Anisakissimplex third stage larvae in Norwegian spring spawning herring (Clupea harengus L.), with emphasis on larval distribution in the flesh

The third stage larvae of the parasitic nematode Anisakis simplex commonly occur in most commercially important fish species of the North Atlantic, including Norwegian spring spawning herring (Clupea harengus L.). The presence of nematode larvae in the flesh of fish may significantly lower the aesthetical quality of the product, or even pose a consumer health risk, especially with regard to the possible allergenic nature of the larvae or molecular traces thereof. In this study, the occurrence and spatial distribution of A. simplex larvae in comparable size groups of Norwegian spring spawning herring caught in the north-eastern Norwegian Sea in October 2004 and in the outer basin of Vestfjorden, northern Norway, in November 2007, was investigated. Emphasis was put on manually- and industrially produced, i.e. automatically trimmed and skinned fillets of herring. The overall larval prevalence was 98–100% in the herring of all size groups and the abundance increased with increasing body weight in both sampling years. On an average 3.5% of the larvae were found in the belly flaps, i.e. the ventral portion of the body musculature covering the visceral cavity on both sides, while 0.5% occurred in the dorsal part of the fillets. The larval prevalence varied from 42 to 70% and 8 to 10% in the manually- and industrially produced fillets, respectively. Thus, any product that is based on industrially produced fillets of Norwegian spring spawning herring may still carry nematode larvae when put on the market. However, compared to the manually produced ones, especially those untrimmed, the probability of A. simplex larvae to be present in industrially produced fillets appears to be approximately 5–8 times lower.