Proliferative kidney disease (PKD) has been responsible for extensive mortalities among young rainbow trout, especially in Europe, during the summer months. We examined 25 trout in October, 1980, and found obvious signs of the disease in the kidneys of two fish. In one specimen, the kidney was swollen and discolored, and in the other it was normal in color and size except for an opaque area less than 1 cm in length. The liver, spleen, and kidney from these fish were fixed in neutral formalin and brought to the Oxford Laboratory for histo-pathological study. The kidneys were in advance stages of the disease and few parasites were found. The spleens were severly congested with only slight evidence of parasitism. In contrast, the liver tissue demonstrated numerous life cycle stages of protozoan parasites. Seagrave et al, (J. Fish. Biol., 16:453-459, 1980) illustrated numerous stages of a similar parasite in the kidney and spleen of trout and proposed that the PKD organism resembled haplosporidan parasites possibly related to the genus Marteilia. They did not find definitive spore stages but did describe primary, secondary, and tertiary cells, and probably haplosporosomes in the primary cells. The liver from one fish collected in Italy contained many primary, secondary, and tertiary cells similar to those reported by Seagrave and co-workers. Spherical bodies containing up to four smaller spheres or a vermiform body and one or more small spheres were observed only in the liver. Spheres containing four small bodies have been identified as sporangia in the life cycle of Marteilia refringens, a parasite of European oysters, Ostrea edulis. Spheres containing vermiform bodies which resemble sporozoites have not been reported as life cycle stages of haplosporidians, suggesting that the diseased fish from Italy may have been simultaneously infected with several different parasites. Preliminary electron microscopic studies showed various stages of the parasite previously described from PKD- infected fish, but it remains an open question as to whether the haplosporidan-like protozoans belong to a genus other than Marteilia.

We gratefully acknowledge Ms. Jane Wade for her excellent technical assistant in preparing tissues for electron microscopy. Dr. Joel Bodammer provided the guidance and instruction that enabled us to make preliminary observations on ultrastructural characteristics of the parasites.