Morphological changes in rainbow trout (Oncorhynchus mykiss) gill epithelia following repeated intermittent exposure to chloramine-T

Chloramine-T is a widely used prophylactic and therapeutic agent in freshwater aquaculture (mainly against bacterial gill disease). The effects of repeated intermittent exposure of healthy rainbow trout (Oncorhynchus mykiss) fingerlings to sublethal concentrations of chloramine-T (0,5, 10, or 20 mg/litre) twice weekly in 1-h pulses at 11 degrees C for 4 weeks in a replicate-tank facility were examined. Gills were excised from subsamples of fish before exposure and at the end of the 4-week experimental period. Tissue was fixed and processed for light microscopy and transmission and scanning electron microscopy. The gill epithelium from fish treated with 10 and 20 mg/litre chloramine-T appeared swollen and vacuolated, with extensive intercellular oedema. There was a significant reduction in the number of lamellar mucous cells and an apparent increase in the numbers of chloride cells. Chloride cells from both the base of the lamella and the lamellar surface of gills exposed to chloramine-T had an increase in the area of the apical plasmalemma after treatment with 10 and 20 mg/litre, and a reduction in the thickness of the apical plasmalemma-associated glycocalyx. These morphological changes are consistent with a compensatory mechanism for the remedial uptake of ions, suggesting that chloramine-T increased epithelial ion permeability coincident with a possible influx of water leading to intercellular oedema. Chloride cell proliferation and intercellular oedema may also have affected gas exchange across the branchial epithelium..