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Pathogen characterisation and assessment of risks to wild and farmed fish in the UK - FC1188

Ensuring a high health status of wild animal populations is a priority for Defra. This project will employ a range of approaches and tools to improve the scientific basis for aquatic animal disease management in both wild and farmed fish and shellfish populations to facilitate that high health status. Risk assessments are being used increasingly to assess the threat to wild and farmed fish and shellfish populations from current and potential disease threats. However, even whilst conducting risk assessments for diseases that we have been aware of for many years it is apparent that much basic data required for risk assessments do not exist, or are very poor. Also new strains or variants of pathogens arise, which may differ in certain properties from the original strains. The properties of those strains may need to be assessed for risk assessment purposes. One example of where information is lacking or limited in nature is pathogen survival data, which is of vital importance for risk assessments. That was highlighted in a recent report (The European Food Safety Authority Journal [2007] 584: 1-163) considering vector species of the exotic and non-exotic pathogens listed in Council Directive 2006/88/EC (specifying requirements for the prevention and control of certain disease in aquatic animals). The data on survival important pathogens in UK conditions will be reviewed, and any knowledge gaps with respect to strains of the pathogens present or posing an immediate threat to the UK will be determined by experimentation.

In order to assess the impact that those exotic and non-exotic pathogens may have on our native fish species it is important to assess the relative susceptibility of different hosts to those pathogens, including different strains or variants. It is important to determine the characteristics and behaviour of the pathogens in those hosts including development of the disease over time. This will be done by experimental transmission of the pathogens and assessment using histopathology and ultrastructure, pathogen location using methods such as in situ hybridisation and/or immunolabelling. The use of such techniques will provide information on the target tissues of the pathogens, and potentially the location of the pathogen in fish that become carriers of the disease. For some exotic pathogens it may be necessary to develop or adapt challenge models initially using known hosts of the disease, and it will be necessary to ensure that methods are available for identification of the pathogens in those known hosts. An example of the need for continued monitoring of the host range of pathogens is given by viral haemorrhagic septicaemia virus (VHSV), of which there are four genotypes differing in their pathogenicity for marine or freshwater fish. In North America there is one genotype (type IV) which mainly affected marine fish, but in recent years there have been disease episodes with high mortality in a wide range of freshwater fish in the Great Lakes caused by a subtype (type IVb) of that genotype. It will be important to determine the effect of VHSV genotype IVb on our native fish species, particularly as there has been a report that carp is a host of the virus.

Other information already identified as lacking for conducting risk assessments is the minimum infectious dose of virus, and the likelihood that recovered animals will be infected, and a) be a source of re-infection by excretion of virus and b) may be a source of infection as a commodity. The location and quantity of virus is not known with confidence for all pathogens listed in Council Directive 2006/88/EC (specifying requirements for the prevention and control of certain disease in aquatic animals). The infectiousness of a pathogen, as measured by the number of new cases resulting from one infection under different conditions e.g. different age groups, different hosts and different temperatures is also required for risk assessments. Following a literature review, a priority list for conducting experiments will be drawn up and then executed. This will complement work being planned under new proposal C3386 (Commodity products).

The SVCV report produced under Defra contract F1165 suggested further work to include comparison of different diagnostic tests for different purposes e.g. detection of virus in clinical disease or identification of populations of carrier fish. In addition the immunological response of fish to the pathogen needs to be determined under field conditions to complement work conducted under Defra project F1170. Should new SVCV outbreaks occur on sites that will not be disinfected, it is hoped that longitudinal surveys can be undertaken to follow the immunological response of the fish to the virus.

As more risk assessments are conducted and further gaps in our knowledge are identified there will be a requirement for further research. Field collected data are vital for providing information for risk assessments and disease models, but such data may be of limited value because of confounding variables. One of the best ways to control for these is by factorial experimentation so that the effects of each factor can be identified. That research will be undertaken in this project. Priorities for new research under this project will be set in consultation with Defra, and targets for existing work will be revised accordingly.
Project Documents
• EVID4 - Final project report : FC1188 EVID4 Final Report   (403k)
Time-Scale and Cost
From: 2009

To: 2012

Cost: £599,647
Contractor / Funded Organisations
Farmed fish              
Fish Disease              
Risk assessment and management              
Fields of Study
Fish Health and Aquaculture