2018 Embarrass River Microsporidia

During an epidemiologic survey following a mortality event of freshwater mussels in 2018 in the Embarrass River, Wisconsin, USA, we identified a novel microsporidian parasite in the ovary of mucket (Actinonaias ligamentina), plain pocketbook (Lampsilis cardium), and fatmucket (Lampsilis siliquoidea) (Unioinidae). Histopathology showed round-to-oval microsporidial spores in the cytoplasm of oocytes in 60% (3/5) of mucket, 100% (4/4) of plain pocketbook and 50% (1/2) of fatmucket. On transmission electron microscopy, mature spores were round to oval, measured 4.13 +/- 0.64 µm (3.14–5.31) long by 2.88 +/-0.37 µm (2.36–3.68) wide. Spores had a thin electron dense exospore with a spiky coat, a thick electron lucent endospore, diplokaryotic nuclei, a polar vacuole, and 27–28 polar filaments arranged in 1 to 3 rows. Sequencing of the small subunit rRNA produced a 1356 bp sequence most similar to Pseudonosema cristatellae, and phylogenetic analysis grouped it with freshwater Neopereziida. The morphologic and ultrastructural characteristics did not match those of Pseudonosema sp. and a new genus and species, Hirsutonosema embarrassi n. gen., n. sp., were designated. Additional studies could evaluate host susceptibility, distribution, seasonality, transmission, and lethal or sub-lethal effects of this parasite to freshwater mussels.

2018年美国威斯康星州恩巴拉斯河（Embarrass River）发生淡水贻贝死亡事件后，研究团队在该事件后的流行病学调查中，从珠蚌（Actinonaias ligamentina）、普通荷包蚌（Lampsilis cardium）以及胖珠蚌（Lampsilis siliquoidea）这三种隶属于蚌科（Unionidae）的淡水双壳类的卵巢内，分离得到一株新型微孢子虫寄生虫（microsporidian parasite）。组织病理学检测显示，60%（3/5）的珠蚌、100%（4/4）的普通荷包蚌以及50%（1/2）的胖珠蚌的卵母细胞胞质中，可见圆形至卵圆形的微孢子虫孢子。透射电子显微镜（transmission electron microscopy）观察结果显示，成熟孢子呈圆形至卵圆形，大小为4.13±0.64 μm（长3.14~5.31 μm）、宽2.88±0.37 μm（2.36~3.68 μm）；孢子带有覆有棘状涂层的薄电子致密外孢子膜、厚电子透亮内孢子膜，含双核细胞核与极囊泡，且具有27~28根排列成1~3列的极丝。对小亚基核糖体RNA（small subunit rRNA）进行测序得到1356 bp的序列，该序列与Pseudonosema cristatellae的同源性最高；系统发育分析（phylogenetic analysis）显示其与淡水类Neopereziida聚为一支。该寄生虫的形态学与超微结构特征均与Pseudonosema属物种不符，因此将其定为新属新种：Hirsutonosema embarrassi n. gen., n. sp.。后续研究可进一步评估该寄生虫对淡水贻贝的宿主易感性、分布范围、季节性规律、传播途径，以及致死或亚致死效应。