Field and Experimental Evidence of Vibrio parahaemolyticus as the Causative Agent of Acute Hepatopancreatic Necrosis Disease of Cultured Shrimp (Litopenaeus vannamei) in Northwestern Mexico

Abstract

ABSTRACTMoribund shrimp affected by acute hepatopancreatic necrosis disease (AHPND) from farms in northwestern Mexico were sampled for bacteriological and histological analysis. Bacterial isolates were molecularly identified asVibrio parahaemolyticusby the presence of thetlhgene. Thetdh-negative,trh-negative, andtlh-positiveV. parahaemolyticusstrains were further characterized by repetitive extragenic palindromic element-PCR (rep-PCR), and primers AP1, AP2, AP3, and AP and an ems2 IQ2000 detection kit (GeneReach, Taiwan) were used in the diagnostic tests for AHPND. TheV. parahaemolyticusstrains were used in immersion challenges with shrimp, and farmed and challenged shrimp presented the same clinical and pathological symptoms: lethargy, empty gut, pale and aqueous hepatopancreas, and expanded chromatophores. Using histological analysis and bacterial density count, three stages of AHNPD (initial, acute, and terminal) were identified in the affected shrimp. The pathognomonic lesions indicating severe desquamation of tubular epithelial cells of the hepatopancreas were observed in both challenged and pond-infected shrimp. The results showed that differentV. parahaemolyticusstrains have different virulences; some of the less virulent strains do not induce 100% mortality, and mortality rates also rise more slowly than they do for the more virulent strains. The virulence ofV. parahaemolyticusstrains was dose dependent, where the threshold infective density was 104CFU ml−1; below that density, no mortality was observed. The AP3 primer set had the best sensitivity and specificity. Field and experimental results showed that theV. parahaemolyticusstrain that causes AHPND acts as a primary pathogen for shrimp in Mexico compared with theV. parahaemolyticusstrains reported to date.