Effects of Water Temperature on the Susceptibility of Rainbow Trout to Streptococcus agalactiae

Background: The outbreak of diseases is a limiting factor in fish culture. Among different bacterial agents, Streptococcus agalactiae is a major problem, causing heavy loss in cultured and wild fish species from fresh and marine water. Outbreaks with considerable mortalities in different fish species have been associated with multiple environmental factors, including warm water temperatures, increased ammonia levels and low dissolved oxygen levels. The effects of various water quality factors on streptococcal infections have been previously shown. To date, there is no available data on the effects of water temperature on the infection of rainbow trout with S. agalactiae. Therefore, present study evaluated the effect of two water temperatures (12 and 18 degrees C) on clinical signs, relative percent survival (RPS), cumulative mortality and LD50 in rainbow trout against S. agalactiae. Materials, Methods & Results: Three hundred and sixty rainbow trout (56 +/- 4.1 g) were acclimatized for 2 weeks and fed the commercial fish diet at 2% body weight twice daily. The experiment was conducted in two different water temperatures (12 +/- 1 degrees C and 18 +/- 1 degrees C). In each temperature, fish were injected with 0.2 mL of five different concentrations (10(8), 10(7), 10(6), 10(5) and 10(4) CFU mL(-1)) of S. agalactiae (RTCC2051). In control group, the same amount of physiological saline was injected instead of the bacterial suspension. Mortality rates and clinical signs were recorded for 14 days. Throughout the experiment, different signs such as eye abnormalities, erratic swimming and body lesions were observed in fish which were exposed to high bacterial doses at 18 degrees C while fish kept at 12 degrees C were completely asymptomatic. No mortality was observed within 2 or 3 days in fish at 12 degrees C while at 18 degrees C, the mortality started growing rapidly even from first day of bacterial injections. Results also revealed that the trend of mortalities in 18 degrees C was quite different from 12 degrees C. The median lethal dose (LD50) on the 14th day was also calculated at 12 and 18 degrees C as 7.22 x 10(7) CFU mL(-1) and 1.711 x 10(7) CFU mL(-1), respectively. Discussion: In this study, the relationship between temperature and infection with Streptococcus agalactiae was pronounced. When the water temperature was 18 degrees C, mortality due to S. agalactiae was higher in rainbow trout than 12 degrees C. It was previously showed that the optimum temperature for growth provide the best performance of fish immune system. Non-specific and specific immune parameters in teleost decrease significantly when the water temperature is lower or higher than normal physiological range of the fish. According to current study, the higher mortality in fish kept at 18 degrees C might be affected by the reduction of rainbow trout immunogenic activity. On the other hand, the effects of water temperature on the progress of infection in fish species are also important. For example, manipulation of temperature and/or salinity can be used to reduce the growth rate of Flavobacterium psychrophilum. The change of virulent S. agalactiae related to water temperature in fish species needs more consideration. In conclusion, there was a clear relationship between temperature and mortality in rainbow trout against Streptococcus agalactiae and the total cumulative mortality at 18 degrees C was higher compared to 12 degrees C for every single concentration used.