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Density and dose: factors affecting mortality of Streptococcus iniae infected tilapia (Oreochromis niloticus)
Shoemaker, C.A.; Evans, J.J.; Klesius, Ph.H. (2000). Density and dose: factors affecting mortality of Streptococcus iniae infected tilapia (Oreochromis niloticus). Aquaculture 188: 229-235
In: Aquaculture. Elsevier: Amsterdam; London; New York; Oxford; Tokyo. ISSN 0044-8486, more
Peer reviewed article  

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  • Shoemaker, C.A.
  • Evans, J.J.
  • Klesius, Ph.H.

    Fish density and infectious dose have been suspected to affect the mortality rate of cultured fish exposed to Streptococcus iniae. We determined the effects of S. iniae dose and tilapia (Oreochromis niloticus) density on streptococcal disease mortality. Tilapia with a mean weight of 12.7 g were used and maintained at 25±1°C in aquaria supplied with flow-through water at 0.5 l/min with a 12 h light: 12 h dark cycle. Density and dose were evaluated by stocking tilapia at low (5.6 g/l), medium (11.2 g/l) and high (22.4 g/l) density and administering 2.5×107, 5×107 and 1×108 colony-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 low (4.8%) and medium (28.4%) and low and high (25.6%) density treatments. No significant difference was observed when comparing medium -and high-density treatments. Two-way analysis of variance demonstrated density had a significant 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 interaction (P=0.001). We have demonstrated density has a significant effect on streptococcal disease mortality in tilapia exposed to S. iniae by immersion. We also evaluated infection of susceptible tilapia using dead/moribund S. iniae infected fish (i.e., cohabitation by placing five dead/moribund fish into tanks for 48 h). No significant difference in mortality pattern was observed between immersion in 8.6×107 CFU/ml S. iniae (37.6% and 34.6%) and cohabitation with S. iniae infected tilapia (24.0%). Although, densities 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 mortality of tilapia infected with S. iniae. A health-management strategy would be to reduce fish density thus lowering streptococcal disease mortality.

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