Occurrence of larval anisakids in horse mackerel (Trachurus trachurus) caught in Portuguese waters.

The third-stage larvae (L3) of the Anisakidae family are parasitic nematodes with zoonotic impact and are frequently encountered in the organs and musculature of various fish intended for human consumption. Since Anisakis simplex (s.s.) and A. pegreffii are the major aetiological agents of human disease, this study aims to combine the morphological and molecular data on the recovered anisakid larvae to contribute to a simplified morphological distinction of those species and conducted a survey of anisakid larvae infection in horse mackerel (Trachurus trachurus). Here, 116 horse mackerel caught in Portuguese waters were analysed for the presence of L3 of anisakids, and 3148 larvae were collected, of which only 30% were retrieved during visual inspection. As such, visual inspection does not appear to be very effective in anisakid detection. A prevalence of 84.5% of infected fish was found, and the mean intensity and mean abundance were 32.1 and 27.1 parasites per fish, respectively. The morphological and molecular analyses of 196 L3 randomly chosen from the total sample of parasites demonstrated the presence of L3 of mostly Anisakis spp., with only one L3 of Hysterothylacium aduncum. Relative frequencies of 62.9% for A. pegreffii and 37.1% for A. simplex (s.s.) were obtained. The morphometry differences between these two sibling species were evaluated, and the results demonstrated significant differences between the length of the ventriculus and the length of the oesophagus. Precisely, A. simplex (s.s.) has a longer oesophagus and ventriculus than A. pegreffii. As such, these differences may be used to distinguish the two species through morphological analysis.