EXPERIMENTAL INFECTION OF PACU Piaractus mesopotamicus WITH Aeromonas hydrophila
Pacu (P. mesopotamicus) is a tropical fish endemic to the Paraguay-Paraná River basin (South America) and constitutes one of the most important reared freshwater fish in Brazil. Considering the problems associated with pacu farming, bacterial diseases have an important negative impact due to slow growth and high mortality rates. Bacteria of the genus Aeromonas are common agents in tropical fish farms, and the strains of A. hydrophila are the most frequent isolates from natural infections. This study aimed to determine the survival rate, frequency of bacterial reisolation in tissues and macroscopic clinical changes in pacu experimentally infected with A. hydrophila.
A total of 50 fish of average weight 150 g were distribuited into five groups (n = 10): 1 hour post-inoculation (hpi), 3 hpi, 6 hpi and 9 hpi groups were injected by coelomic route with 0.5 mL of A. hydrophila (1.8 x 109 CFU) and an uninfected control group (buffer saline). At each time point, fish were anesthetized to perform the macroscopic examination of external and internal organs, collecting fragments of spleen, heart, brain, liver and kidney to bacterial reisolation. At the same time, another 28 fish were distribuited into infected and uninfected groups (n = 14), to observe the survival rate.
The macroscopic clinical findings related to infection were more frequent in groups of 6 and 9 hpi, with major injuries found in both external and internal organs: skin bleeding (petechiae and suffusions), subconjunctival hemorrhage, sero-bloody fluid in the coelomic cavity, hepatomegaly, splenomegaly, gut bleeding and adhesions of organs to the walls of the coelom. The frequency of reisolation of A. hydrophila in pacu tissues was higher in all groups challenged with the bacteria compared to control group (FIGURE 1). The survival rate of challenged fishes had decreased to 57.14% five days post-inoculation, while the control group had a survival rate of 100%. These results indicate that the experimental infection by A. hydrophila in pacu leads to bacterial dissemination in several tissues and organs, causing the induction of macroscopic lesions in both internal and external organs and a substantial mortality.
We are also performing further analyses to better understand the physiopathology of A. hydrophila in pacu.