity. Cohen's kappa was also calculated and revealed that the agreement between the 2 tests was no better than random. In contrast to previously published results the IFAT did not perform better than histology in the presence of bacterial kidney disease. The results emphasize the importance of evaluating tests in the field conditions in which they are to be used. The possible reasons for the shortcomings of the IFAT are discussed.     

The role of fish movements and the spread of infectious salmon anemia virus (ISAV) in Chile, 2007–2009

Infectious salmon anemia virus (ISAV) infection is a constant major threat to farmed and wild Atlantic salmon worldwide. Many epidemics have recently been reported in the most important salmon farming regions of the world, including Chile (2007–2009), where ISAV generated the most important disease and economic crisis in history of the salmon industry of the country. The spread of ISAV within a region is most likely by local or neighborhood spread from an infected farm; however, there is evidence that anthropogenic activities, such as movement of live or harvested fish or their byproduct, may have played a more important role than environmental or passive transmission in the 2007–2009 outbreak. Atlantic salmon farms (n = 421) were retrospectively followed from stocking to harvesting in southern Chile at the time of the ISAV epidemic (2007–2009). The effect of husbandry and spatial risk factors, in addition to contact-network risk factors, which were obtained from the social network analyses, on time to first ISAV infection was estimated using a multivariable Cox proportional hazards model. Five variables were retained in the final fitted model: co-existing multiple generations on a farm (hazard ratio [HR] = 2.585), mean smolt weight at stocking greater than 120 g (HR = 1.165), farm area (per km²) (HR = 1.005), and increased number of shipments entering a farm, i.e. the farm input degree (HR = 1.876) were associated with reduced time to infection; whereas time-to-infection was longer for farms located farther from an ongoing ISAV outbreak (HR = 0.943). It was demonstrated that movements of latently infected fish resulted in approximately 7 outbreaks, and potentially explain about 6% of the total number of cases during the epidemic. Results from this study provide new information about the mechanisms of spread of ISAV in one the largest documented ISAV epidemics in the world. Findings may be used to support the design and implementation of risk-based surveillance and control programs that may help to prevent, detect and control future ISAV outbreaks.     

A New Eye Disease in Pen-Reared Chinook Salmon Caused by Metacestodes of Gilquinia squali (Trypanorhyncha)

Abstract

Eye infections by metacestodes of Gilquinia squali (Cestoda: Trypanorhyncha) were associated with mortality in chinook salmon Oncorhynchus tshawytscha reared in net-pens in seawater at two separate locations in British Columbia. Mortality associated with the disease was about 10% at one site and undetermined at the second site. The metacestode was found in the vitreous humor of moribund fish with eye lesions, whereas healthy fish from the same pen exhibited neither eye lesions nor the infection. The lesions were characterized by retinal folding and apparent retinal duplication, and cataractous changes; in severe cases, the cornea ruptured, and the lens was extruded. The spiny dogfish Squalus acanthias i s the definitive host for the cestode. Spiny dogfish were prevalent in and around the affected net-pen sites, thus providing coracidia to infect the first intermediate host, which is most likely a crustacean. Although the metacestode is common in the eyes of European whiting Merlangius merlangus in the northeast Atlantic region, this is the first report of the infection in a salmonid.     

Hydrographics and the timing of infectious salmon anemia outbreaks among Atlantic salmon (Salmo salar L.) farms in the Quoddy region of Maine, USA and New Brunswick, Canada

Infectious salmon anemia (ISA) has caused severe morbidity and mortality in farmed Atlantic salmon in North America, Norway, Scotland and the Faroe Islands. The Quoddy region of Maine, United States of America (USA), and New Brunswick (NB), Canada is characterized by extensive tidal mixing and close proximity between farms. This region is also prone to recurrent appearances of ISA, though control measures limit disease spread and severity on infected farms. We conducted a retrospective longitudinal analysis of the apparent impact of hydrographics on the incidence and timing of ISA outbreaks on Atlantic salmon (Salmo salar L.) farms in the Quoddy region from May 2002 to August 2004. A time-series cross-sectional regression of 32 farms over 28 months demonstrated a limited, but statistically significant, spatio-temporal clustering of ISA outbreaks linked hydrographically. New outbreaks correlated temporally with those occurring on-site 1 and 3 months prior, and those occurring within one tidal-excursion upstream the same month. Other risk factors included holdover of previous year-class fish, wharf sharing, and possibly harvests of cages infected in previous months. Conclusions suggest that tidal dispersion does play a role in ISAV transmission in the Quoddy region. Dispersal of free virus and/or tidal distribution of lice or other hydrographically influenced vectors or fomites could all contribute to the spatio-temporal patterns described.     

