Spleen melanomacrophage centers response of Nile tilapia during Aeromanas hydrophila and Mycobacterium marinum infections

In order to understand the pathophysiology of melanomcrophage centers (MMCs) formation during the tilapia defense response to bacterial infections, the present study evaluated the response, in terms of area, number and pigment constitution, of splenic MMCs of Oreochromis niloticus subjected to intraperitoneal (i.p.) infection with Aeromonas hydrophila and Mycobacterium marinum. Eighty-four fish (396.9 +/- 21.0 g) were randomly distributed into twelve plastic tanks (300 L), to constitute three treatments with 28 animals each: control group (inoculated with PBS); Infected with A. hydrophila (1 x 10(7) UFC mL(-1)); Infected with M. marinum (1 x 10(6) UFC mL(-1)). The spleen was collected in seven fish per treatment on the 3rd, 7th, 14th and 21st day post-infection (DPI). The results revealed the participation of MMCs in the defense response of tilapia during bacterial infection by A. hydrophila and M. marinum, since there was an increase in the number and size of these cell aggregates. Variation of pigment accumulation with significant increase of hemosiderin, in infected tilapias by A. hydrophila, bacteria responsible for causing hemolytic anemia in fish was also found. On the other hand, M. marinum-infected tilapia had high amount of melanin in MMCs. In general, mycobacterial infections are notoriously difficult to treat, being characterized as a chronic disease. These findings demonstrate different strategies of fish response during the evolution of these bacterial diseases.