Diagnostic and pathological studies of epizootic ulcerative syndrome (EUS) outbreak in cultured leucistic Channa argus in Sichuan Province of China
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摘要: 四川地区养殖的白乌鳢 (Channa argus) 在2021年秋冬阴雨季节频繁发生溃疡性死亡事件。为确定白乌鳢溃疡性疾病的原因，通过临床观察、分离培养、组织学和分子生物学等技术开展了研究。大体病理研究发现，患病鱼主要表现为皮肤脱落并暴露皮下肌肉层，肌肉出现深层溃疡的情况；在光镜下，发现肌肉触片中有直径为10~20 μm的无隔膜分枝状菌丝，通过霉菌分离纯化和ITS基因序列分析，鉴定该病原菌为侵入性丝囊霉菌 (Aphanomyces invadans)。组织病理学显示，在皮肤真皮层和肌肉中的溃疡边缘包围着成熟的肉芽肿，而这些肉芽肿可侵袭至肉眼所观察到的正常肌肉处，对病鱼造成进一步的影响。研究结果表明，此次白乌鳢暴发性死亡的原因是侵入性丝囊霉菌引起的流行性溃疡综合征 (Epizootic Ulcerative Syndrome, EUS)，这是关于白乌鳢流行性溃疡综合征的首次报告。研究结果为侵入性丝囊霉菌的鉴定和临床诊断提供了理论参考，也是侵入性丝囊霉菌宿主进一步扩大的证明。Abstract: In order to determine the causes of the frequent occurrence of diseases in leucistic northern snakehead (Channa argus) in Sichuan Province of China during the cloudy, rainy autumn and winter, we carried out the reseach by techniques such as clinical observation, isolation and culture, as well as histology and molecular biology, etc.. Generally, the pathological study shows that the main manifestations of the diseased fish were skin peeling, exposing the subcutaneous muscle layer, and the deep ulcer of the muscle. Moreover, non-septate branching fungal hyphae with a diameter of 10–20 µm were observed in squash preparations under light microscopy. The pathogen was identified as invasive Aphanomyces invadans by isolation and purification and ITS sequence analysis. Histopathology reveals that the ulcer edge in the skin dermis and muscle was surrounded by mature granuloma. These granulomas could invade the normal muscles observed by naked eye and caused a further impact on the diseased fish. The findings indicate that the disease was Epizootic Ulcerative Syndrome (EUS), which is the first report of EUS in leucistic C. argus, providing the basic data for the identification and clinical diagnosis of A. invadans. The findings also suggest that a risk of host expansion of this fungus may exist.
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图 2 患病白乌鳢的大体病变特征
Figure 2. General pathological features of diseased leucistic C. argus
图 3 病鱼肌肉的组织病理学分析
注：a. 真菌入侵后形成的肉芽肿变化，箭头表示被肉芽肿占据的残余肌纤维；b. 肉芽组织入侵肌肉的概况；b'. 肉芽组织入侵皮肤和肌肉示意图，黄色区域表示肉芽组织入侵区域，绿色区域表示正常组织。
Figure 3. Histopathological analysis of muscles of diseased fish
Note: a. Change in granulomas formed after fungal invasion, and the arrows indicate remnant muscle fibres occupied by granulomas; b. Overview of the invasion of muscle by granulation tissue; b'. Schematic illustration of granulation tissue invading skin and muscle. The yellow area indicates the area occupied by granulation tissue and the green area indicates the unoccupied area.
图 5 侵入性丝囊霉菌的形态特征和分子鉴定
注：a. 临床样本的肌肉压片；b. 纯化的真菌菌株；c. 基于菌株bwl1ITS基因序列构建的系统进化树。
Figure 5. Morphological characteristics and molecular identification of A. invadans
Note: a. Muscle compression of clinical samples; b. Purified fungal strains; c. Phylogenetic tree based on ITS gene sequence of the strain bwl1.
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