Occurrence of rare genotypes of Streptococcus agalactiae in cultured red tilapia Oreochromis sp. and Nile tilapia O. niloticus in Thailand—Relationship to human isolates?
Introduction
In Thailand, intensive fish culture has been established over the past decade. At present, more than ten species of fish are cultured, with Nile tilapia Oreochromis niloticus as the major species produced (Fishery Information Technology Center, 2004). However, the success of aquaculture may be impeded by the prevalence of infectious diseases. Among Gram-positive bacteria, Streptococcus spp. infect sand goby Oxyeleotris marmoratus (Kasornchan et al., 1986) and Asian sea bass Lates calcarifer (Danayadol et al., 2000) causing serious damage to fish farming in Thailand.
Traditional classifications of streptococci have been based on the serogrouping of the carbohydrate antigens of the cell wall and their haemolytic activities. Lancefield serogroup B (GBS) corresponds to a single streptococcal species, S. agalactiae (Evans et al., 2002). GBS has been reported as an important fish (Baya et al., 1990, Duremdez et al., 2004, Evans et al., 2002, Rasheed and Plumb, 1984, Rasheed et al., 1985), bovine (Martinez et al., 2000, Martinez et al., 2001, Meiri-Bendek et al., 2002) and human (Hickman et al., 1999, Ip et al., 2006, Persson et al., 2004, Straková and Motlová, 2004) pathogen. Based on the composition of the capsular polysaccharide, GBS can be divided into ten serotypes (Ia, Ib and II to IX) (Chaffin et al., 2000, Persson et al., 2004, Slotved et al., 2007). Only serotype Ib has been previously reported in fish (Vandamme et al., 1997). In Thailand, GBS infection in fish was reported by our group (Suanyuk et al., 2005) from infected tilapia cultured in Surat Thani province, southern Thailand. Subsequently, mortalities of cultured red tilapia Oreochromis sp. and Nile tilapia O. niloticus showing typical signs of streptococcal infection occurred in earthen ponds and cages throughout the country. Our previous study reported the biochemical characteristics and pathogenicity of these bacteria (Suanyuk et al., 2005). In this study, the distribution of GBS and their genetic diversity were investigated. GBS strains used in this study were isolated from infected tilapia cultured in Thailand including the isolates reported in our previous study.
Section snippets
Fish
Infection chronology, sources of isolation and a list of fish hosts are summarized in Table 1. The first case occurred in 2003 in a stock of red tilapia from a commercial fish farm in Surat Thani province, southern Thailand, and was submitted to Aquatic Animal Health Research Center's laboratory (AAHRC) for disease examination. Other subsequent cases included both red tilapia and Nile tilapia grown in eight provinces in Thailand. During the disease outbreaks, clinical signs in infected fish
Water quality parameters
Water quality parameters during the disease outbreak were as follows; fish samples were from freshwater farm except for the Songkhla sample, which was from brackish water (salinity 1.2%), pH ranged from 7.08–8.15, alkalinity from 61–200 mg/l, hardness from 200–600 mg/l, ammonia from 0–0.2 mg/l, nitrite from 0–0.1 mg/l and temperature from 25–32 °C.
Clinical signs
During April 2003–March 2006, large numbers of Gram-positive cocci in short chains were isolated from infected and moribund tilapia. In naturally
Discussion
In 2003, high mortality of red tilapia and Nile tilapia with typical signs of streptococcosis occurred in fish farms in Surat Thani, southern Thailand. Similar phenomena were observed in additional farms in 2005 and 2006. At the time of sample collection, a mortality rate of up to 30% was observed, affecting very small to large fish weighing from 0.3–500 g, including broodstock. Economic losses as a result of this type of infection were significant.
The taxonomic status of these isolates was
Acknowledgement
This work was supported by Thailand Research Fund, The Royal Golden Jubilee Ph.D. program to Naraid Suanyuk. Thanks to Bureau of the Budget, Thailand for financial support for some part of this work (2005–2006). The authors thank R. L. Thune for his valuable comments in this research work. Thanks also to C. Wanman, K. Duwao, S. Jongyotha, M. Pochatarn, S. Jomsawat, A. Songpradit, H. Kanghear, B. Viriyapongsutee and W. Srijong for their help in collection of samples.
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