The aquaculture disease network model (AquaNet-Mod): A simulation model to evaluate disease spread and controls for the salmonid industry in England and Wales.

Infectious disease causes significant mortality in wild and farmed systems, threatening biodiversity, conservation and animal welfare, as well as food security. To mitigate impacts and inform policy, tools such as mathematical models and computer simulations are valuable for predicting the potential spread and impact of disease. This paper describes the development of the Aquaculture Disease Network Model, AquaNet-Mod, and demonstrates its application to evaluating disease epidemics and the efficacy of control, using a Viral Haemorrhagic Septicaemia (VHS) case study. AquaNet-Mod is a data-driven, stochastic, state-transition model. Disease spread can occur via four different mechanisms, i) live fish movement, ii) river based, iii) short distance mechanical and iv) distance independent mechanical. Sites transit between three disease states: susceptible, clinically infected and subclinically infected. Disease spread can be interrupted by the application of disease mitigation measures and controls such as contact tracing, culling, fallowing and surveillance. Results from a VHS case study highlight the potential for VHS to spread to 96% of sites over a 10 year time horizon if no disease controls are applied. Epidemiological impact is significantly reduced when live fish movement restrictions are placed on the most connected sites and further still, when disease controls, representative of current disease control policy in England and Wales, are applied. The importance of specific disease control measures, particularly contact tracing and disease detection rate, are also highlighted. The merit of this model for evaluation of disease spread and the efficacy of controls, in the context of policy, along with potential for further application and development of the model, for example to include economic parameters, is discussed.

Serology in Finfish for Diagnosis, Surveillance, and Research: A Systematic Review.

Historically, serological tests for finfish diseases have been underused when compared with their use in terrestrial animal health. For years the nonspecific immune response in fish was judged to make serology unreliable and inferior to the direct measurement of agent analytes. We conducted a systematic review of peer-reviewed publications that reported on the development, validation, or application of serological tests for finfish diseases. A total of 168 articles met the screening criteria; most of them were focused on salmonid pathogens (e.g., Aeromonas spp. and viral hemorrhagic septicemia virus). Before the 1980s, most publications reported the use of agglutination tests, but our review indicates that enzyme-linked immunosorbent assay (ELISA) has more recently become the dominant serological test. The main application of serological tests has been in the assessment of vaccine efficacy, with few applications for surveillance or demonstration of freedom from disease, despite the advantages of serological tests over direct detection at the population level. Nonlethal sampling, low cost, and postinfection persistence of antibodies make serological assays the test of choice in surveillance, especially of valuable broodstock. However, their adoption has been constrained by poor characterization and validation. The number of publications in our review reporting diagnostic sensitivity and specificity of serological tests in finfish was small (n = 7). Foreseeing a wider use of serological tests in the future for diagnostic end purposes, we offer recommendations for mitigating deficiencies in the development and evaluation of serological tests, including optimization, control of nonspecific reactions, informed cutoff points, diagnostic accuracy, and serological baseline studies. Achieving these goals will facilitate greater international recognition of serological testing in programs supporting aquatic animal health. Received March 21, 2016; accepted September 24, 2016.

Puffy Skin Disease Is an Emerging Transmissible Condition in Rainbow Trout Oncorhynchus mykiss Walbaum.

The transmission of puffy skin disease (PSD) to rainbow trout Oncorhynchus mykiss Walbaum was tested in the laboratory by conducting co-habitation challenges with puffy skin (PS)-affected fish (Trojans) collected from the field. Two separate challenges were conducted using Trojans sourced from two different sites and diploid (first trial) or triploid (second trial) naïve fish. PSD-specific clinical signs were observed in both groups of naïve fish, with 66% of the fish sampled during the challenges showing signs of varying severity. The first clinical features of PSD were presented as white oval skin patches on one or both flanks 15-21 days post-challenge (dpc). The extent of the lesions ranged from 10 to 90% of the body surface, depending on the severity of the lesion. Both the severity and number of affected fish increased during the challenge. Macroscopically, oedema of the skin and multifocal petechial haemorrhaging were observed towards the end of the trials. Abnormal fish behaviour consisting of "flashing" and excessive mucous production was noted from 15 dpc onwards. Fish with severe PSD lesions also displayed inappetence and associated emaciation. Rodlet cells were observed in 41% of the fresh skin scrapes analysed from the second trial. Histologically epidermal oedema was observed in 31% of the naive fish showing gross pathology, with additional 12% displaying epidermal hyperplasia, mostly observed at the end of the challenge. Other concomitant features of the PSD lesions in challenged fish were epithelial erosion and sloughing, and occasionally mild or focal inflammation. No consistent pathology of internal organs was observed. The parasites Ichthyophthirius multifiliis and Ichthyobodo necator were observed in skin samples of a proportion of naïve challenged fish and in Trojans but not in control fish. The presence of these and other known fish pathogens in the skin of PSD-fish was confirmed by high-throughput sequencing analysis. In summary, we have demonstrated that PSD is a transmissible condition. However, even though a number of known fish pathogens were identified in the skin tissues of PSD-fish, the actual causative infectious agent(s) remain(s) unknown.

