Abstract
Food poisoning of humans, caused by the ingestion of raw flesh of the olive flounder Paralichthys olivaceus infected with Kudoa septempunctata, has recently become a public health concern in Japan. The present study investigated the infection dynamics of K. septempunctata in two cohorts of olive flounder produced in a hatchery, where K. septempunctata infection was enzootic, by PCR assay and light microscopy. In less than 1-year-old juveniles of the 2011 cohort (hatched in February 2011), K. septempunctata was not detected in either June or July 2011 even by conventional PCR, but light microscopy detected a heavy infection (> 1 × 106 spores/g) in October 2011. In 2-year-old fish of the 2009 cohort (hatched in February 2009), the prevalence of infection varied from 30% to 90% from April to December 2011, although no clear pattern was observed in the fluctuation of prevalence and intensity. Fish-to-fish transmission of K. septempunctata was not possible orally or by cohabitation. To investigate the infection period and early development of K. septempunctata, uninfected fish were exposed monthly for 2 weeks to the seawater at the infected hatchery. The results indicated that the peak period of infection was July, and that K. septempunctata was detectable in the heart by quantitative PCR assay as early as 1 week post-exposure, then in the blood and somatic muscle at 2 weeks post-exposure.
