Discussion
In the present study, PCR analysis of the stool samples of seven patients and leftover sashimi (prepared from juvenile PBT) samples indicated K. hexapunctata infection. Previous studies have confirmed the presence of K. hexapunctata in both Japanese bluefin and yellowfin tuna, especially in PBT, with a higher rate of infection in juvenile than in adult fish. The K. hexapunctatapositivity rate in juvenile PBT from Japanese waters is generally high from May to July [4]. In the current study, juvenile PBT consumed by the patients were caught in early August, which is proximal to the aforementioned time-frame. Therefore, concurrent with the previous reports, we considered the K. hexapunctatapositivity rate in juvenile PBT consumed by the patients to be high[4]. Surveys of the clinical diarrhea due to tuna ingestion in Tokyo indicated a 5–7-h incubation period until symptom onset [4]. In vitro studies have reported that the time lag until transepithelial electrical resistance (TER) across the Caco-2 cell monolayer decreases by 80% in cell monolayer permeability assays, which are performed to assess K. hexapunctata toxicity in vitro . These results correlated with the incubation period reported in an epidemiological survey[4]. Rapid reduction in TER in an assay using the Caco-2 cell line, which has been widely used as an in vitro model for intestinal transport and enterotoxin studies, indicates the loss of human intestinal epithelial monolayer integrity, which is believed to cause diarrhea.[7]. The incubation period and primary symptom (diarrhea) observed herein are consistent with those reported previously [4]. K. hexapunctatadoes not cause gastrointestinal inflammatory symptoms, unless it is abundantly present in the raw fish consumed [4]. Hence, in the current study, some patients may not have developed gastrointestinal symptoms despite consuming the sashimi. However, few studies have evaluated the cases of food poisoning that are thought to be caused by K. hexapunctata . Therefore, it is necessary to investigate the suspected food poisoning cases caused by ingestion of adult or juvenile PBT.
Symptoms of the patients were resolved within a few days without specific treatments. However, in case of patients with severe symptoms, supportive care (infusion fluids, antiemetics, and antifebrile treatment) may be required. Currently, Japanese cuisine is the second-most popular cuisine worldwide [8]; therefore, further experimental, and epidemiological studies are required to elucidate the pathogenicity of K. hexapunctata . A patient’s history and physical examination usually form the basis of diagnosis for acute gastroenteritis, as culture or blood tests are rarely performed. This increases the chances of underdiagnosis or misdiagnosis of acute gastroenteritis due to parasites, includingK. septempunctata, and the condition is more commonly diagnosed as acute viral gastroenteritis. Thus, it is important to acquire information about the fish used in sashimi and sushi preparation (raw flounder and adult or juvenile PBT) from the patients for clarifying whether their gastrointestinal symptoms (e.g., vomiting and diarrhea) are associated with a Kudoa infection.