凡纳滨对虾虾苗细菌性玻化症(BVS)的病原、病理分析

为进一步查明对虾"玻璃苗"的主要致病原,实验通过对河北省沧州市凡纳滨对虾苗种玻化症进行了流行病学调查及病原、病理分析。结果发现,患病虾苗表现为活力降低、厌食直至空肠、空胃,虾体消瘦、暗浊;肝胰腺组织坏死性萎缩、轮廓模糊、颜色变浅呈淡黄色,甚至肝胰腺区由正常的饱满褐色组织变为无组织结构的"玻璃化"状态。组织病理观察结果显示,患病对虾肝小管上皮细胞坏死、脱落,肝小管中充斥大量的碎片组织,并逐步褐化、黑化,甚至肝小管组织大面积坏死,留有连片玻璃样均质化区域;肠道内充斥大量的组织碎片,绒毛膜脱落消失。超微组织病理观察发现,患病对虾肝小管上皮细胞的细胞膜消融,细胞器解体,细胞核固化;其后细胞解体、脱落,甚至肝小管组织结构解体消融;肝胰腺、肠道、胃黏膜周围发现大量细菌,优势菌株为杆状菌且呈弧形,未发现病毒粒子的存在。从患病虾苗分离出2株优势菌(Lv-A和Lv-B),经人工浸染实验发现,Lv-A和Lv-B可致凡纳滨对虾PL7苗种出现与自然患病相同的玻璃化症状,其半致死浓度分别为1.62×10~3和5.38×10~3 CFU/mL,致病力强。根据16S rDNA和gyrB序列分析结果发现,Lv-A和Lv-B与溶藻弧菌、新喀里多尼亚弧菌和副溶血弧菌均有较高相似性。初步将该病命名为虾苗细菌性玻化症(shrimp postlarva bacterial vitrified syndrome,BVS)。药敏实验结果显示,Lv-A和Lv-B均对米诺环素、多西环素、萘啶酸等敏感,而对新霉素、吡哌酸、利福平等耐药。本研究为BVS的有效防控、保障对虾行业健康发展提供理论基础和技术支撑。

Pathogens and histopathological characteristics of shrimp postlarvae bacterial vitrified syndrome (BVS) in the Litopenaeus vannamei

The shrimp postlarva vitrified syndrome broke out in the spring of 2020 and spread explosively along the coastal areas from south to north of China. In this study, the pathogen was isolated and identified, and the histopathology was investigated. The postlarva symptoms included emaciation, dark cloud, decreased activity, anorexia, empty intestinal tract and stomach. The hepatopancreatic was atrophy, blurring of contour, paleness and even vitrified syndrome. Histopathological analysis showed that the epithelial cells of liver tubule were necrotic and exfoliated, liver tubule filled with a large amount of debris tissue, and leaving continuous glassy homogeneous areas. The intestinal tract was filled with tissue fragments, chorionic membrane falls off and even disappears. Ultrastructural pathological examination showed that the membranes of epithelial cells were ablated, organelles disintegrated, and nuclei were solidified. Subsequently, the cells disintegrated, fell off, and even the hepatic tubular tissue structure was ablated. The bacteria were found in the hepatopancreatic, intestinal tract and gastric mucosa. The dominant strain was rod-shaped and curved, and no virions were found. Two dominant bacteria (Lv-A, Lv-B) were isolated from the diseased shrimp postlarva. Artificial infection experiments illustrated that the strains of Lv-A and Lv-B were the causative pathogens with a median lethal dose of 1.62×103 CFU/mL and 5.38×103 CFU/mL, respectively. Based on molecular analyses (16S rDNA and gyrB), Lv-A and Lv-B were high similar to Vibrio alginolyticus, Vibrio neocaledonicus and Vibrio parahaemolyticus, preliminarily named as Shrimp postlarva bacterial vitrified syndrome (BVS). The chemotherapeutant sensitivity tests illustrated that Lv-A and Lv-B was sensitive to minocycline, doxycycline, nalidixic acid, etc. and resistant to neomycin, pipemidic, rifampicin, etc. This study provides theoretical basis and technical support for the effective prevention of BVS and the healthy development of shrimp industry.