Outbreak Of Gill Disease Research Articles

Genotyping tool for salmonid gill pox virus (SGPV) obtained from farmed and wild Atlantic salmon (Salmo salar)

Poxviruses are common viruses found in vertebrate species. In 2006, the first poxvirus associated with salmon, salmonid gill poxvirus (SGPV), was identified during an outbreak of gill disease at a smolt production site in northern Norway and at two marine farms in western Norway. Poxviruses had previously been detected in ayu (Plecoglossus altivelis) and koi carp (Cyprinus carpio). In all three fish species, poxviruses are associated with gill disease. It has not been possible to culture SGPV from Norway, and little is known about its virulence. However, the association between SGPV and gill disease in salmon has shown the need for molecular tools to identify reservoirs and transmission routes. Sequencing the genome of a second isolate of SGPV has made it possible to compare variable regions between two strains of the virus, showing the presence of a large number of variable regions that exhibit both variable numbers of tandem repeats and intra-ORF variation. We present eight regions that are suitable for distinguishing strains of SGPV and determining their phylogenetic relationship, and these were used to compare SGPV isolates obtained from both farmed and wild salmon in fresh and sea water. The prevalence of the virus was found to be higher in wild salmon in rivers than in returning wild salmon collected from traps in Norwegian fjords. Genotyping based on the eight selected variable regions, suggests the presence of geographically distinct isolates in freshwater among both farmed and wild salmon, while SGPV from marine farms shows high local diversity and a wide geographical distribution of similar strains of the virus.

‘Snorkel’ lice barrier technology reduced two co- occurring parasites, the salmon louse (Lepeophtheirus salmonis) and the amoebic gill disease causing agent (Neoparamoeba perurans), in commercial salmon sea-cages

Diverse chemical-free parasite controls are gaining status in Atlantic salmon sea-cage farming. Yet, the intricacies of their use at commercial scale, including effects on co-occurring parasites, are seldom reported. A new salmon lice prevention method involves installing a deep net roof and ‘snorkel’ lice barrier in cages to shelter salmon from free-living infective larvae which concentrate at shallow depths, and allows salmon to jump and re- inflate their buoyancy-regulating swim bladder by swallowing air. We document use of snorkel cages (10m deep barrier) in commercial farms, where their effects on salmon lice levels, amoebic gill disease (AGD)-related gill scores, the cage environment, fish welfare and farm management practices were compared to standard cages. During an autumn–winter study involving only snorkel cages, high AGD-related gill scores were observed to decline when freshwater was pumped into snorkels, creating a freshwater surface layer for salmon to enter for self-treatment. In a spring–summer study incorporating snorkel and standard cages, snorkel cages were found to reduce new lice infestations by 84%. The deployment of snorkels and intermittent oxygen depletion detected within them in the spring–summer study did not alter fish welfare parameters. Overall, the results suggest snorkel technology has a place in the toolkit of commercial salmon sea-cage farmers co-managing salmon lice and amoebic gill disease outbreaks − two principal parasite issues facing the industry.

A prospective matched nested case–control study of bacterial gill disease outbreaks in Ontario, Canada government salmonid hatcheries

Early-rearing salmonids in Ontario, Canada government fish hatcheries have been persistently affected by bacterial gill disease (BGD), and outbreaks at these locations have often been associated with high morbidity and mortality. The causative agent of BGD, Flavobacterium branchiophilum, is ubiquitous in fresh water, and outbreaks of BGD are considered to be associated with deleterious environmental conditions. This paper summarizes a 14-month rearing unit-level prospective nested matched case–control investigation at six Ontario government hatcheries (raising a total of six different salmonid species) to identify, and quantify the effects of, important predictors of BGD outbreaks. Ongoing husbandry data were collected on all early-rearing (<9 months of age) fish tank-lots (“tank-lot”=a group of fish from a specific lot existing in a single hatchery tank for a given period during the study time frame) at participating hatcheries, and all outbreaks of BGD were confirmed by light microscopy during the study period. Control tank-lots were selected at the end of the study and matched to individual cases based on time, hatchery, and species. Data were analyzed using logistic regression modeling, controlling for fish age. The final multivariable model indicated that affected tank-lots were significantly more likely to have had lower fish numbers, lower individual fish weights, higher mortality levels and higher feeding rates during the week preceding observed BGD outbreaks than were asymptomatic control tank-lots. Refinements in the observation and manipulation of these factors could therefore aid in the prevention of fish losses associated with observable BGD outbreaks. The predictive (as opposed to causal) nature of the identified factors needs to be considered, and further research is required to understand the relationships between these factors and BGD.

