Proliferative kidney disease in rainbow trout: Time- and temperature-related renal pathology and parasite distribution

Abstract

Proliferative kidney disease is a parasitic infection of salmonid fishes caused by Tetra-capsuloides bryosalmonae. The main target organ of the parasite in the fish is the kidney. To investigate the influence of water temperature on the disease in fish, rainbow trout Oncorhynchus mykiss infected with T. bryosalmonae were kept at 12°C and 18°C. The number of parasites, the type and degree of lesions in the kidney and the mortality rate was evaluated from infection until full development of disease. While mortality stayed low at 12°C, it reached 77% at 18°C. At 12°C, pathological lesions were dominated by a multifocal proliferative and granulomatous interstitial nephritis. This was accompanied by low numbers of T. bryosalmonae, mainly located in the interstitial lesions. With progression of the disease, small numbers of parasites appeared in the excretory tubuli, and parasite DNA was detected in the urine. Parasite degeneration in the interstitium was observed at late stages of the disease. At 18°C, pathological lesions in kidneys were more severe and more widely distributed, and accompanied by significantly higher parasite numbers. Distribution of parasites in the renal compartments, onset of parasite degeneration and time course of appearance of parasite DNA in urine were not clearly different from the 12°C group. These findings indicate that higher mortality at 18°C compared to 12°C is associated with an enhanced severity of renal pathology and increased parasite numbers. © Inter-Research 2009.