Chinook Salmon Epizootics in Lake Michigan: Possible Contributing Factors and Management Implications

Abstract

Stability of the Lake Michigan fishery for chinook salmon Oncorhynchus tshawytscha at high levels became questionable after stocks declined dramatically following spring epizootics in which bacterial kidney disease (BKD) was a major factor. Initially stocked in 1967, favorable survival and growth of chinook salmon through the 1970s led to increases in abundance and in popularity with anglers. Returns of chinook salmon improved annually until the late–1980s, when, with little warning, spring epizootics reduced the abundance of adult salmon by 50% or more. Reduced abundance of alewives (Alosa pseudoharengus), coupled with an increase in chinook salmon density and heavy parasite infection rates were hypothesized to have reduced chinook salmon growth and fitness and to have increased their susceptibility to BKD. Evidence of slower growth exists and low food availability may be the stressor that triggered the epizootics. Chinook salmon were a major component of the economic development and subsequent hardship of the sportfishing industry on Lake Michigan. Sustaining the chinook salmon fishery at previous levels may require managing for high abundance of alewives, which may be inconsistent with overall fish community management goals. The future sustainability and role of chinook salmon needs to be reevaluated in the context of the entire Lake Michigan fish community.     

Farm-level reproduction number during an epidemic of infectious salmon anemia virus in southern Chile in 2007-2009

An epidemic of infectious salmon anemia virus (ISAV) has greatly impacted salmon production in Chile with devastating social and economic consequences. The epidemic is analyzed here and is likely the largest ISAV outbreak reported affecting one of the most productive regions for salmon farming activities in the world. After re-emerging in 2007, ISAV rapidly expanded the following two years, both in magnitude and geographic range, affecting about 65% and 50% of salmon farms located at the 10th and 11th regions of Chile, respectively. A useful metric for the control of infectious diseases that quantifies the progression of an epidemic is the reproduction number at the farm level (R(f)), which describes the mean number of secondary cases generated by an infectious farm. The parameter in this study was estimated for individual farms (R(fi)), specific phases (R(tf)), and for the entire epidemic (R(f)) by using several analytical approaches based on the characterization of the epidemic curves for the two regions. For the initial spread and the epidemic growth phase, initial and intrinsic growth rates were used to estimate R(tf). In addition, two approaches (epidemic final size and nearest neighbor analyses) were used to obtain an individual (R(fi)) and overall estimate of R(f) for the complete epidemic. In general, two distinct regional patterns of spread were identified. In the 10th region, after an explosive initial spread of ISAV in which R(tf) reached 12.0-16.9, a smaller epidemic growth of 1.6≤R(tf)≥2.5 and a final burnout with R(tf)<1 were observed. For the 11th region, R(tf) only reached 2.4 during the initial spread phase, ranged from 1.6≤R(tf)≥4.4 during the epidemic growth phases and ended when R(tf) was <1.0. The epidemic was characterized by clustering of ISAV 'superspreaders' farms i.e., farms with statistically significantly (P<0.047) higher R(fi) values. Distances between pairs of infected farms were statistically significantly (P=0.003) shorter in the 10th compared to the 11th region. Overall, R(f) ranged from 1.6 to 2.5 and 1.3 to 1.7 in the 10th and 11th regions, respectively. Our findings suggest that control efforts were able to protect 38-60% and 23-41% of the farms in the 10th and 11th regions, respectively, and may have resulted in the epidemic not spreading further. In addition, control strategies in highly populated areas using a control zone of at least 10km radius may be more effective than the 5km zone recommended by the World Animal Health Organization.     

Fish Processing Facilities: New Challenge to Marine Biosecurity in Canada

Abstract

The transmission of pathogens is a common consequence of animal food production. Marine salmon farms and their processing facilities can serve as sources of virulent fish pathogens; our study is the first to confirm the broadcast of a live fish pathogen from a farmed salmon processing facility into the marine waters of Canada's Pacific coast. We found live salmon lice Lepeophtheirus salmonis, mucus, and fish tissue in effluent from the processing facility. Sea lice transmitted from this source may pose a threat to wild salmon populations, and the release of untreated offal, including blood water, is of considerable concern. Further research is needed to quantify the extent to which processing facilities release sea lice and to determine whether more virulent fish pathogens are present in effluent. These data underscore the need for fish farming nations to develop mandatory biosecurity programs to ensure that farmed salmon processing facilities will prevent the broadcast of infectious fish pathogens into wild fish habitat.

Received April 26, 2012; accepted September 7, 2013     

Transmission of Loma salmonae (Microsporea) to chinook salmon in sea water.

