Towards better survival: Modeling drivers for daily mortality in Norwegian Atlantic salmon farming.

Mortality in the production of farmed salmonids is a major constraint to the sustainability of this form of animal husbandry. We have developed a model for the daily mortality in salmon farming over a full production cycle from stocking to harvest, considering different environmental and production factors. These factors included sea temperature, salinity, day within year, fish weight at stocking, stocking day, four types of lice treatments and the possible occurrence of pancreas disease (PD). We considered a generalized additive model following full production cycles, allowing for non-linear descriptions of how relevant factors relate to the daily mortality. We saw a high overall mortality rate immediately after stocking, which decreased the first three months in the cycle and thereafter increased. We found that the total mortality could be reduced by 21% if avoiding all lice treatments, and similarly reduced by 20% if no PD infections occurred. If avoiding jointly PD and all lice treatments, the accumulated mortality could be reduced by 34%. A single thermal or hydrogen peroxide treatment was associated with a mortality of around 1.6% and 1.3%, respectively. This modeling approach gave a unique opportunity to model how different factors interact on the overall global mortality and can easily be extended by other factors, such as additional fish diseases.

The DECIDE project: from surveillance data to decision-support for farmers and veterinarians.

Farmers, veterinarians and other animal health managers in the livestock sector are currently missing sufficient information on prevalence and burden of contagious endemic animal diseases. They need adequate tools for risk assessment and prioritization of control measures for these diseases. The DECIDE project develops data-driven decision-support tools, which present (i) robust and early signals of disease emergence and options for diagnostic confirmation; and (ii) options for controlling the disease along with their implications in terms of disease spread, economic burden and animal welfare. DECIDE focuses on respiratory and gastro-intestinal syndromes in the three most important terrestrial livestock species (pigs, poultry, cattle) and on reduced growth and mortality in two of the most important aquaculture species (salmon and trout). For each of these, we (i) identify the stakeholder needs; (ii) determine the burden of disease and costs of control measures; (iii) develop data sharing frameworks based on federated data access and meta-information sharing; (iv) build multivariate and multi-level models for creating early warning systems; and (v) rank interventions based on multiple criteria. Together, all of this forms decision-support tools to be integrated in existing farm management systems wherever possible and to be evaluated in several pilot implementations in farms across Europe. The results of DECIDE lead to improved use of surveillance data and evidence-based decisions on disease control. Improved disease control is essential for a sustainable food chain in Europe with increased animal health and welfare and that protects human health.

Mortality patterns during the freshwater production phase of salmonids in Norway.

Mortality in Norwegian salmonid aquaculture has a major influence on fish welfare and represents economic losses for producers. We reviewed the estimated monthly mortality for freshwater farms with Atlantic salmon and rainbow trout between 2011 and 2019. We built a regression model for mortality which included the variables year, weight group, season, region and farm. Additionally, we distributed questionnaires to farmers to gather information regarding potential causes of mortalities. The analysis of data for Atlantic salmon showed that median monthly mortality increased from 0.15% in 2011 (interquartile range [IQR]: 0.06-0.39) to 0.25% (IQR: 0.1-0.67) in 2019. Mortality was highest in the North (0.27%, IQR: 0.11-0.72) and lowest in the Southwest region (0.16%, IQR: 0.07-0.4). The season with highest mortality was summer (0.24%, IQR: 0.1-0.64), while winter had the lowest (0.12%, IQR: 0.05-0.35). The smallest fish (3-12 g) showed highest mortality (0.31%, IQR: 0.14-0.69) compared to heavier fish. Results from the questionnaire showed that infectious or non-infectious diseases were the most commonly reported causes of mortality. The mortality patterns described in this study identifies several important risk factors. Highlighting causal links is an important step to reducing mortality and improving welfare in the freshwater production phase of salmonids in Norway.

Monitoring infection with Piscine myocarditis virus and development of cardiomyopathy syndrome in farmed Atlantic salmon (Salmo salar L.) in Norway.

