The Implications of Ranaviruses to European farmed and wild freshwater fish

The present thesis explores the implications of ranaviruses to European farmed and wild freshwater fish. The work presented was carried out as a part of the EU project “Risk assessment of new and emerging systemic iridoviral diseases for European fish and aquatic ecosystems” which was initiated in 2005 as a reaction to the speculation that ranaviruses might pose a serious threat to both farmed and wild-living freshwater fish and amphibians within the European community.

In the present thesis, the purpose is to determine the implications of ranaviruses to European freshwater farmed and wild-living fish. The following specific objectives are addressed: Objective 1: To determine the susceptibility of selected European freshwater fish to a panel of ranaviruses Objective 2: To determine whether ornamental fish are susceptible to or can be carriers of ranaviruses Objective 3: To develop a model describing the risk of introduction and spread of exotic ranaviruses in European wild and farmed aquatic ecosystems Objectives 1 and 2 have been addressed by experimental trials involving bath challenges of both European farmed and wild fish species and ornamental fish species.

The results showed that some fish species are susceptible to some or all of the ranaviruses tested, while other fish species were not found to be susceptible to infection with ranaviruses. Furthermore, virus could be re-isolated from several of the challenged fish even if no significant mortalities were observed.

A separate study demonstrated that European stocks of the two fish species (redfin perch and rainbow trout) listed as susceptible to infection with epizootic haematopoietic necrosis virus did not show significant mortalities in bath challenges. These results are compared to published literature on ranavirus outbreaks and susceptibility trials in fish, and discussed in the context of taxonomy and legislation.

Objective 3 was addressed by using the principles from the OIE risk assessment framework to develop a generic risk model for assessing the risk of introducing exotic ranaviruses into the EU via imports of infected ornamental fish from Asia. In the process of creating the model, a pathway for introduction was identified, and the potential risks of several ranaviruses were evaluated as part of a hazard identification process.

This model used expert opinion to provide estimates of the probabilities of each step in the introduction pathway. The pathway and the hazard identification were important outcomes of this model, together with the estimates of the risk of an exotic ranavirus being introduced into the EU by importation of ornamental fish from Asia.

The risk for farmed fish is considered low, whereas the risk for wildlife is low to medium. The work presented in the present thesis provides valuable information for evaluating the legislation regarding ranaviruses in fish and when considering the taxonomy of ranaviruses in general. The thesis also provides a solid starting point for further investigations into the pathogenicity of ranaviruses to European farmed and wild freshwater fish species and for a risk analysis of the introduction of exotic ranaviruses into the EU.