The presence of sublethal concentrations of noxious chemicals in freshwater environments can promote the emergence and development of infectious diseases in fish (Carballo and Munoz, 1991;Carballo et al., 1995). Rainbow trout exposed for 24 hours to 0.24 mg
(corresponding to 50% of lethal concentration) were challenged after toxin exposure with Saprolegnia parasitica causing mycotic dermal infection. The acute stress response provoked by the toxin exposure accounts for the main contribution to the increase in saprolegniosis susceptibility, representing approximately a 100% increase in the percentage of infected fish when compared with the control group
Bromide, which is chemically similar to chloride, was studied by Eddy et al. (1983), who found that 1mM of sodium bromide (80 mg
/l) was enough to offset the presence of 0.7mM nitrite (32 mg
/l nitrite-N) almost completely for Atlantic salmon (Salmo salar) in fresh water.
Calcium chloride was more effective than sodium chloride at reducing nitrite toxicity in striped bass (Morone saxatilis) (Mazik et al., 1991) or shortnose sturgeon fingerlings (Acipenser brevirostrum)(Fontenot and Isely, 1999). On the other hand, Bowser et al. (1983) found that sodium chloride and calcium chloride provided equivalent protection against nitrite toxicity for channel catfish, suggesting that the identity of the metal cation was of small importance.
Krous et al. (1982) pointed out that high concentrations of calcium generally reduced the loss of chloride through the gills. This in turn diminished the requirement for nitrite uptake. Thus there are theoretical reasons to expect that calcium ions will reduce toxicity although experimental work that has been done so far proves that the effect is a weak one.
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