Factors affecting experimental Streptococcus agalactiae infection in tilapia, Oreochromis niloticus

Abstract

Streptococcus agalactiae infection is one of the major disease problems affecting farmed tilapia (Oreochromis niloticus) worldwide. Tilapia are highly susceptible to this disease which results in mortality of up to 70% over a period of around 7 days and significant economic losses for farmers. Affected tilapia commonly present with an irregular behaviour associated with meningoencephalitis and septicaemia. Currently, factors affecting the virulence and transmission of S. agalactiae in fish including tilapia are poorly understood. Reports from natural outbreaks of S. agalactiae infection on tilapia farms have suggested larvae and juvenile or fish smaller than 20 g are not susceptible. In addition, there is variability in individual response to experimental inflammatory challenge associated with coping styles (bold, shy) in common carp (Cyprinus carpio). The central hypotheses of this thesis were that weight, age and coping style might affect the development and progression of this bacterial disease. This study investigated these three factors with experimental S. agalactiae infection in Nile tilapia.

A range of bacterial isolates recovered from farmed tilapia, presenting with clinical sign of streptococcosis during natural disease outbreaks were identified and characterised as S. agalactiae by standard conventional methods, biochemical characteristic tests, Lancefield serogrouping and species-specific PCR assay. These isolates were Gram-positive cocci, either β- or non-haemolytic (γ), non-motile, oxidase negative and all of serogroup B. In addition, they were able to grow on Edwards medium (modified) agar as blue colonies and growth was observed in broth from 22 to 37 oC and with 0.5-5% NaCl. The biochemical profiles showed some differences in reactions while all the PCR samples showed similarities to the S. agalactiae type strain. These data confirmed that these strains were identified as group B S. agalactiae.

A challenge model for S. agalactiae in Nile tilapia was developed and the LD50 estimated prior to performing subsequent experimental challenge studies. Two exposure routes, immersion and intraperitoneal injection (i.p.), were tested with various concentrations of S. agalactiae. Only i.p. injection produced significant mortalities (9 × 108 CFU/ml = 48% mortality, 9 × 107 = 48% and 8 × 106 = 26%). Streptococcus agalactiae was recovered and identified from all the dead and moribund fish during these experiments, where affected fish showed similar clinical signs and pathology to those reported from natural S. agalactiae infections. The study results showed that an experimental i.p. challenge model for S. agalactiae infection had successfully infected healthy Nile tilapia. In the immersion challenges, only 1 fish died despite testing a range of bacterial concentrations, exposure times, stocking density, water system and bacterial preparations.

The experimental studies were conducted to investigate the association between weight or age of fish and susceptibility to S. agalactiae infection in Nile tilapia. This was performed under experimental conditions including control groups and a single population of 8 months old fish from one set of parents divided into 7 weight categories. These fish received a single i.p. injection of 6 × 107 CFU/ml of S. agalactiae. Controls and fish of 4 or 8 months old with a mean weight of 5 g received an i.p. injection of 7 × 107 CFU/ml of S. agalactiae. Clinical signs, lesions and histopathological changes in the affected fish were consistent with those reported in natural infection. Streptococcus agalactiae was recovered and identified from all moribund or dead fish. The mortality in the study of different weights varied from 0 to 33% between the groups but the association with weight was weak (R2 = 0.02). In the study of different ages the 4 months old fish group had a total mortality of 24%, and the 8 months old fish group a total mortality of 4%. This study produced no evidence for an association between the weight and susceptibility to S. agalactiae infection but suggested an association between the age or growth rate of fish and this disease.

Different coping styles and susceptibility to S. agalactiae infection in Nile tilapia was examined. Fish were screened and scored depending on their risk-taking behavioural responses to a range of different environmental conditions. Individual differences in behavioural responses were evident but only consistent across behavioural trials for some individuals. A selection of fish with consistent responses across trials was exposed to the 6 × 107 CFU/ml of S. agalactiae by i.p. injection. Fewer bold than shy fish died suggesting that the bold fish might be less susceptible to the infection than shy fish.

In conclusion, this study characterised a number of S. agalactiae isolates and developed an experimental bacterial challenge model. Subsequent experiments suggested that age (or growth rate) and coping style in fish but not the fish weight may affect susceptibility to S. agalactiae infection in Nile tilapia.