Speaking at a recent conference, Dr Anne-Marie Brady of the VMD said that adhesions (the 'threads' of tissue which form between the body-wall and internal organs, or from organ to organ, in vaccinated fish, and which are an indicator that the vaccine has 'taken') impact on two areas - animal welfare and customer/retail issues.

A necessary evil Vets look to find some sign of adhesions to show that vaccine has taken successfully, but where threads are thick or widely-dispersed, their removal during processing can leave marks on the fillet - and as Martin Cooke of Tesco pointed out to delegates: "If the customer can see it, he won't buy".

At present, the two main tools used for assessing the extent of adhesions are the Speilberg scale (which assigns numerical scores to indicate the extent and location of adhesions, as well as the degree of melanin spotting on the flesh), and the EU monographs, which are based on verbal descriptions rather than numerical scores. EC regulations specify that if 25% of fish sampled score higher than the equivalent of a Speilberg 4 (substantial and widespread adhesions) in vaccine trials, the vaccine is deemed to fail safety tests.

Effective scoring system essential Aquaculture vet Bernt Melgard told the conference that an effective adhesion scoring system is essential, not only to monitor the performance of vaccines, but also to give a tool for predicting the likely extent of adhesions and pigmentation. He argued that the Speilberg scale offers a workable and easily interpreted system. "We normally find that a population will show a spread along the scale. What vets look for is a 1 or a 2," he said. "A zero would indicate that the vaccine had not taken. A few 3's are acceptable, but we wouldn't want to see 4's or above, because with that there are animal welfare issues, as well as commercial implications if fillets are marked, and thus devalued. The situation is complicated by the fact that as the adhesions 'mature', the score may come down."

Scoring has subjective element However, one aspect of adhesion scoring which came over clearly from discussion during the conference, as well as from Bernt Melgard's presentation, was that not every fish vet will agree on whether a particular adhesion represents, for example, a 2 or a 3 on the Speilberg scale. As it stands at present, adhesion scoring is a somewhat subjective science.

"The situation is complicated by the fact that as the adhesions 'mature', the score may come down," said Melgard. However, he remains adamant that the Speilberg scale, with its ease of adaptation to diagrammatic form as scoring sheets, offers the best tool for monitoring adhesions.

Adhesion scoring: a subjective science?