Model for ranking freshwater fish farms according to their risk of infection and illustration for viral haemorrhagic septicaemia.

We developed a model to calculate a quantitative risk score for individual aquaculture sites. The score indicates the risk of the site being infected with a specific fish pathogen (viral haemorrhagic septicaemia virus (VHSV); infectious haematopoietic necrosis virus, Koi herpes virus), and is intended to be used for risk ranking sites to support surveillance for demonstration of zone or member state freedom from these pathogens. The inputs to the model include a range of quantitative and qualitative estimates of risk factors organised into five risk themes (1) Live fish and egg movements; (2) Exposure via water; (3) On-site processing; (4) Short-distance mechanical transmission; (5) Distance-independent mechanical transmission. The calculated risk score for an individual aquaculture site is a value between zero and one and is intended to indicate the risk of a site relative to the risk of other sites (thereby allowing ranking). The model was applied to evaluate 76 rainbow trout farms in 3 countries (42 from England, 32 from Italy and 2 from Switzerland) with the aim to establish their risk of being infected with VHSV. Risk scores for farms in England and Italy showed great variation, clearly enabling ranking. Scores ranged from 0.002 to 0.254 (mean score 0.080) in England and 0.011 to 0.778 (mean of 0.130) for Italy, reflecting the diversity of infection status of farms in these countries. Requirements for broader application of the model are discussed. Cost efficient farm data collection is important to realise the benefits from a risk-based approach.

Expert consultation on risk factors for introduction of infectious pathogens into fish farms.

An expert consultation was conducted to provide quantitative parameters required to inform risk-based surveillance of aquaculture holdings for selected infectious hazards. The hazards were four fish diseases endemic in some or several European countries: infectious salmon anaemia (ISA), viral haemorrhagic septicaemia (VHS), infectious haematopoietic necrosis (IHN), and koi herpes virus disease (KHD). Experts were asked to provide estimates for the relative importance of 5 risk themes for the hazard to be introduced into and infect susceptible fish at the destination. The 5 risk themes were: (1) live fish and egg movements; (2) exposure via water; (3) on-site processing; (4) short distance mechanical transmission and (5) distance independent mechanical transmission. The experts also provided parameter estimates for hazard transmission pathways within the themes. The expert consultation was undertaken in a 2 step approach: an online survey followed by an expert consultation meeting. The expert opinion indicated that live fish movements and exposure via water were the major relevant risk themes. Experts were recruited from several European countries and thus covered a range of farming systems. Therefore, the outputs from the expert consultation have relevance for the European context.

Policy, phylogeny, and the parasite.

Animal diseases gain political attention by their inclusion on lists of global bodies such as those of the World Organisation for Animal Health (OIE). Inclusion requires national governments to report outbreaks promptly but may lead to trading restrictions between nations in an attempt to limit spread. Detection therefore has consequences that may have direct impact from farm to state levels. We consider here current approaches to discriminating listed parasites from related but unlisted counterparts. We outline necessary drivers for the discrimination of important taxa and how these may be influenced by national policies. Further, we propose a set of 'best practice' measures, broadly based upon current taxonomic philosophies for protists and metazoans, that should be applied when defining taxa for listing as notifiable.

Investigation of mortality in Pacific oysters associated with Ostreid herpesvirus-1 µVar in the Republic of Ireland in 2009.

High levels of mortality in Pacific oysters Crassostrea gigas in the Republic of Ireland were recorded during the summer of 2009. The new variant of Ostreid herpes 1 (OsHV-1 µVar) which first emerged in France in 2008 was identified from affected stocks. Retrospective data was collected from 70 oyster farmers through an interviewer-administered questionnaire to investigate the distribution and determinants of the mortality. Based on farmer recall, data were recorded at the batch level for cumulative mortality during 2009, start dates and duration of the mortality event and the age of animals affected. Observable mortalities were recorded in 109 out of 346 batches at 47 sites; 104 of the 109 batches were located in bays where OsHV-1 µVar had been detected. The records from bays where OsHV-1 µVar had been detected were analysed to characterize the pattern of mortality and potential risk factors. Batch mortality averaged 37% (18-65% quartiles) but showed a bimodal distribution (half the batches had mortality less than 45%). Mortalities started at the end of May and continued until early August, peaking in early July. On average oysters died over a period of 18 days. Mortality varied considerably both between and within bays. Mortality started in recently introduced batches and occurred later in the summer in established oysters, which is consistent with the introduction of an infectious agent. Mortality was significantly lower in adults compared with other age groups, which supports observations from France. Three variables were significantly (P<0.05) associated, in both bivariate screening and a logistic regression, with high batch-level mortality (>40%): oysters (i) introduced as juveniles, (ii) during or since the winter of 2008/9 and (iii) which spent less than 8h out of water (in a tidal cycle) (compared with oysters introduced as adults before the winter of 2008/9 and spending more than 8h out of water). Twenty-one percent of triploid batches experienced "high" (>40%) mortality compared with 10% for diploid batches which was significant (P<0.05) in the initial bivariate screening but not in the final logistic regression model. Future studies should develop improved methods to assess oyster mortality and follow stocks over time to better determine the influence of management and environmental factors on mortality.

