The immune system of cyprinid fish. The immunosuppressive effect of the antibiotic oxytetracycline in carp ( Cyprinus carpio L.)

Abstract

The effect of oxytetracycline (oxyTC) upon the immune system of carp was investigated. OxyTC was administered by feeding with oxyTC-containing pellets or by intraperitoneal injection. In order to study cellular immunity, allogeneic scale transplantation was carried out. Oral administration of oxyTC had no influence upon the median survival time (MST) of the scales. However, injections with oxyTC significantly prolonged the MST from 8.5 to 11–20 days. Thus, cellular immunity was not affected by oral administration of oxyTC, but injections did have a dramatic immunosuppressive effect. To investigate the effect of oxyTC upon humoral immunity, animals were injected with rabbit red blood cells (RaRBC). During the primary and secondary response the number of rosette forming cells (RFC) in the spleen was determined. Control animals (not treated with oxyTC) developed an anti-RaRBC response up to 25000 RFC/10 6 white spleen cells but oxyTC-treated animals always showed reduced RFC numbers. In some cases the RFC number in oxyTC-treated animals was comparable with background levels in non-immunized control animals (4000 RFC/10 6 white spleen cells). Thus, irrespective of the route of administration, the humoral immune response is depressed by oxyTC. It is concluded that both humoral and cellular immune responses of carp are suppressed during treatment with oxyTC. Preliminary observations showed an increased number of granulocytes in the spleen of oxyTC-treated animals. It is tempting to speculate that in those cases where specific lymphoid defence mechanisms are blocked, the phagocytic defence system becomes more active.