Transmission studies were conducted to determine if Loma salmonae was transmissible in sea water. Transmission of L. salmonae to chinook salmon (Oncorhynchus tshawytscha) held in sea water was achieved by exposing fish to macerated, infected gill tissue. Fish were exposed in seawater in a flow-through aquarium, and the infection was detected as soon as 5 wk after exposure. Heavily infected fish exhibited numerous xenomas in the branchial arteries, central venous sinusoids, and within the blood channels of the lamellae. The pathological changes were similar to those seen in pen-reared salmon with L. salmonae infections. The infection was not observed in Atlantic salmon (Salmo salar), Pacific herring (Clupea pallasi, family Clupeidae), or shiner perch (Cymatogaster aggregata, family Embiotocidae), experimentally exposed using identical methods. This study suggests that L. salmonae is transmissible to chinook salmon in seawater netpens. Fish farmers and fish health specialists should consider this possibility when developing and implementing strategies to control the infection.     

Haemobaphes disphaerocephalus, an accidental parasite of seawater pen-reared Atlantic salmon.

Gill infections by the parasitic copepod Haemobaphes disphaerocephalus (family Pennellidae) were observed on a few Atlantic salmon reared at 2 seawater netpen sites in British Columbia. This is the first report of this parasite affecting salmonids. The terminal holdfast penetrated into a branchial artery and was associated with anemia.     

Infectious Salmon Anemia Virus (ISAV) in Brown Trout

Abstract

Infectious salmon anemia (ISA) is a viral disease of Atlantic salmon Salmo salar that have been exposed to seawater in fish farms or hatcheries. This disease was previously believed to be exclusively one of salmon. However, it has been shown that anadromous brown trout Salmo trutta may carry the ISA virus (ISAV). Propagation of the ISAV in brown trout without the trout's showing any gross clinical signs of disease could be a result of a longstanding host-pathogen relationship between the virus and brown trout. A brown trout population isolated from the sea during the last 5,000 years and expected to be naive to the virus was challenged. These fish did not develop any gross signs of disease, but a few ISAVs were present as late as 46 d postchallenge. It was also shown that the ISA virus was present in brown trout as late as 7 months after challenge.     

The Gill Pathogen Dermocystidium salmonis in Oregon Salmonids

Abstract

Intense infections of the gill pathogen Dermocystidium salmonis were associated with mortality of prespawning chinook salmon Oncorhynchus tshawytscha in several Oregon rivers in 1988. The occurrence of the pathogen in returning adult chinook salmon was monitored in several coastal Oregon stocks from 1989 to 1993. Although the prevalence of the pathogen was high in these fish (up to 66.6%), infection intensities were generally low, and no mortality attributable to D. salmonis was observed. In 1988, the pathogen was associated with a lethal epizootic among juvenile chinook salmon smolts at the Trask State Fish Hatchery near Tillamook, Oregon. Histological examination of gills from heavily infected fish revealed hyperplasia of gill epithelium and fusion of gill lamellae. When naturally infected smolts were transferred from fresh to salt water, the most heavily infected fish died within 10 d, and the number of D. salmonis cysts declined and disappeared from previously infected salmon after 21–42 d.     

Infectious Hematopoietic Necrosis Virus (IHNV) in Coho Salmon

Abstract

Infectious hematopoietic necrosis virus (IHNV) causes important losses of chinook salmon Oncorhynchus tshawytscha, sockeye salmon Oncorhynchus nerka, and rainbow trout and steelhead Oncorhynchus mykiss on the west coast of North America. Although coho salmon Oncorhynchus kisutch are considered resistant to IHNV infection, the virus was detected in numerous adult coho salmon returning to Trinity River Hatchery, California, in 1985 and 1986. The virus was isolated from internal organs and ovarian fluids of these fish. Antigenic and structural polypeptides of the viruses were identical in adult coho and chinook salmon collected at the same location. Chinook salmon and rainbow trout alevins exhibited high degrees of susceptibility to IHNV obtained from adult coho and chinook salmon. Coho salmon alevins were resistant to both virus isolants.     

Ranking freshwater fish farms for the risk of pathogen introduction and spread

A semi-quantitative model is presented to rank freshwater rainbow trout farms within a country or region with regards to the risk of becoming infected and spreading a specified pathogen. The model was developed to support a risk-based surveillance scheme for notifiable salmonid pathogens. Routes of pathogen introduction and spread were identified through a process of expert consultation in a series of workshops. The routes were combined into themes (e.g. exposure via water, mechanical transmission). Themes were weighted based on expert opinion. Risk factors for each route were scored and combined into a theme score which was adjusted by the weight. The number of sources and consignments were used to assess introduction via live fish movements onto the farm. Biosecurity measures were scored to assess introduction on fomites. Upstream farms, wild fish and processing plants were included in assessing the likelihood of introduction by water. The scores for each theme were combined to give separate risk scores for introduction and spread. A matrix was used to combine these to give an overall risk score. A case study for viral haemorrhagic septicaemia is presented. Nine farms that represent a range of farming practices of rainbow trout farms in England and Wales are used as worked examples of the model. The model is suited to risk rank freshwater salmonid farms which are declared free of the pathogen(s) under consideration. The score allocated to a farm does not equate to a quantitative probability estimate of the farm to become infected or spread infection. Nevertheless, the method provides a transparent approach to ranking farms with regards to pathogen transmission risks. The output of the model at a regional or national level allows the allocation of surveillance effort to be risk based. It also provides fish farms with information on how they can reduce their risk score by improving biosecurity. The framework of the model can be applied to different production systems which may have other routes of disease spread. Further work is recommended to validate the allocated scores. Expert opinion was obtained through workshops, where the outputs from groups were single point estimates for relative weights of risks. More formal expert opinion elicitation methods could be used to capture variation in the experts' estimates and uncertainty and would provide data on which to simulate the model stochastically. The model can be downloaded (in Microsoft(?)-Excel format) from the Internet at: http://www.cefas.defra.gov.uk/6701.aspx.     

