Avaliação da resistência à estreptococose em diferentes variedades de tilápias

Abstract

Nile tilapia (Oreochromis niloticus) has been always considered disease resistant compared to other fish species; however, it has been shown in recent decades that tilapia is also susceptible to many bacterial, parasitic, fungal and nutritional diseases. In Brazil, streptococcal infection is one of the major diseases that attacks fish causing many economic losses. Thus, this study aimed to evaluate resistance to experimental inoculation with Streptococcus agalactiae in Nile tilapia strains. A resistance test was performed within 180 fish of three commercial strains (VC1, VC2 and VC3, the latter red tilapia - RT) and UFLA strain. Each strain was distributed in five aquariums containing nine fish each, animals from four aquariums were inoculated intraperitoneally with 107 CFU of S. agalactiae per fish, and animals from the remaining aquariums received 0.1 mL of sterile BHI (Brain Heart infusion) as control group. One animal from each strain was sacrificed at 24 h, 48 h, 72 h, 96 h and 360 h after inoculation, to perform serum and tissue evaluations. Survival was, showing significant reduction between UFLA compared to other strains and these did not differ among themselves, through 15 days of challenge. A completely randomized design (CRD) was performed in a factorial scheme (4 strains x 5 times) for lysozyme and antiprotease activity, Scott-Knott and Dunnett tests were practiced to evaluate means and contrasts, respectively, between inoculated animals and non-infected fish at 5% probability. Infected strains showed higher lysozyme activity than non-infected ones (p = 0.001), but did not differ among them. As a function of time after inoculation, there was linear increase (p = 0.001) from 24 h to 72 h, and over 24 h after inoculation, lysozyme activity of inoculated groups was similar to non-infected fish. Subsequently, infected fish exhibited higher activities when compared to uninoculated ones (p<0.05). All infected strains showed similar anti-protease activity to non-infected fish (p = 0.269), but there were differences as a function of the time between inoculated strains (p = 0.042), within first 48 h. Histopathological lesions of gills, spleen and liver were analyzed by Chi-square test, finding differences in the number of events during the challenge between inoculated fish from UF and the other strains, but there were not meaningful differences within presence of the bacteria at spleen immunohistochemistry. These results demonstrate that exist animals with a better response to resistance challenges than others. Lysozyme activity is inversely proportional to the survival rate. However, fish that are not resistant to the pathogen should not be discarded, as these may exhibit other criteria of genetic selection. Nevertheless, understanding different approaches to indirect selection may allow genetic selection programs to be used in field conditions where they could be more prone to disease outbreaks.