津冀地区养殖三疣梭子蟹大量死亡的病原和病理分析

2014年7月,津冀沿海地区池塘养殖三疣梭子蟹(Portunus trituberculatus)暴发疾病,导致大量死亡。患病梭子蟹有两种不同表现症状:一种症状为停止摄食,空胃、空肠,行动迟缓;肌肉白浊、不透明,最后乳化呈牙膏状;该病死亡率达60%~70%,称之为"肌肉白浊病"。另一种症状为壳软,停止摄食,行动迟缓;肌肉变白浊、水样变化、萎缩;体腔液及组织坏死液化后从组织中大量渗出,不凝固;死亡率可达90%以上,称之为"牛奶病"。通过疾病流行特征、组织病理、病原超微结构等分析,初步判断肌肉白浊病由一种微孢子虫感染而致,虫体大小(1544±250)nm,呈椭球形,极丝圈数8~9圈,具孢壁厚(130±22)nm],纤毛(长120~500 nm)、极体(726±200)nm]、极管、固定盘、质膜、核等结构;牛奶病则是由一种血卵涡鞭虫(Hematodinium sp.)感染而致,虫体大小8~10μm,卵圆形,体表多皱褶、凸起,由细胞壁、多核、纤毛等组成。两种患病蟹的肌肉组织病变明显不同:患肌肉白浊病的蟹肌纤维断裂、溃散、浆质化,肌束减少并被团状集聚的寄生虫充斥、占据。电镜观察显示肌纤维细胞破碎、融合,出现大量空泡结构;肌纤维细胞的线粒体、内质网等细胞器解体、消失。患牛奶病的蟹肌束间隙变大,大量血卵涡鞭虫或充斥在间隙中或附着在肌纤维上,导致肌纤维断裂、离散解体,甚至溶解性坏死,组织呈水样变化。电镜观察也证实这两种寄生虫可同时感染三疣梭子蟹个体的同一组织;当肌纤维细胞破碎融合后,两种寄生虫均能寄生在融合后的胞浆中。从发病史来看,肌肉白浊病和牛奶病的发生有蔓延趋势,死亡率趋向更高,已逐步成为三疣梭子蟹养殖的重要疾病。

Etiological and pathological analyses of massive mortality in cultured crab Portunus trituberculatus along the coasts of Tianjin and Hebei, China

In July 2014, the widely cultured pond crabPortunus trituberculatus (commonly called Chinese swim-ming crab) had a disease outbreak along the coasts of Tianjin and Hebei provinces, China, resulting in massive mortalities. Symptoms of two different illnesses appeared in the crabs. One was white muscle disease (WMD), with its symptoms being anorexia, slowed movement, and the muscles turning opaque white and finally emulsify-ing into a toothpaste-like consistency. Mortality among the crabs displaying WMD was 60%?70%. The other was milky disease (MD), which displayed as a softened carapace, cessation of feeding, slowed movement, and the muscles similarly turning opaque white but then becoming watery ('milky') and finally shrinking; the coelomic fluid and the deteriorated tissue leaked from the body and had not coagulated. Mortality among the crabs display-ing MD exceeded 90%. According to the characteristics of the epidemic, and histopathological and ultrastructural analyses, the supposed cause of WMD was infection with a microsporidian. The parasite was ellipsoid, measured (1544±250) nm, and had the following structure: pole filament (8 or 9 laps), spore wall thickness (130±22) nm], cilium (length 120?500 nm), polaroplast (726±200) nm], pole tube, anchoring disk, plasma membrane and nu-cleus. The MD outbreak was caused byHematodinium sp., a monad, sized 8?10μm, with an oval shape, and wrinkled and raised surface; its basic structure was a cell wall, multiple nuclei, cilium and other structures. Histo-pathological examination showed that the two kinds of diseased crabs had significantly different muscle lesions. The crabs associated with WMD had many microsporidia adhering to their muscle fibers, causing the fibers to break, collapse and necrosis, finally reducing them to muscle bundles. SEM observations showed that the muscle cells became fused and acquired a large number of vacuoles, while the mitochondria and endoplasmic reticulum disintegrated and disappeared. In comparison, the gap in the muscle bundles was bigger in cases of MD, in which numerous parasites occupied or adhered to the muscle fibers, resulting in the fibers rupture, disintegration, and resolvable necrosis. Electron microscopy also confirmed that the two different parasites could infect the same tis-sue in the same crab, and both parasites could enter the disintegrated cytoplasm after muscle cell fusion. Since the initial epidemic of these diseases, other occurrences of WMD and MD have tended to cause a higher level of mor-tality, causing concern as significant diseases currently effecting the crab-farming industry.