The effect of high total ammonia concentration on the survival of channel catfish experimentally infected with Flavobacterium columnare.

Ammonia concentrations in water can affect the severity of Flavobacterium columnare infections in fish. Two trials lasting 7 d each were conducted to determine the effect of a single immersion flush treatment of total ammonia nitrogen (TAN; 15 mg/L) on the survival of channel catfish Ictalurus punctatus infected with E columnare; the chemical was added while the water flowed continuously through the tanks. Both trials consisted of four treatments: (1) no ammonia exposure and no bacterial challenge (control), (2) ammonia exposure only, (3) bacterial challenge only, and (4) both ammonia exposure and bacterial challenge. Two hours after exposure to ammonia, the highest un-ionized ammonia level was 0.43 mg/L. The percent un-ionized ammonia is based on TAN, temperature, and pH. Caudal fins from three fish in each treatment were sampled at 24 h posttreatment to be analyzed by quantitative real-time polymerase chain reaction (qPCR). No significant difference in survival (mean +/- SE) was noted between the channel catfish in treatment 1 (95.2 +/- 1.2%) and those in treatment 2 (95.6 +/- 1.0%); however, survival in both treatments 1 and 2 differed significantly from that in treatments 3 (8.5 + 4.5%) and 4 (41.8 +/- 12.7%). Treatment 4 catfish had significantly higher survival than treatment 3 catfish. Quantitative PCR data showed that treatment 4 fish had significantly less F. columnare (7.6 x 10(5)) than did treatment 3 fish (1.2 x 10(7)), and treatment 2 fish (8.5 x 10(3)) had significantly less bacteria than did treatment 1 fish (6.9 x 10(4)), indicating that ammonia limited the F. columnare infection. The highest mean concentration of the bacteria (3.9 x 10(7)) was found on moribund fish. The ammonia concentrations tested did not negatively influence fish survival but interfered with the infection process. An in vitro assay was also conducted to evaluate the direct effects of ammonia on F columnare.