a Tilapia were passively immunized by i.p. injection with serum from control fish (CON), tryptic soy broth (TSB group), serum from 90 d vaccinated fish (VAX), serum from 90 d vaccinated - 25 d challenged fish (VCH), heat-inactivated serum from 90 d vaccinated fish (HIVAX), or heat-inactivated serum from 90 d vaccinated - 25 d challenged fish (HIVCH).