Kasai Akihiro, Tsuduki Hideaki, Jimenez Lea Angsinco, Li Ying-Chun, Tanaka Shuhei, Sato Hiroshi
Laboratory of Parasitology, Joint Faculty of Veterinary Medicine, Yamaguchi University, 1677-1 Yoshida, Yamaguchi, 753-8515, Japan.
Aichi Prefectural Food Inspection Center, Nishi-Kasugai-gun, Toyoyama-cho, Aichi, 480-0202, Japan.
Parasitol Res. 2017 Apr;116(4):1137-1150. doi: 10.1007/s00436-016-5369-7. Epub 2017 Jan 9.
A variety of tunas of the genus Thunnus are consumed daily in Japan as sliced raw fish (sashimi and sushi). The consumption of fresh sliced raw fish, i.e., unfrozen or uncooked, can sometimes cause food poisoning that is manifested by transient diarrhea and vomiting for a single day. One of the causes of this type of food poisoning has been identified as live Kudoa septempunctata (Myxosporea: Multivalvulida) in the olive flounder (Paralichthys olivaceus). Furthermore, raw slices of fresh tunas are highly suspected to be a possible causative fish of similar food poisoning in Japan. In the present study, we conducted a survey of kudoid infections in tunas (the yellowfin tuna Thunnus albacares, the Pacific bluefin tuna Thunnus orientalis, and the longtail tuna Thunnus tonggol) fished in the western Pacific Ocean off Japan and several East Asian countries and characterized morphologically and genetically the kudoid myxospores in pseudocysts or cysts dispersed in the trunk muscles. Pseudocysts of solely Kudoa hexapunctata were identified in the Pacific bluefin tuna (four isolates), whereas in the yellowfin tuna (21 isolates) pseudocysts of Kudoa neothunni and K. hexapunctata were detected at a ratio of 15:6, respectively, in addition to cyst-forming Kudoa thunni in five yellowfin tunas. In the trunk muscles of six longtail tunas examined, pseudocysts of K. neothunni (all six fish) and K. hexapunctata (two fish) were densely dispersed. The myxospores of K. neothunni found in these longtail tunas had seven shell valves and polar capsules (SV/PC) instead of the more common six SV/PC arranged symmetrically. Nucleotide sequences of the 18S and 28S ribosomal RNA gene (rDNA), some with the internal transcribed spacer regions as well, of K. hexapunctata and K. neothunni from the three Thunnus spp., including the seven-SV/PC morphotype, were very similar to previously characterized nucleotide sequences of each species, whereas the 18S and 28S rDNA of four isolates of K. thunni from yellowfin tunas showed a range of nucleotide variations of 99.0-99.9% identity over 1752-1763-bp long partial 18S rDNA and 97.4-99.9% identity over 797-802-bp long partial 28S rDNA. Therefore, this rather high variation of the rDNA nucleotide sequences of K. thunni proved to be contrary to the few variations of K. neothunni and K. hexapunctata rDNA nucleotide sequences. The present study provides a new host record of the longtail tuna for K. neothunni and K. hexapunctata and reveals a high prevalence of the seven-SV/PC myxospore morphotype of K. neothunni in this tuna host.
在日本，人们每天都会食用各种金枪鱼属的金枪鱼，通常是切成薄片的生鱼片（刺身和寿司）。食用新鲜的切片生鱼，即未冷冻或未烹饪的生鱼，有时会导致食物中毒，症状表现为短暂的腹泻和呕吐，持续一天。这种食物中毒的原因之一已被确定为牙鲆鱼（Paralichthys olivaceus）体内的活七斑库道虫（Kudoa septempunctata，粘孢子虫纲：多壳目）。此外，新鲜金枪鱼的生鱼片被高度怀疑是日本类似食物中毒的可能致病鱼类。在本研究中，我们对在日本以西的西太平洋以及几个东亚国家捕捞的金枪鱼（黄鳍金枪鱼Thunnus albacares、太平洋蓝鳍金枪鱼Thunnus orientalis和长尾金枪鱼Thunnus tonggol）进行了库道虫感染调查，并对分散在躯干肌肉中的假囊肿或囊肿内的库道虫粘孢子进行了形态学和遗传学特征分析。在太平洋蓝鳍金枪鱼中仅鉴定出六斑库道虫（Kudoa hexapunctata）的假囊肿（四个分离株），而在黄鳍金枪鱼中（21个分离株），新金枪鱼库道虫（Kudoa neothunni）和六斑库道虫的假囊肿检出率分别为15:6，此外在五条黄鳍金枪鱼中还检测到形成囊肿的金枪鱼库道虫（Kudoa thunni）。在检查的六条长尾金枪鱼的躯干肌肉中，新金枪鱼库道虫（所有六条鱼）和六斑库道虫（两条鱼）的假囊肿密集分布。在这些长尾金枪鱼中发现的新金枪鱼库道虫的粘孢子有七个壳瓣和极囊（SV/PC），而不是更常见的六个对称排列的SV/PC。来自三种金枪鱼属的六斑库道虫和新金枪鱼库道虫的18S和28S核糖体RNA基因（rDNA）的核苷酸序列，有些还包括内部转录间隔区，与先前鉴定的每个物种的核苷酸序列非常相似，而来自黄鳍金枪鱼的四个金枪鱼库道虫分离株的18S和28S rDNA在1752 - 1763 bp长的部分18S rDNA上显示出99.0 - 99.9%的核苷酸同一性范围，在797 - 802 bp长的部分28S rDNA上显示出97.4 - 99.9%的同一性范围。因此，金枪鱼库道虫rDNA核苷酸序列的这种相当高的变异被证明与新金枪鱼库道虫和六斑库道虫rDNA核苷酸序列的少量变异相反。本研究提供了新的宿主记录，表明长尾金枪鱼是新金枪鱼库道虫和六斑库道虫的宿主，并揭示了新金枪鱼库道虫的七SV/PC粘孢子形态型在这种金枪鱼宿主中的高流行率。
