Host Sex as Factor on the Prevalence and Count of Streptococcus spp. in Nile Tilapia (Oreochromis niloticus) During the Dry and Rainy Seasons

The group B streptococci, particularly Streptococcus agalactiae, cause significant mortality and morbidity in a wide variety of freshwater and saltwater fish species in a number of countries worldwide. The possible influence of host sex in the prevalence and count of bacteria are less studied in fishes. A total of 20 Nile tilapia grow-out farms in Lubao, Pampanga, Philippines served as the collection sites during the dry and rainy seasons. The collected tilapia samples were segregated based on sex. Two series of 10-fold dilutions (10-1 and 10-2) of tissue homogenates of tilapia organs (1 g) were separately made in Phosphate-Buffered Saline (PBS). One hundred microliters (100 µL) of the diluted organ homogenate was plated onto a selective Edwards Modified medium. The plates were incubated for 18 to 24 h at 35 to 37°C and blue to colorless colonies of Streptococcus spp. on plates were counted. The prevalence and count of Streptococcus spp. in the different tilapia organs based on sex and season were computed. Although not statistically significant, the results of the study showed that female tilapia were more prone to infect by Streptococcus spp. compared to the male fish because of higher recorded bacterial prevalence in organs such as skin, liver and brain during the dry season and in organs such as skin, liver and foregut during the rainy season. In addition, higher counts of Streptococcus spp. were recorded on the skin, liver and brain of female tilapia samples during both seasons.