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.
Highlights
Litopenaeus vannamei (L. vannamei) shrimp is the most extensively cultivated species worldwide for its high-yield and low-demand for concentration of salt
With the continual expanding and intensifying of aquaculture, more and more serious viral diseases are emerging such as white spot syndrome virus (WSSV), infectious myonecrosis virus (IMNV) and Penaeus vannamei nodavirus (PvNV)
Histopathological and ultrastructural analysis 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 artificiallyinfected L. vannamei shrimp (Fig. 2)
Summary
Litopenaeus vannamei (L. vannamei) shrimp is the most extensively cultivated species worldwide for its high-yield and low-demand for concentration of salt. IMNV and PvNV were documented to be causative pathogens of ‘‘white tail disease’’ (WTD)-like disease in marine shrimp following the Macrobrachium rosenbergii nodavirus (MrNV) identified in freshwater prawn [1,2,3]. The two viruses both primarily targeted the skeletal muscle and resulted in very similar gross signs (a white or opaque tail) and histopathological changes (focal to extensive areas of muscle necrosis and the formation of prominent lymphoid organ spheroids) in Penaeid shrimp. With increasing global-warming and intensive aquaculture, bacterial diseases especially vibrioses are becoming another important threatening to the sustainable development of the Penaeid shrimp aquaculture industry [5]
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