Over the last six years, a series of food poisoning outbreaks occurred in the Tokyo metropolitan area. Officials linked the outbreaks to consumption of flatfish (Paralichthys olivaceus), Pacific bluefin tuna (PBT) (Thunnus orientalis), or amberjack (Seriola dumerili). As Suzuki et al describe in their recent publication, the sickness caused severe diarrhoea and vomiting within a short time after ingestion of the fish.1
To help determine the direct cause, Japan’s General of Department of Food Safety in the Ministry of Health, Labour, and Welfare investigated these food poisoning incidents and positively identified a Kudoid parasite called Kudoa septempunctata in flatfish samples. However, this pathogen has not previously been identified in tuna, or amberjack.
To gain more insight into these food poisoning cases, Suzuki et al surveyed the detection rates of kudoid parasites in 12 tuna samples that were related to clinical cases of diarrhoea in Tokyo from 2009-2012. They combined this information with a genetic analysis of samples purchased in Tokyo.
The team obtained 104 samples of juvenile PBT ranging in size from 1.5 to 12.7 kg. Samples originated from the Tokyo Metropolitan Central or Tama branch of the Wholesale Market in Japan between April 2011 and December 2013. They also purchased meat parts from 80 adult PBTs/Northern bluefin tuna (T. thynnus), 43 yellowfin tunas (YFT) (T. albacares), 21 Bigeye tuna (T. obesus), and 9 Southern bluefin tunas (T. maccoyii) from the same markets.
The team separated the muscle and internal organs of juvenile PBT. They collected six somatic muscle samples per fish from the ventral, dorsal, and tail parts of the right and left sides of each fish for morphological and molecular examinations. They also collected samples from two points in proportion from the other tuna species. From this, they determined the morphology was consistent with those previously reported for K. hexapunctata.
Suzuki et al. surveyed the morphological characteristics by flattening PBT tissue between two plates and observing the samples under a dissecting microscope. They were able to visualize Kudoa spores from fish somatic muscle tissue in wet mount preparations at ×1000 magnification. They also collected spore samples and visualized them at x 400.
Next, the team isolated DNA and performed Real-Time PCR using primer sets designed to amplify 18S rDNA of kudoid parasites along with the Taqman minor groove binding (MGB) probe (Thermo Fisher Scientific). They then sequenced the PCR products and found the sequences were also consistent with those of K. hexapunctata.
Interestingly, the team found differences in the distribution of K. hexapunctata. Looking at the somatic muscle of juvenile PB compared with adult PBT, they saw homogeneous distribution, (65.5%) in juvenile PBT. In adult PBT; however, the K. hexapunctata-positive rate was markedly lower (10.4%).
Based on the data survey, more than 70% of clinical diarrhoea cases due to tuna ingestion occurred between June and September, with K. hexapunctata detected in 9 out of 12 tuna samples associated with clinical diarrhoea cases. Combining this with the DNA data, the authors were able to conclude that K. hexapunctata in juvenile PBT from was likely to blame for the food poisoning.
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1. Suzuki, J. et al. (2015) “Detection rate of diarrhoea-causing Kudoa hexapunctata in Pacific bluefin tuna Thunnus orientalis from Japanese waters.” International Journal of Food Microbiology Feb 2;194:(pp. 1-6) doi: 10.1016/j.ijfoodmicro.2014.11.001. Epub 2014 Nov 7.