A study of sequential histopathology of Trypanoplasma borreli (Protozoa : Kinetoplastida) in susceptible common carp Cyprinus carpio

The tissue response of common carp Cyprinus carpio to the kinetoplastid blo od parasite Trypanoplasma borreli Laveran & Mesnil, 1901 was investigated d uring a laboratory infection of a highly susceptible carp line. With the de velopment of the parasitaemia an increased proliferation of the lymphoid re nal interstitial tissue was induced, which resulted in a progressive depres sion and deterioration of renal tubules. In heavily infected carp at Days 2 0 to 28 post inoculation (PI), a tubulonephrosis, a glomerulitis caused by a massive accumulation of leukocytes in glomerular capillaries, and large n umbers of trypanoplasms in blood vessels and renal interstitium were observ ed. Corresponding with rising T. borreli numbers in the peripheral blood, s plenic lymphocytes showed increasing proliferation rates, and the capillari es of the liver, gills, heart and intestine were infiltrated with lymphocyt es and trypanoplasms. In heavily infected carp, congestion of liver sinusoi ds, focal necroses of hepatic tissue, extensive accumulations of erythrocyt es in the spleen and in the blood marked anaemia were observed. These carp often showed abdominal distension, exophthalmus and swimming disorders desc ribed as 'sleeping sickness of carp'. Proliferation of cells from the inter stitial lymphoid tissue of the kidney, which bears a close resemblance to t he bone marrow of higher vertebrates, is considered a normal immune respons e of fish to antigen challenge. We here describe the unique case of a sever e but ineffective immune reaction which results in the destruction of excre tory renal structures. This has to be considered a severe disturbance of os moregulation in affected carp, which, together with a decrease in oxygen up take due to anaemia, is likely a major cause of death in these carp.