Clubbing and necrosis gill (CNG) syndrome in sea‐caged Atlantic salmon, Salmo salar L., in Tasmania: an initial report

In autumn 1993, an outbreak of gill disease occurred in Atlantic salmon, Salmo salar L., smolts farmed in the Huon estuary in southern Tasmania, Australia. Histologically, the presumptive primary lesion appeared as focal, sometimes full thickness, gill necrosis, which showed no host reaction in the initial stages. Healing of these damaged areas gave rise to conspicuous expanded tips or clubbing of gill filaments. The term clubbing and necrosis gill (CNG) syndrome is suggested to include both these histological features. Fusion of lamellae proximally along the filament, often with vascular dilation, was also a feature of the condition. Similar lesions have been described previously in the literature, but the initial necrosis followed by the very prominent clubbing of filaments was sufficiently different from previous descriptions to warrant designation of this condition as a distinct entity.

Occurrence of Flavobacterium branchiophila associated with bacterial gill disease at a trout hatchery.

A field survey was carried out for four years in order to investigate the occurrence of Flavobacterium branchiophila infection at the Okutama Trout Hatchery affiliated to Tokyo Metropolitan Fisheries Experimental Station. The gills and the skin of fish collected from rearing ponds were examined by indirect fluorescent antibody technique (IFAT), plating method and light microscopy. The water from the fish ponds was also examined.IFAT were more sentitive and specific in detection of F.branchiophila than plating method or light microscopy. F. branchiophila was always associated with bacterial gill disease outbreaks that exhibited typical clinical signs. In other words it was detected from both fish and water during the period of the outbreaks only. Outbreaks of F. branchiophila gill disease occurred intermittently in the same fish population. Therefore the recovered fish appeared not to obtain immunity against the pathogen.

Dermocystidiosis–a gill disease of the carp due to Dermocystidium cyprini n.sp.

A new species of Dermocystidium is reported to be the causative agent of three outbreaks of gill disease in carp observed in recent years in Czechoslovakia. No other parasites or influences could be held responsible for the disease. It is suggested that at least some of gill disease of unknown aetiology in carp is due to dermocystidia, and the disease might be distributed in many regions of Europe. A description of a greater part of the developmental cycle of the species D. cyprini n.sp. Červinka et Lom, is presented, supplemented by a discussion of other species of the genus. A summary of known facts on the host‐parasite relation in these parasites is also given.

Infectious Diseases of Pacific Salmon

Investigations on infectious diseases of Pacific salmon due to micro-organisms other than viruses are reviewed. The etiological agents include trematodes, fungi, protozoa and bacteria. Bacteria have been found to be the most important agents of disease in the several species of Pacific salmon. Kidney disease, due to a small, unnamed Gram-positive diplobacillus, causes serious mortalities in young salmon reared in hatcheries. The disease has also been found in wild fish. Aquatic myxobacteria are important agents of disease both in the hatchery and in the natural habitat. One of the myxobacteria, Chondrococcus columnaris, causes disease at relatively high water temperatures. The problem of the taxonomy of this organism is discussed. Another myxobacterium, Cytophaga psychrophila, has been found responsible for epizootics in coho salmon at lower water temperatures, i.e., in the range of 40° to 55° F. In outbreaks of gill disease in young salmon, myxobacteria of several kinds have been implicated. A variety of bacteria has been found responsible for outbreaks of disease in salmon in sea water. The most important of these is a species of Vibrio. Tuberculosis has been found in adult chinook salmon and the evidence indicates that the disease was contracted at sea.