UN-IONIZED AMMONIA EXPOSURE IN NILE TILAPIA: TOXICITY, STRESS RESPONSE, AND SUSCEPTIBILITY TO STREPTOCOCCUS AGALACTIAE

Authors: Evans, Joyce; Pasnik, David; BRILL, GREGG - WASHINGTON COLLEGE; Klesius, Phillip

Submitted to: North American Journal of Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2005
Publication Date: 1/1/2006
Citation: Evans, J.J., Pasnik, D.J., Brill, G.C., Klesius, P.H. 2006. Un-ionized ammonia exposure in Nile Tilapia: Toxicity, Stress Response, and Susceptibility to Streptococcus agalactiae. North American Journal of Aquaculture. 68(1):23-33.

Interpretive Summary: Ammonia is a well-known aquatic pollutant and toxin of fish, and it is produced as an end-product of nitrogenous metabolism. Most teleost fish are ammoniotelic and excrete ammonia as their principal waste product. Environmental ammonia concentrations increase due to ammonia excretion or because of the breakdown of organic matter in the water. The two forms of ammonia in the environment are unionized and ionized ammonia, and unionized ammonia (UIA) is toxic to fish because it can easily diffuse across gill membranes. Increased UIA levels are known to cause behavioral, physiological, and histologic changes in fish and can cause increased susceptibility to pathogens due to post-exposure stress responses and to immune mechanism impairment. High concentrations or increased durations of exposure to UIA may also directly result in mortalities. Because of the possible affects on fish health and survival, ammonia accumulation is of particular concern in aquaculture. Though the effects of UIA have been studied in other tilapia (Oreochromis mossambicus x O. niloticus and O. aureus), the median lethal concentration (LC50), behavioral responses, stress responses, and disease resistance following UIA exposure have not been examined in Nile tilapia, O. niloticus. The purpose of this study was to determine the effects of UIA on stress, disease resistance, and mortality in this economically-important aquaculture species. Special emphasis was placed on the effect of UIA on fish challenged with Streptococcus agalactiae, an important emerging pathogen of wild and cultured fish A series of experiments were conducted to determine the toxicity, behavior, blood glucose stress response and disease susceptibility in Nile tilapia Oreochromis niloticus following UIA exposure. The acute toxicity of unionized ammonia to Nile tilapia was measured in a 96 hour static test. The LC50 was 1.46 mg/L UIA at 24 and 48 hours post-exposure, 1.33 mg/L at 72 hours post-exposure, and 0.98 mg/L UIA at 96 hours post-exposure. No mortalities were noted in unexposed (0 mg/L UIA) control fish or fish exposed to 0.5 mg/L UIA. However, 93-100% mortalities were observed within 24 h among fish exposed to 2.0, 3.0, or 4.0 mg/L UIA. In additional UIA exposure experiments, tilapia were exposed to sub-lethal concentrations of UIA (0.32 to 0.37 mg/L UIA) for 24 h and then administered an intraperitoneal injection with 750 colony forming units (CFU) Streptococcus agalactiae per fish. Mortalities of UIA exposed and control fish were not significantly different 21 d post-challenge. Blood glucose levels were not significantly different between exposed and control fish 24 h after the beginning of UIA exposure or between pre-exposure fish and 24 h post-exposure fish. Glucose levels in both groups increased significantly after UIA exposure and subsequent bacterial challenge, suggesting that tilapia experienced handling or infection stress and not necessarily UIA exposure stress alone. During a time-course study with 24 h UIA exposure, sequential blood glucose samples indicated acute stress responses 1-4 h post-exposure that decreased by 24 h post-exposure. The results of this study indicate that exposure to increased UIA concentrations alone had acute, transient effects on stress responses in Nile tilapia and that 24 h exposure to sub-lethal UIA concentrations up to 0.37 mg/L did not increase susceptibility to S. agalactiae.

Technical Abstract: A series of experiments were conducted to determine the toxicity, behavior, blood glucose stress response, and disease susceptibility in Nile tilapia, Oreochromis niloticus, following unionized ammonia (UIA) exposure. The acute toxicity of unionized ammonia to Nile tilapia was measured in a 96 h static test. The median lethal concentration (LC50) was 1.46 mg/L UIA at 24 and 48 h post-exposure, 1.33 mg/L at 72 h post-exposure, and 0.98 mg/L UIA at 96 h post-exposure. No mortalities were noted in unexposed (0 mg/L UIA) control fish or fish exposed to 0.5 mg/L UIA. However, 93-100% mortalities were observed within 24 h among fish exposed to 2.0, 3.0, or 4.0 mg/L UIA. In additional UIA exposure experiments, tilapia were exposed to sub-lethal concentrations of UIA (0.32 to 0.37 mg/L UIA) for 24 h and then administered an intraperitoneal injection with 750 colony forming units (CFU) Streptococcus agalactiae per fish. Mortalities of UIA exposed and control fish were not significantly different 21 d post-challenge. Blood glucose levels were not significantly different between exposed and control fish 24 h after the beginning of UIA exposure or between pre-exposure fish and 24 h post-exposure fish. Glucose levels in both groups increased significantly after UIA exposure and subsequent bacterial challenge, suggesting that tilapia experienced handling or infection stress and not necessarily UIA exposure stress alone. During a time-course study with 24 h UIA exposure, sequential blood glucose samples indicated acute stress responses 1-4 h post-exposure that decreased by 24 h post-exposure. The results of this study indicate that exposure to increased UIA concentrations alone had acute, transient effects on stress responses in Nile tilapia and that 24 h exposure to sub-lethal UIA concentrations up to 0.37 mg/L did not increase susceptibility to S. agalactiae. Key words: Nile tilapia; Oreochromis niloticus; unionized ammonia; stress; blood glucose; Streptococcus agalactiae