Density and dose: factors affecting mortality of Streptococcus iniae infected tilapia (Oreochromis niloticus)

Fish density and infectious dose have been suspected to affect the mortalit y rate of cultured fish exposed to Streptococcus iniae. We determined the e ffects of S. iniae dose and tilapia (Oreochromis niloticus) density on stre ptococcal disease mortality. Tilapia with a mean weight of 12.7 g were used and maintained at 25 +/- 1 degrees C in aquaria supplied with flow-through water at 0.5 1/min with a 12 h light:12 h dark cycle. Density and dose wer e evaluated by stocking tilapia at low (5.6 g/l), medium (11.2 g/l) and hig h (22.4 g/l) density and administering 2.5 x 10(7) 5x10(7) and 1x10(8) colo ny-forming units (CFU)/ml of S. iniae by immersion (5 tanks per density and dose, 45 total tanks). Mortality was monitored for 28 days post challenge. A significant difference (P < 0.05) was seen in mortality when comparing l ow (4.8%) and medium (28.4%) and low and high (25.6%) density treatments. N o significant difference was observed when comparing medium- and high-densi ty treatments. Two-way analysis of variance demonstrated density had a sign ificant effect on S. iniae mortality (P = 0.0001). Doses had little effect on mortality, except at high density by dose which did show a significant i nteraction (P = 0.001). We have demonstrated density has a significant effe ct on streptococcal disease mortality in tilapia exposed to S. iniae by imm ersion. We also evaluated infection of susceptible tilapia using dead/morib und S. iniae infected fish (i.e., cohabitation by placing five dead/moribun d fish into tanks for 48 h). No significant difference in mortality pattern was observed between immersion in 8.6 x 10(7) CFU/ml S. iniae (37.6% and 3 4.6%) and cohabitation with S. iniae infected tilapia (24.0%). Although, de nsities used were less than in most water-reuse production systems (30-290 g/l), tilapia density of 11.2 g/l and above was an important factor in mort ality of tilapia infected with S. iniae. A health-management strategy would be to reduce fish density thus lowering streptococcal disease mortality. P ublished by Elsevier Science B.V.