Infection of barramundi Lates calcarifer with Streptococcus iniae: effects of different routes of exposure

Abstract

The use of various challenge techniques has allowed the formation of a hypothesis for the mode of infection of Streptococcus iniae in barramundi. A bacterial dose of 1 × 10 3 colony forming units (cfu), corresponding to the LD 50 , delivered orally to barramundi could initiate the sub-acute form of the disease observed at the farms. The acute form of the disease could be initiated through bath exposure to the pathogen. S. iniae was equally as infective in freshwater, saltwater or when fish were subject to skin trauma prior to exposure, with LD 50 values of 3.2 × 10 4 , 2.0 × 10 4 , 3.2 × 10 4 cfu, respectively, when observed over a 10 d period. It is suggested that sub-acute infection occurs orally, with mass mortalities occurring through the increased presence of the bacterium in the environment. KEY WORDS: Barramundi · Streptococcus iniae · Route of infectionResale or republication not permitted without written consent of the publisher Dis Aquat Org 52: 199-205, 2002 bacterial exposure by immersion. Taniguchi (1982) suggested that bacterial infection in cultured yellowtail Seriola quinquinradiata was occurring orally, through being fed infected trash fish. More recently, Evans et al. (2000) suggested that nares infection might be important to the establishment of the disease in striped bass Morone saxtilis.These studies were all dependent on having high levels of Streptococcus iniae present in the environment for infection to take place. During preliminary research to determine the level of culturable S. iniae in the marine environment, it was found that the bacterium rarely reached numbers sufficient to become pathogenic to barramundi (Bromage 1997). This finding suggested that another mode of infection might be important. While barramundi culture relies on processed feeds, we hypothesized that the fish were becoming infected orally through the consumption of infected carrier fish or through the faecal-oral route. This study was devised to test the possible routes of entry of S. iniae into susceptible hosts in the marine environment, and to examine the potential of S. iniae as a pathogen of freshwater cultured barramundi. MATERIALS AND METHODSFish selection and maintenance. Barramundi of 100 ± 5 mm total length were obtained from a farm located outside the endemic area of the pathogen. All experimental trials were performed in 170 l glass aquaria. Each aquarium was filled with seawater (35 parts per thousand, unless otherwise stated) and heated to 27°C.Preparation of bacterial inoculum. All studies were conducted with isolate number 28-97-L-K, isolated in a previous study and known to be pathogenic to barramundi (Bromage et al. 1999). The bacterial culture was grown for 24 h in brain-heart infusion broth (BHI, Oxoid) with continual shaking to provide bacteria in the exponential growth phase. The culture was pelleted by centrifugation (4000 × g) and washed 3 times in phosphate buffered saline (PBS) before being resuspended in PBS until an optical density of 1.0 was obtained at A 600 (absorbance ...

Development of Streptococcus agalactiae vaccines for tilapia

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First Report of Streptococcus Agalactiae and Lactococcus Garvieae From a Wild Bottlenose Dolphin (Tursiops Truncatus)

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