Differential pathogenicity of fiveStreptococcus agalactiaeisolates of diverse geographic origin in Nile tilapia (Oreochromis niloticusL.)

Abstract

Streptococcus agalactiae is an emerging pathogen of fish and has caused significant morbidity and mortality worldwide. The main objective of this study is to assess whether pathogenic differences exist among isolates from different geographic locations. Nile tilapia (Oreochromis niloticus L.) were administered an intraperitoneal injection of suspension containing USA, Brazil, Honduras, Israel, or Kuwait S. agalactiae isolates at concentrations ranging from 102 to 107 cfu mL−1. The LD50 values 7 days after challenge were as follows: USA (1.0 × 102 cfu mL−1), Brazil (1.5 × 103 cfu mL−1), Honduras (6.8 × 103 cfu mL−1), Israel (1.0 × 104 cfu mL−1) and Kuwait (7.2 × 105 cfu mL−1). Fish from all groups exhibited lethargy, anorexia, exophthalmia, corneal opacity, erratic swimming, petechiae and mortality. Opercular clearing and ascites were only found after infection with certain geographic isolates. The findings in this study indicate that S. agalactiae isolates of different geographic origin can cause significant mortalities after experimental challenge and can have different pathogenic capacities. Isolates from the Americas (USA, Brazil and Honduras) were more pathogenic to Nile tilapia than isolates from the Middle East/Asia (Israel and Kuwait).