Studies on the toxic and immunosuppressive effects of cadmium on the common carp

In the last couple of decades, research on cadmium extended from its toxicologic aspects to its role in environmental pollution. The toxicity of cadmium to different species of fish varies widely. The cadmium sensitivity of batches of common carp, the most important cultured fish species of Hungary, was studied. Lethality, histopathologic changes and effect of cadmium on the antibody response, lysozyme level and microbicidal capacity were monitored. Wide variation was found in individual sensitivity to the toxicity of cadmium applied at concentrations of 5 to 35 mg/l with 96 h exposure time and LC50 of 21.07 mg/l. Histopathologic changes were observed mainly in the gills, kidney and liver. Oedema of the secondary lamellae of the gills was accompanied by a decrease in the number of mucus cells that could be seen also in the skin and gut sections at high and lower concentrations of cadmium. Distension of the Bowman's capsules in the kidney and degeneration of the renal tubular and liver cells were apparent at higher concentrations of cadmium. No histopathologic change specific of cadmium was found. The mean agglutinin titre to rabbit red blood cells of carp immunized after 96-h exposure to 5 mg/l cadmium was significantly lower than in the control group. Secondary anti-Aeromonas hydrophila-agglutinin response of carp treated with cadmium in the same way before immunization was practically absent. Lysozyme level in the kidney decreased to less than half of the control. Microbial killing of Microtorula albicans (syn.: Candida albicans) yeast cells increased slightly. The latter two parameters did not change significantly. The diverse sensitivity of carp to cadmium toxicity is emphasized. Changes in the immunological parameters are believed to reflect a strong stress effect of cadmium on the fish, since stress has been shown to depress specific immune response and increase neutrophil activity. These observations should call more attention to the dual nature of the toxic effect exerted by cadmium: direct toxicity resulting in fish mortality and immunosuppressive effect that probably promotes disease outbreaks.