A survey on some risk factors and evaluation of their impacts on streptococcosis incidence in rainbow trout farms in west of Mazandaran province

Abstract

This study aimed to investigate the effects of 5- parameter dissolved oxygen, PH, nitrite, nitrate and temperature of the water on Streptococcosis incidence in two groups of fry and grower fish. Research was performed at west Mazandaran province -Tonkabon region in 12 rainbow trout selected farms. Research conducted in 3 farms with well source water , 8 farms with water source of the river of Dohezar and one farm with water source of the river of Azarood, during 12 consecutive months.From 1390/04/01 to 1391/04/01, once time each month , and in each time 10 fish randomly sampled, inlet water were sampled simultaneously. Of 1350 sampled fish 607 fish with an average weight 22.04 gr , average length 12.59 cm were in fry category and 743 fish with an average weight 156.25 gr, average length 23.32 cm in were grower category. Streptococcosis observed only in grower category. Of 72 fish with clinical signs of the disease, 14 numbers were positive Streptococcosis (19.44%) and 58 numbers were negative Streptococcosis(80.55%). Three fish from grower category has not any clinical signs of disease and seemed to be healthy but were positive Streptococcosis in examinations (0.22% of total fish sampled. Fish with clinical signs of the disease but negative Streptococcosis were of at least 55.1 gr and at most weight 417 gr respectively. The results showed that 47.07% cases of Streptococcosis happened at 15.6 °C water temperature, 35.29% at 16.98 °C and 17.64% cases happened at 18.04 °C so that 100% of Streptococcosis cases was observed at the average temperature of 16.99 °C. In addition, the survey results show that despite relatively high levels of nitrite in source water of farms from wells, nitrite does not effect on the disease incidence. It seems that an optimal level of dissolved oxygen in water is effective in reducing the effectiveness of nitrite in this disease. According to equation coefficients logit model is as follows: According to Logit model, it seems that for every degree change in temperature and pH of water, morbidity change will diminish 0.37 % and 1.74 % respectively. 80.56% of fish sampled that had the clinical symptoms, was ranging from an average weight of 5.1 gr (fry) and 417 gr (grower fish), that despite having clinical signs of the disease were negative Streptococcosis. Isolation of Staphylococcus bacteria as well as Gram-negative bacilli from fish with clinical symptoms similar to the symptoms of Streptococcosis, may indicate the involvement of other pathogens in fish clinical signs.