Following a request from the European Commission as regards certain fish diseases, the Panel on Animal Health and Welfare (AHAW) was asked to deliver a scientific opinion on possible vector species and live stages of susceptible species not transmitting the diseases. The mandate was accepted by the Panel on Animal Health and Welfare on the 1st of February 2007. The mandate included diseases affecting 3 different groups of aquatic animals: molluscs, crustaceans and fishes. This opinion focuses exclusively on finfish diseases. The mandate was composed of two questions: i) which species may be responsible for the transmission of a specific disease by acting as vector species for a disease listed in Annex IV Part II to Directive 2006/88/EC, and in particular which life stages and under which conditions those species can transmit disease, and, ii) which life stages of the susceptible species listed in column II of the table in Annex IV part II to Directive 2006/88/EC may not transmit the diseases listed in the same annex. Article 17 of the Directive that regulates the introduction of live aquaculture animals of vector species into disease-free areas requires that a list of vector species is drawn up. Such vectors should be: i) aquatic animals, ii) traded for farming purpose, and, iii) non-susceptible to the infection by the listed disease agent. No published scientific evidence of the role of nonsusceptible aquatic animals as vectors in the transmission of the listed diseases was found. Following a qualitative release and exposure assessment, potential species or groups of species and the conditions under which they may act as vectors for listed pathogens were identified. The likelihood of transfer and also the establishment of the hazards ranked from negligible to moderate under stated conditions. Significant lack of data on prevalence, distribution and infectivity of the listed diseases/agents, as well as pathogen survival parameters outside the host, contributed to a high degree of uncertainty about the likelihood estimates of transfer and establishment of the hazards. This could lead to overestimation of the consideration of aquatic animal species as vectors, although it was recognised that all aquatic animals may potentially act as mechanical vectors for the listed diseases. However, it was not possible to consider whether the likelihood of contamination and transmission through aquatic animals acting as vectors exceeded the acceptable level of risk. Although the consequences of establishment were assumed to be high, the actual acceptable level of risk did not form part of the mandate. Concerning the question of life stages of susceptible species not able to transmit the listed diseases it was concluded that any life stage, with the possible exception of eggs, of the listed susceptible species is susceptible to infection. Intra ovum infection may lead to 'true' vertical transmission. Due to absence of evidence, vertical transmission cannot be excluded for any of the listed diseases. In addition, surface contamination may lead to egg associated transmission but such transmission may be significantly reduced by disinfection. Consequently, the following was recommended: i) further research on possible vertical transmission of listed pathogens, ii) better collection of data at the Community level concerning the volume and origins of traded eggs and other life stages, and, iii) epidemiological studies on the introductory routes of infectious agents. During the development of the report a number of issues were identified which were not included in the terms of reference: i) a group of non-listed susceptible species was identified, for which scientific evidence exists and which demonstrates their susceptibility; therefore, they were excluded from the risk assessment as potential vectors. Nevertheless, they clearly represent an un-controlled route of introduction and spread of listed pathogens, ii) given the very wide variation in pathogenicity of the many strains of listed fish viruses, and the wide distribution of many of the non pathogenic ones, there is currently great imprecision and lack of uniformity in the definition of listed aquatic diseases and their causative agents, which creates confusion for pathogen characterization, classification, and diagnostic tools and, iii) trade of ornamental fish has received poor attention given the volume and dynamics of this sector. There is a significant lack of data in relation to trade of live fish both intra community and with third countries, and ornamental fish trade has been associated with the introduction of several serious diseases to farmed aquatic species. Furthermore, it is recommended that the European Community and other international organisations, including the OIE (World Animal Health Organization), use harmonised, scientifically-based definitions. These should ensure a clear differentiation between vectors, susceptible and non-susceptible species, mechanical and biological carrier states, agent and disease, since this is not the case at the moment for aquatic diseases.