Costs and benefits of freedom from shrimp diseases in the European Union.

The growth in penaeid shrimp aquaculture has been mirrored by the emergence of a number of serious diseases, some of which (e.g. white spot syndrome virus - WSSV) spread rapidly across the globe through movement of infected stock. The World Organisation for Animal Health (OIE) lists six penaeid shrimp pathogens of which three are notifiable in the EU: WSSV (listed as non-exotic to the EU), Taura syndrome virus (TSV) and yellow head disease (YHD) (both listed as exotic). EU Member States (MS) must determine a status for non-exotic diseases (e.g. disease free, unknown, infected). In developing a policy for WSSV, import risk analysis (IRA) can be used to systematically assess the risks of introduction and justify risk mitigation to maintain freedom. OIE guidelines recommend that countries assess the risk of disease introduction via commodities, not listed by the OIE as safe, and apply sanitary measures if necessary. The sanitary measures necessary to maintain freedom from WSSV may not be compatible with current EU animal health legislation. The recent revision by OIE of products listed as safe for international trade strengthens the case for the risks of TSV and YHD introduction into the EU to be assessed. Freedom from WSSV is an important criterion for the development of shrimp aquaculture in the EU. However, in developing disease control policy, governments need to balance the potentially competing interests of all stakeholders, including consumers. Thus economic modelling of the impact of possible sanitary measures on consumer prices of imported products is needed to support decision making. The creation of disease free compartments and post-import risk mitigation for commodities may create the conditions conducive to the development of shrimp aquaculture whilst minimising the costs of maintaining disease freedom.

The application of epidemiology in aquatic animal health -opportunities and challenges.

Over recent years the growth in aquaculture, accompanied by the emergence of new and transboundary diseases, has stimulated epidemiological studies of aquatic animal diseases. Great potential exists for both observational and theoretical approaches to investigate the processes driving emergence but, to date, compared to terrestrial systems, relatively few studies exist in aquatic animals. Research using risk methods has assessed routes of introduction of aquatic animal pathogens to facilitate safe trade (e.g. import risk analyses) and support biosecurity. Epidemiological studies of risk factors for disease in aquaculture (most notably Atlantic salmon farming) have effectively supported control measures. Methods developed for terrestrial livestock diseases (e.g. risk-based surveillance) could improve the capacity of aquatic animal surveillance systems to detect disease incursions and emergence. The study of disease in wild populations presents many challenges and the judicious use of theoretical models offers some solutions. Models, parameterised from observational studies of host pathogen interactions, have been used to extrapolate estimates of impacts on the individual to the population level. These have proved effective in estimating the likely impact of parasite infections on wild salmonid populations in Switzerland and Canada (where the importance of farmed salmon as a reservoir of infection was investigated). A lack of data is often the key constraint in the application of new approaches to surveillance and modelling. The need for epidemiological approaches to protect aquatic animal health will inevitably increase in the face of the combined challenges of climate change, increasing anthropogenic pressures, limited water sources and the growth in aquaculture.

Effect of microbial pathogens on the diversity of aquatic populations, notably in Europe.

The expansion of aquaculture and the demand for ornamental fish have resulted in the large-scale movements of aquatic animals and their pathogens. Here we review the most important non-native fish and shellfish pathogens in European waters and their global impacts on wild fish host populations. The role of theoretical models in the study of the impact of microbial pathogens is discussed, including its integration into risk assessments.

A framework for understanding the potential for emerging diseases in aquaculture.

Numerous diseases have emerged as serious economic or ecological problems in aquaculture species. The combination of factors behind the emergence of each disease is unique, but various common factors are apparent. We combine risk-analysis methods and virulence theory with historical examples (mainly from salmonid production) to identify key disease-emergence risk factors. Diseases have emerged through pathogen exchange with wild populations, evolution from non-pathogenic micro-organisms and anthropogenic transfer of stocks. Aquacultural practices frequently result in high population densities and other stresses (such as intercurrent disease) which increase the risk of infection establishment and spread. As aquaculture expands and new species are farmed, diseases will continue to emerge and affect both wild and farmed fish adversely. The rate and extent of emergence can be reduced by the application of biosecurity programmes designed to mitigate the risk factors for disease emergence.