Risk assessment for the design of a risk-based surveillance programme for fish farms in Switzerland (in accordance with Council Directive 2006/88/EC of the European Union).

Swiss aquaculture farms were assessed according to their risk of acquiring or spreading viral haemorrhagic septicaemia (VHS) and infectious haematopoietic necrosis (IHN). Risk factors for the introduction and spread of VHS and IHN were defined and assessed using published data and expert opinions. Among the 357 aquaculture farms identified in Switzerland, 49.3% were categorised as high risk, 49.0% as medium risk and 1.7% as low risk. According to the new Directive 2006/88/EC for aquaculture of the European Union, the frequency of farm inspections must be derived from their risk levels. A sensitivity analysis showed that water supply and fish movements were highly influential on the output of the risk assessment regarding the introduction of VHS and IHN. Fish movements were also highly influential on the risk assessment output regarding the spread of these diseases.

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.

The emergence of epitheliocystis in the upper Rhone region: evidence for Chlamydiae in wild and farm salmonid populations

We present the first study comparing epitheliocystis in a wild and farmed salmonid in Europe. Sampling three tributaries to the Lake Geneva, including one from headwaters to river mouth, revealed an unequal distribution of epitheliocystis in brown trout (Salmo trutta). When evaluated histologically and comparing sites grouped as wild versus farm, the probability of finding infected trout is higher on farms. In contrast, the infection intensities, as estimated by the number of cysts per gill arch, were higher on average and showed maximum values in the wild trout. Sequence analysis showed the most common epitheliocystis agents were Candidatus Piscichlamydia salmonis, all clustering into a single clade, whereas Candidatus Clavichlamydia salmonicola sequences cluster in two closely related sub-species, of which one was mostly found in farmed fish and the other exclusively in wild brown trout, indicating that farms are unlikely to be the source of infections in wild trout. A detailed morphological analysis of cysts using transmission electron microscopy revealed unique features illustrating the wide divergence existing between Ca. P. salmonis and Ca. C. salmonicola within the phylum Chlamydiae