Bacterial Muscle Necrosis in Cultured Penaeus vannamei in Latin America

A pathology characterized by multifocal reddish patches on the cuticle of shrimp has been reported by shrimp farmers in Latin America. These red patches are commonly associated with small black necrotic spots and white muscle necrosis. One significant impact of this pathology is the reduction in shrimp prices due to the appearance of the animals before and after cooking. The disease is more prevalent in areas with salinities above 5 ppt, with one farm reporting a 45% incidence in one pond. Histopathological examination revealed infiltration of hemocytes in the muscle tissue, melanosis, muscle necrosis, and the presence of Gram-negative rods in the muscle tissue. Given the presence of bacteria and negative polymerase chain reaction (PCR) results for muscle-infecting viruses such as infectious myonecrosis virus (IMNV) and Penaeus vannamei nodavirus (PvNV), this pathology is likely due to bacterial infection, though environmental stressors cannot be ruled out as contributing factors in this complex condition. Several Vibrio strains were isolated from the affected tissue and identified using API 20E and PCR. Two strains were selected for in vivo experimentation: Vibrio parahaemolyticus (C8Cg, PirAB toxin-positive) and V. alginolyticus (C3Ver, PirAB toxin-negative). Shrimp injected with both live and autoclaved bacteria showed similar histopathological changes, suggesting that the pathology is due to virulence factors within the bacterial cells that can still provoke a strong immune or toxic response even in the absence of live, actively replicating bacteria.