ABSTRACT: We assessed the effect of chronic stress using a group of potent, broad-spectrum antimicrobial polypeptides, called histone-like proteins (HLPs), which appear to be an important component of non-specific immunity in channel catfish Ictalurus punctatus skin. An enzyme-linked immunosorbent assay (ELISA) was developed to measure the predominant HLP (HLP-1) in channel catfish skin. Catfish were then exposed to a chronic stress consisting of overcrowding and elevated ammonia. Healthy unstressed fish had consistently high HLP-1 levels, but fish that had been stressed for 1 wk had significantly depressed HLP-1 levels; HLP-1 levels declined further in fish stressed for 3 or 4 wk. The time-dependent decline in HLP-1 levels was not accompanied by any gross signs of disease. In contrast to HLP-1 levels, antibacterial activity in the skin was significantly greater in fish stressed for 1 wk compared with unstressed fish; in addition, antibacterial activity was the same in fish that were unstressed or stressed for 3 or 4 wk. This suggests that other antibiotics besides HLP-1 may be induced in the skin, especially during early stages of stress, that may compensate for depressed HLP-1 levels. Our results indicate that chronic stress has a significant suppressive effect on HLP-1 levels in channel catfish skin. The reduction of HLP-1 in the absence of clinical signs of disease, combined with evidence that its levels are not affected by the acute stressors of capture or sampling, suggests that HLP levels may be a promising indicator for monitoring fish health.

KEY WORDS: Endobiotics · Non-specific immunity · Diagnostic stress test · ELISA

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Published in DAO Vol. 44, No. 2 (2001) on March 9
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 2001