Factors affecting experimental Streptococcus agalactiae infection in tilapia, Oreochromis niloticus

摘要: Streptococcus agalactiae infection is one of the major disease problems affecting farmed tilapia (Oreochromis niloticus) worldwide. Tilapia are highly susceptible to this which results in mortality up 70% over a period around 7 days and significant economic losses for farmers. Affected commonly present with an irregular behaviour associated meningoencephalitis septicaemia. Currently, factors virulence transmission S. fish including poorly understood. Reports from natural outbreaks on farms have suggested larvae juvenile or smaller than 20 g not susceptible. In addition, there variability individual response experimental inflammatory challenge coping styles (bold, shy) common carp (Cyprinus carpio). The central hypotheses thesis were that weight, age style might affect development progression bacterial disease. This study investigated these three Nile tilapia. A range isolates recovered tilapia, presenting clinical sign streptococcosis during identified characterised as by standard conventional methods, biochemical characteristic tests, Lancefield serogrouping species-specific PCR assay. These Gram-positive cocci, either β- non-haemolytic (γ), non-motile, oxidase negative all serogroup B. they able grow Edwards medium (modified) agar blue colonies growth was observed broth 22 37 oC 0.5-5% NaCl. profiles showed some differences reactions while samples similarities type strain. data confirmed strains group B agalactiae. A model developed LD50 estimated prior performing subsequent studies. Two exposure routes, immersion intraperitoneal injection (i.p.), tested various concentrations agalactiae. Only i.p. produced mortalities (9 × 108 CFU/ml = 48% mortality, 9 107 8 106 26%). dead moribund experiments, where affected similar signs pathology those reported infections. had successfully infected healthy challenges, only 1 died despite testing concentrations, times, stocking density, water system preparations. The studies conducted investigate association between weight susceptibility Nile…