Gill disease in marine farmed Atlantic salmon at four farms in Ireland

A study of four marine salmon farms was undertaken in Ireland in 2008, with a focus on gill health and disease. All four farms suffered severe gill disease resulting in mortalities and, in some cases, failure to thrive. The aetiology of the gill pathologies in some cases was associated with small gelatinous zooplankton and bacteria, but also involved epitheliocystis and parasites such as marine costia (Ichthyobodo species) and amoebae (Neoparamoeba species). Treatments with oral broad-spectrum antibiotics and/or freshwater baths had equivocal benefits. There was a strong association of susceptibility to gill disease with one genetic strain of salmon.     

Use of observed wild bird activity on poultry farms and a literature review to target species as high priority for avian influenza testing in 2 regions of Canada

The risk of avian influenza outbreaks in poultry is partially dependent on the probability of contact between domestic poultry and wild birds shedding avian influenza (AI) virus. The major objective of this study was to document wild bird activity on poultry farms to determine which wild bird species should be targeted for AI surveillance in Canada. We collected data in 2 major poultry producing regions of Canada, southwestern Ontario and the Fraser Valley of British Columbia, on the relative abundance of various wild bird species found on poultry farms and on how these species utilized habitat around poultry farms. We reviewed the published literature to determine what was known about AI pathobiology in the species we observed. From these results we created a list of 10 wild bird species that are a priority for further study. These species are the European starling, barn swallow, rock dove, American crow, northwestern crow, American robin, dark-eyed junco, song sparrow, horned lark, and common grackle. Abundance of these and other species varied between provinces and seasons.     

Development and Evaluation of a Monoclonal-Antibody-Based Enzyme-Linked Immunosorbent Assay for the Diagnosis of Renibacterium salmoninarum Infection

Abstract

A monoclonal-antibody-based enzyme-linked immunosorbent assay (ELISA) was developed for the diagnosis of bacterial kidney disease (BKD). This ELISA can detect Renibacterium salmoninarum antigen at concentrations as low as 0.05–0.1 μg/mL. During the 1988–1989 spawning season, 60 coho salmon Oncorhynchus kisutch, 60 chinook salmon O. tshawytscha, and 60 steelhead O. mykiss (Great Lakes rainbow trout) were caught and screened for BKD with the developed ELISA and a direct fluorescent antibody technique (FAT). Serum agglutination titers for R. salmoninarum were measured to determine any relationship between presence of antigen (R. salmoninarum) and humoral antibody to R. salmoninarum. Twelve of the coho salmon (20%), 48 of the chinook salmon (80%), and 7 of the steelhead (11.7%) were BKD-positive according to the ELISA. Only one steelhead (1.7%) was BKD-positive by FAT, whereas none of the coho salmon or chinook salmon were BKD-positive. It was concluded that the monoclonal-antibodybased ELISA was more sensitive than FAT. Antibody titers of these asymptomatic fish were variable. There was no correlation between antigen level and antibody titer.     

Extensive diversity in New Zealand Dichelobacter nodosus strains from infected sheep and goats

Footrot is a contagious bacterial disease of ruminants spread by the Gram-negative, anaerobic organism, Dichelobacter nodosus. It is endemic in New Zealand and throughout sheep and goat farming regions of the world. Using the polymerase chain reaction (PCR) to amplify fragments of the fimbrial gene (fimA), D. nodosus was detected in 14 hoof scrapings, sampled from six farming regions within New Zealand. DNA sequencing revealed 15 strains covering eight serogroups on the New Zealand farms. The predominant serogroup was B which contained six strains, followed by serogroups F, H and G. No strains from serogroups D and I were detected in this investigation. Eleven out of the 15 D. nodosus strains had fimbriae sequences different to those previously reported and the presence of multiple strains on a single hoof was common (86% samples). Individual sheep from the same farm, or the same paddock, were often infected by a different range of strains, which suggests a host role in mediating footrot infection.