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Necrotizing hepatopancreatitis in Penaeus vannamei
molecular phylogeny and detection of the etiologic agent and pathogenesis in an experimental model of the disease
xiii, 130 leaves : illustrations ; 28 cm.
Necrotizing hepatopancreatitis (NHP) is a severe disease of
farm-raised Pacific white shrimp (Penaeus vannamei) that has
been associated with mortalaties approaching 95%. The
etiology of NHP is a pleomorphic, Gram-negative bacterium.
The NHP bacterium has not been cultivated in vitro and little
is known about the life cycle of the NHP bacterium outside
the shrimp host epithelial cell. In this study, we used the
polymerase chain reaction (PCR) to amplify the gene that
encodes the 16S ribosomal RNA (16S RDNA) of the NHP
bacterium. The amplified 16S RDNA was then cloned and
sequenced. Phylogenetic analysis of the sequenced 16S RDNA
enabled placement of NHP bacterium in the (X-Proteobacteria.
As phylogenetic categories are predictive, the information
that is known about one organism can generally be applied to
its relatives. Using information derived from the
phylogenetic classification.of the NHP bacterium, potential
reservoir hosts and environmental sources of the organism are
currently being investigated. In addition, the 16S RDNA
sequence was used to develop a sensitive and specific PCR
detection assay and a NHP bacterium-specific 16S RDNA probe.
In this study, the PCR assay was used to confirm NHP in
shrimp cultured in Texas and Peru. The DNA probe was tested
in an in situ hybridization assay and was shown to be
efficacious for early detection of the NHP bacterium in
infected shrimp. Using electron microscopy and in situ
hybridization, the temporal pathogenesis of NUP was
investigated. In this study, we demonstrated that the
initial infection most likely occurs through endocytosis of
the NHP bacterium by the epithelial cell and the NHP
bacterium is spread to adjacent hepatopancreatic epithelial
cells through lateral transmission. In this study, we
demonstrated that the NHP bacterium can be degraded by the
shrimp epithelial cell, provided that the organism is
confined to the phagosome. If, however, the NHP bacterium
escapes from the phagosome it can replicate within the
cytoplasm to reach the massive numbers seen in the natural
infections. These findings have important implicafions in
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