Transmission of Necrotizing Hepatopancreatitis in Penaeus vannamei

Abstract

Necrotizing hepatopancreatitis (NHP) has been an important disease of shrimp mariculture in Texas since it was first recognized in 1985. This disease has resulted in mass mortalities of Penaeus vannamei in commercial growout ponds. Though the cause has remained an enigma, results of a recent morphologic study suggested that the etiology was of one or possibly two intracellular bacteria. The predominant organism associated with necrotic hepatopancreatic tubules was a small, pleomorphic rod, while small numbers of a unique helical organism was sometimes also present. In the present study, successful transmission of necrotizing hepatopancreatitis was accomplished in P. vannamei. Shrimp were injected with a suspension of small, pleomorphic, Gram-negative organisms isolated from spontaneously infected farmed shrimp. Experimentally infected shrimp demonstrated characteristic histologic lesions of NHP and intracellular bacteria in hepatopancreatic epithelial cells. Ultrastructurally, moderate numbers of small pleomorphic rods were demonstrated within hepatopancreatic cells of four infected shrimp, while only a single helical organism was identified. The findings indicate that NHP is the result of a bacterial infection and that the small, pleomorphic, Gram-negative bacterium plays a major role in the pathogenesis of the disease. The relationship of the helical organism in development of the disease and its relationship to the pleomorphic bacterium could not be determined from this study. Shrimp maintained for 20 weeks in a tank that had previously held NHP-infected shrimp failed to develop histologic evidence of hepatopancreatic intracellular infection or evidence of disease. The findings suggest that transmission of this intracellular bacterium may involve a reservoir host and/or that it requires specific environmental conditions.