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Histopathological changes in whitish muscles.(A and C) Normal (black arrows) and broken muscle fibers (white arrows). (B and D) Coagulative to liquefactive muscle necrosis and infiltration of a great number of rod-shaped bacteria (black arrows). Tissue A and B were sampled from L. vannamei shrimp with a WTD-like disease in the farm, while tissue C and D were sampled from L. vannamei shrimp laboratory-infected with the V. harveyi strain HLB0905(A: bar = 35 µm; B, C and D: bar = 10 µm).
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pone-0029961-g002: Histopathological changes in whitish muscles.(A and C) Normal (black arrows) and broken muscle fibers (white arrows). (B and D) Coagulative to liquefactive muscle necrosis and infiltration of a great number of rod-shaped bacteria (black arrows). Tissue A and B were sampled from L. vannamei shrimp with a WTD-like disease in the farm, while tissue C and D were sampled from L. vannamei shrimp laboratory-infected with the V. harveyi strain HLB0905(A: bar = 35 µm; B, C and D: bar = 10 µm).

Mentions: Histopathological analysis showed that muscle fibers composing the whitish tail muscle were damaged in different degrees with focal to extensive fiber necrosis in both naturally- and artificially-infected L. vannamei shrimp (Fig. 2). The data indicated that the opaque or whitish appearance of diseased shrimp was due to muscle necrosis. Furthermore, an electron microscopical analysis showed that fiber cells composing the whitish muscle were damaged, including nuclear pyknosis, cell vacuolation, mitochondrial damage and myofibrils damaged in different degrees (Fig. 3). Notably, light and electron microscopical analysis both demonstrated that there were lots of rod-shaped bacteria (Fig. 2B, 3C and 3D) with a flagellum at one end (Fig. 3D) infiltrating in these necrotic muscles, and except for these, there were not any kinds of microorganisms such as viruses and parasites being observed over a large number of ultrathin sections cut consecutively. These analyses suggested that a Vibrio-like bacterium may be associated with the WTD-like disease.

A Nonluminescent and Highly Virulent Vibrio harveyi Strain Is Associated with “Bacterial White Tail Disease” of Litopenaeus vannamei Shrimp

Zhou J, Fang W, Yang X, Zhou S, Hu L, Li X, Qi X, Su H, Xie L - PLoS ONE (2012)

Bottom Line: To differentiate from such diseases as with a sign of "white tail" but of non-bacterial origin, the present disease was named as "bacterial white tail disease (BWTD)".Present study revealed that, just like IMN and WTD, BWTD could also cause mass mortalities in pond-cultured shrimp.These results suggested that some bacterial strains are changing themselves from secondary to primary pathogens by enhancing their virulence in current shrimp aquaculture system.

Affiliation: Key Laboratory of Marine and Estuarine Fisheries Resources and Ecology, East China Sea Fisheries Research Institute, Chinese Academy of Fisheries Science, Shanghai, China. junfangzhou@yahoo.cn

ABSTRACT
Recurrent outbreaks of a disease in pond-cultured juvenile and subadult Litopenaeus vannamei shrimp in several districts in China remain an important problem in recent years. The disease was characterized by "white tail" and generally accompanied by mass mortalities. Based on data from the microscopical analyses, PCR detection and 16S rRNA sequencing, a new Vibrio harveyi strain (designated as strain HLB0905) was identified as the etiologic pathogen. The bacterial isolation and challenge tests demonstrated that the HLB0905 strain was nonluminescent but highly virulent. It could cause mass mortality in affected shrimp during a short time period with a low dose of infection. Meanwhile, the histopathological and electron microscopical analysis both showed that the HLB0905 strain could cause severe fiber cell damages and striated muscle necrosis by accumulating in the tail muscle of L. vannamei shrimp, which led the affected shrimp to exhibit white or opaque lesions in the tail. The typical sign was closely similar to that caused by infectious myonecrosis (IMN), white tail disease (WTD) or penaeid white tail disease (PWTD). To differentiate from such diseases as with a sign of "white tail" but of non-bacterial origin, the present disease was named as "bacterial white tail disease (BWTD)". Present study revealed that, just like IMN and WTD, BWTD could also cause mass mortalities in pond-cultured shrimp. These results suggested that some bacterial strains are changing themselves from secondary to primary pathogens by enhancing their virulence in current shrimp aquaculture system.

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