ARS | Publication request: DENSITY AND DOSE: FACTORS AFFECTING MORTALITY OF STREPTOCOCCUS INIAE INFECTED TILAPIA (OREOCHROMIS NILOTICUS)

Submitted to: European Association of Fish Pathologists
Publication Type: Abstract Only
Publication Acceptance Date: June 14, 1999
Publication Date: September 19, 1999
Citation: SHOEMAKER, C.A., EVANS, J.J., KLESIUS, P.H. DENSITY AND DOSE: FACTORS AFFECTING MORTALITY OF STREPTOCOCCUS INIAE INFECTED TILAPIA (OREOCHROMIS NILOTICUS). EUROPEAN ASSOCIATION OF FISH PATHOLOGISTS. NINTH INTERNATIONAL CONFERENCE "DISEASE OF FISH AND SHELLFISH". 1999.

Technical Abstract: Fish density and infectious dose have been suspected to affect the mortality rate of cultured fish infected with Streptococcus iniae. We determined the effects of S. iniae dose and tilapia density on streptococcus disease mortality. Tilapia with a mean weight of 12.7 g were used and maintained at 25+1oC in aquaria supplied with flow through water at 0.5 L / min with a 12 light: 12 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 (5 tanks per density and dose, 45 total). Three doses of S. iniae were administered by immersion exposure which were equivalent to 1 X 107, 5 X 107 and 1 X 108 CFU / mL. Mortality was monitored for 28 days post challenge. A significant difference (P<0.05) in mortality was seen between low (4.8%) and medium (28.4%) and high (25.6%) density groups. Two-way analysis of variance demonstrated density had a significant effect on S. iniae mortality (P=0.0001). Doses utilized had no effect on mortality. Low density by increasing dose however, did show a significant interaction (P=0.001). We have demonstrated high density has a significant effect on streptococcus disease mortality in tilapia exposed to S. iniae by immersion. The question of how high fish densities favor the transmission of S. iniae is unanswered. We suspect that either increased contact with a greater concentration of S. iniae in water from dead diseased fish or cannibalism may be responsible. Densities used in this experiment were less than those utilized in commercial water-reuse systems (30 - 290 g / L). Management strategies to reduce fish density may significantly enhance production and economic benefit by reducing mortality caused by S. iniae.