Haemorrhagic septicaemia in the hybrid surubim (Pseudoplatystoma corruscans x Pseudoplatystoma fasciatum) caused by Aeromonas hydrophila

Intensive culture of the hybrid surubim (Pseudoplatystoma corruscans x Pseudoplatystoma fasciatum) in Brazil is responsible for the occurrence of diseases and consequent economic losses. However, the causative agents are not well known. The objective of this study was to isolate and to characterize the pathogenic agent responsible for mortalities in cultured surubim and to demonstrate its virulence. Ten fish from a fish farm located in the Mato Grosso do Sul State (Brazil) were collected and 14 haemolytic bacteria characterized as Aeromonas hydrophila were isolated from the kidneys (eight) and brain (six). As an experimental challenge, fish weighing 98.1 +/- 23.6 g were injected with 1 mL of saline solution and 2 x 10(2), 2 x 10(4), 2 x 10(6) and 2 x 10(8) CFU A. hydrophila mL-1. Fish infected with 2 x 10(8) CFU showed increased external and internal symptoms and mortality of 50 +/- 12.5% after 96 h. Increased A. hydrophila concentration was responsible for a decrease in haematocrit percentage and erythrocyte number, lymphocytes and eosinophils, as well as an increase in monocytes, neutrophils, serum agglutination titre and serum antimicrobial activity. It was concluded that A. hydrophila was responsible for characteristic symptoms of bacterial haemorrhagic septicaemia as well as important haematological and immunological alterations, which led to surubim mortality.