An epidemiological study was carried out in Norway in 2015-2018, investigating the development of infection with Piscine myocarditis virus (PMCV) and development of cardiomyopathy syndrome (CMS) in farmed Atlantic salmon. Cohorts from 12 sites were followed and sampled every month or every other month from sea transfer to slaughter. PMCV was detected at all sites and in all sampled cages, and fish in six sites developed clinical CMS. The initial infection happened between 1 and 7 months post-sea transfer, and the median time from infection with PMCV until outbreak of CMS was 6.5 months. Generally, fish from sites with CMS had higher viral titre and a higher prevalence of PMCV, compared to sites that did not develop clinical CMS. The virus persisted until the point of slaughter at most (11 out of 12) of the sites. The detection of PMCV in all sites suggests that PMCV is more widespread than previously known. Screening for PMCV as a tool to monitor impending outbreaks of CMS must be supported by observations of the health status of the fish and risk factors for development of disease.

Quantitation of ranaviruses in cell culture and tissue samples.

A quantitative real-time PCR (qPCR) based on a standard curve was developed for detection and quantitation of ranaviruses. The target gene for the qPCR was viral DNA polymerase (DNApol). All ten ranavirus isolates studied (Epizootic haematopoietic necrosis virus, EHNV; European catfish virus, ECV; European sheatfish virus, ESV; Frog virus 3, FV3; Bohle iridovirus, BIV; Doctor fish virus, DFV; Guppy virus 6, GV6; Pike-perch iridovirus, PPIV; Rana esculenta virus Italy 282/I02, REV282/I02 and Short-finned eel ranavirus, SERV) were detected with the qPCR assay. In addition, two fish cell lines - epithelioma papulosum cyprini (EPC) and bluegill fry (BF-2) - were infected with four of the isolates (EHNV, ECV, FV3 and DFV), and the viral quantity was determined from seven time points during the first three days after infection. The qPCR was also used to determine the viral load in tissue samples from pike (Esox lucius) fry challenged experimentally with EHNV.

Ranavirus in wild edible frogs Pelophylax kl. esculentus in Denmark.

A survey for the amphibian pathogens ranavirus and Batrachochytrium dendrobatidis (Bd) was conducted in Denmark during August and September 2008. The public was encouraged via the media to register unusual mortalities in a web-based survey. All members of the public that registered cases were interviewed by phone and 10 cases were examined on suspicion of disease-induced mortality. All samples were negative for Bd. Ranavirus was isolated from 2 samples of recently dead frogs collected during a mass mortality event in an artificial pond near Slagelse, Denmark. The identity of the virus was confirmed by immunofluorescent antibody test. Sequencing of the major capsid protein gene showed the isolate had more than 97.3% nucleotide homology to 6 other ranaviruses.

Susceptibility of pike Esox lucius to a panel of Ranavirus isolates.

In order to study the pathogenicity of ranaviruses to a wild European freshwater fish species, pike Esox lucius fry were challenged with the following Ranavirus isolates: epizootic haematopoietic necrosis virus (EHNV), European sheatfish virus (ESV), European catfish virus (ECV), pike-perch iridovirus (PPIV), New Zealand eel virus (NZeelV) and frog virus 3 (FV3). The fry were infected using bath challenge at 12 and 22 degrees C. Significant mortalities were observed at 12 degrees C for EHNV, ESV, PPIV and NZeelV. Background mortality was too high in the experiments performed at 22 degrees C for any conclusions about viral pathogenicity at this temperature to be drawn. Viruses could be re-isolated from samples from all challenged groups, and their presence in infected tissue was demonstrated using immunohistochemistry. The findings suggest that pike fry are susceptible to EHNV, ESV, PPIV and NZeelV and can be a vector for ECV and FV3. Statistical analysis of the factors associated with positive virus re-isolation showed that the number of fish in the sample influenced the outcome of virus re-isolation. Moreover, the likelihood of positive virus re-isolation significantly differed among the 6 viral isolates. The temperature from where the sample was taken and the number of days after infection were not associated with the probability of a positive virus re-isolation.