Prevalence, risk factors and transmission of streptococcus agalactiae in the red hybrid tilapia (Oreochromis Sp.)

Outbreak of Streptococcus agalactiae infection in fish was first reported in Malaysia in late 1990s in Pahang river, Pahang, affecting floating net cage-cultured red tilapia (Oreochromis sp.). Since then, outbreaks of S. agalactiae infection in tilapia have been widespread, covering almost all parts of Peninsular Malaysia. Presently, S. agalactiae infection has become a leading disease that has severe economic impact to the tilapia farming industry not only in this country but all over the world. Therefore, concrete understanding on the epidemiology of this disease is essential in order to control and prevent the infection. This study was conducted to understand the prevalence, clinical signs and pathological changes, risk factors from water quality and transmissions of S. agalactiae in the red hybrid tilapia (Oreochromis sp.) floating net cage culture system.The prevalence of S. agalactiae in red hybrid tilapia cultured in different types of water bodies was investigated for a period of 24 months. The study was conducted at five types of water bodies consisting of two huge-sized reservoirs (Kenyir Lake, Terengganu and Pedu Lake, Kedah), a moderate-sized Terengganu river with four sampling sites along the river (Beladau Selat, Beladau Kepong, Pantai Ali and Kuala Kejir, Terengganu), a small-sized pond (Jitra, Kedah), a small-sized irrigation canal (Kodiang, Kedah) and a small-sized ex-mining pool (Pantai Kamloon, Penang). This study involved monthly sampling of 30 red hybrid tilapias for bacterial isolations, measurement of water quality such as water temperature, dissolved oxygen and pH by using hand-held YSI meter (YSI Incorporated, USA) and ammonia, iron, nitrite and sulfide by using spectrophotometers (HACH Company, USA). For water flow, water clarity and depth at culture sites, the readings were measured using a current water meter (Global Water, California), Secchi disc and ultrasonic depth sensor (Speedtech Instrument, USA), respectively. All of the measurements were taken at 1 m deep involving four consistent sampling points within and surrounding the cages. With regard to the type of water bodies, the mean prevalence of red hybrid tilapia that were cultured positive to S. agalactiae was significantly higher (p0.05). There was also a significant positive correlation (r=0.9312, p<0.05) between isolations of S. agalactiae and mortalities of red hybrid tilapias in Pedu Lake, in the presence of Staphylococcus spp. The results indicate that the water temperature, rate of water flow, size of fish and the presence of other bacterial influence the prevalence of S. agalactiae in cultured red hybrid tilapia. The organs of red hybrid tilapias naturally infected by S. agalactiae were examined for pathological changes, including histopathology. Affected red hybrid tilapias showed consistent gross findings of congestion of internal organs, particularly the livers, spleens and kidneys. Other features included exophthalmos, softening of the brains and occasional accumulation of fluid within the abdominal cavity. Microscopic examination revealed swollen endothelial cells that lined the major blood vessels of livers and occasionally spleens leading to extensive infarction, while bacterial colonies were observed within and immediately surrounding the affected blood vessels. The meninges were thickened by the infiltration of numerous heterophils. Similar infiltrations of heterophils and lymphocytes were observed in the lamina propria of intestine. The kidneys were severely congested and haemorrhagic, with extensive interstitial nephritis. The lesion pattern suggested an acute systemic infection. The effect of water quality on the presence of S. agalactiae in cultured red hybrid tilapias was only done in reservoirs and river, due to the significant isolation of the pathogen in these water bodies. There were significant differences (p0.05) in iron, ammonia and nitrite levels along the river but significant differences (p0.05) lower than downstream. This study revealed significant positive correlation (r>0.5000; p<0.05) between certain water quality parameters and the presence of S. agalactiae, particularly the pH, temperature, clarity and dissolved oxygen in both lakes. While in river, water quality parameters such as iron, ammonia, nitrite, pH, temperature, clarity and dissolved oxygen were observed to have a significant positive correlation with the presence of S. agalactiae. The results suggest that different type of water bodies and location of culture sites can affect water quality, creating stressed environment and increased the presence of S. agalactiae in cultured red hybrid tilapia. The phenomenon on the significance of high water temperature in reservoirs and their effect to the cultured red hybrid tilapia to S. agalactiae infection was also investigated. Readings for water temperature and dissolved oxygen at 1 m intervals were collected in situ by using a hand-held YSI meter (YSI Incorporated, USA) for up to 20 m deep, at four consistent sampling points within and surrounding the cages. In Kenyir Lake, the high water temperature column (≥29oC) was mostly noted at between 0 and 8 m depth, particularly between April and November 2007 and 2008. However, in Pedu Lake, the high water temperature (≥29oC) for up to 8 m deep was recorded between April to September 2007 and 2008. Dissolved oxygen profiling in both lakes, however, remained high at >5 mg L-1 for up to 8 m deep, except for October 2007 in Kenyir Lake and April 2008 in Pedu Lake. Analysis of water quality parameters in Kenyir Lake revealed a significant positive correlation between water clarity (r=0.8823, p<0.05) and the prevalence of S. agalactiae. Significantly, water temperature also showed a negative correlation with water flow (r=-0.8584, p<0.05) and a positive correlation with water clarity (r=0.7510, p