Occurrence and prevalence of fish-borne Anisakis larvae in the spotted mackerel Scomber australasicus from Taiwanese waters.

Anisakid nematodes have been found in a variety of marine fishes throughout the world and they are known to cause anisakiasis in human hosts. The present study investigated the prevalence of potentially zoonotic anisakid larvae in spotted mackerel caught from Taiwanese waters where fish represents an important food sources. Anisakis third-stage larvae (L3, n=502) were isolated from 250 spotted mackerel Scomber australasicus. Anisakis L3 larvae were divided morphologically into two types, Anisakis type I larvae had a longer ventriculus and mucron while type II larvae had a shorter ventriculus and no mucron. Anisakis species were identified by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) of the internal transcribed spacer (ITS) regions of ribosomal DNA and direct sequencing. A simple molecular taxonomic key, utilizing RFLP by two restriction enzymes HinfI and HhaI, enabled the differentiation of the genus Anisakis. The prevalence, mean intensity and mean abundance of Anisakis nematodes recorded for the total specimens were 72.8%, 2.8 (1-15) and 2.0 (0-15), respectively. Anisakis pegreffii was determined to be the dominant species (prevalence=57.2%) and important agent of human anisakiasis. A recombinant genotype (Anisakis simplex sensu stricto × A. pegreffii) was identified as the subdominant species (25.3%) followed by Anisakis typica (10%), Anisakis physeteris (4.0%), Anisakis paggiae (3.0%) and Anisakis brevispiculata (0.5%). The topology of the maximum likelihood and neighbor-joining trees show two well supported clades: one includes the species of A. pegreffii and the other includes A. paggiae, A. physeteris and A. brevispiculata, while A. typica has basal position to all other Anisakis spp. analyzed. This study advances our knowledge of the prevalence of different Anisakis spp. in the spotted mackerel from Taiwanese waters, which is helpful for monitoring the fish populations throughout a diverse array of aquatic ecosystems. More importantly, we provide the concise characterization of multiple Anisakis spp. by PCR-RFLP, which could also be applicable for the rapid diagnosis of human anisakiasis.