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Better Definitions and Data Collection Would Improve Biosecurity

EU - Following a request from the European Commission with regard to 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. Jane Jordan, The FishSite Editor, reviews the outcome.

The report, commissioned by the European Food Safety Authority (EFSA) focussed on diseases affecting three different groups of aquatic animals:

Molluscs Crustaceans Fish

The AHAW opinion looked exclusively at finfish diseases and centred on two questions:

1. 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

2. 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 already regulates the introduction of live aquaculture animals of vector species into disease-free areas. It requires that a list of vector species is drawn up. However, no published scientific evidence of the role of non-susceptible 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 the establishment of the hazards was also ranked from negligible to moderate under stated conditions.

However, AHAW says that there is a significant lack of data on prevalence, distribution and infectivity of the listed diseases/agents, as well as pathogen survival parameters outside the host. This contributes to a high degree of uncertainty and could lead to an over estimation of the consideration of aquatic animal species as vectors. It was however, noted that all aquatic animals may potentially act as mechanical vectors for the transfer of listed diseases.

Investigators said that 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.

In response to the question of life stages of susceptible species unable to transmit the listed diseases, the report concludes that any life stage, with the possible exception of eggs, is susceptible to infection.

Intra ovum infection may lead to ‘true’ vertical transmission, but due to absence of evidence, vertical transmission could not be excluded for any of the listed diseases. Surface contamination was also highlighted as a possible casque of egg associated transmission. However, this pathway could be significantly reduced by disinfection.

Research call
The AHAW report recommends further research on possible vertical transmission of listed pathogens. It also says that data collection needs to be improved on the volume and origins of traded eggs and other life stages at a Community level. Further epidemiological studies on the introductory routes of infectious agents would also be valuable.

During the development of the report a number of issues were also identified. These areas were not included in the terms of reference, but there development should be encouraged, say AHAW panelists.

  • 1. 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.

  • 2. 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.

  • 3. 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.

AWAH is urging the European Community and other international organisations, including the OIE (World Animal Health Organisation), adopt harmonised, scientifically-based definitions. Such a policy would ensure a clear differentiation between vectors, susceptible and non-susceptible species, mechanical and biological carrier states, agent and disease, and significantly improve the biosecurity of aquatic-based industries.

Further Reading

- To view the EFSA scientific report and AHAW opinion click here.