Detection rate of diarrhoea-causing Kudoa hexapunctata in Pacific bluefin tuna Thunnus orientalis from Japanese waters

•Kudoa hexapunctata were detected in 75% of tuna samples with clinical diarrhoea cases.•Kudoa-positive rate in juvenile Thunnus orientalis from Japanese waters was 64.4%.•Kudoa 18S rDNA copies/g in 72.7% of < 5 kg T. orientalis were more than 1 × 109.•K. hexapunctata of 5 × 107 spores were toxic for Caco-2 cells within 2–4 h.

Diffuse outbreaks of food poisoning with unknown aetiologies leading to diarrhoea and vomiting within a short time after ingesting flatfish (Paralichthys olivaceus), tuna (Thunnus spp.), or amberjack (Seriola dumerili) have occurred nationwide in Japan, including the Tokyo metropolitan area. In this study, we surveyed the detection rates of kudoid parasites in 12 tuna samples that caused clinical diarrhoea from 2009 to 2012; we assessed 104 samples of whole juvenile Pacific bluefin tuna (PBT, Thunnus orientalis) and 153 block samples of other tuna distributed in the Tokyo Metropolitan Central Wholesale Market. The survey revealed that more than 70% of clinical diarrhoea cases due to tuna ingestion occurred between June and September, and Kudoa hexapunctata were detected in 9 of 12 tuna samples associated with clinical diarrhoea cases. The numbers of spores and 18S ribosomal DNA (rDNA) copies per gram of fish in 8 of 9 samples were more than 1 × 106 spores and 1 × 109 copies, respectively. Market research revealed that the K. hexapunctata-positive rate in juvenile PBT from Japanese waters was 64.4% (67/104) but that in adult PBT was 10.4% (7/67). The numbers of K. hexapunctata 18S rDNA copies in 64.5% (20/31) samples and 72.7% (16/22) of < 5 kg fish samples collected between May and July were more than 1 × 109 copies/g. On the other hand, kudoid parasites were not detected from 73 tuna samples except for a single sample of Thunnus albacares. Cell monolayer permeability assays performed to examine the toxicity of K. hexapunctata against Caco-2 cells revealed that the transepithelial electrical resistance (TER) in 5 × 107K. hexapunctata spores decreased by 80% within 2–4 h. In conclusion, K. hexapunctata was commonly detected in juvenile PBT from Japanese waters and are a likely cause of the diarrhoea outbreaks.