California Sea Grant College Program

Rainbow trout, Oncorhynchus mykiss (formally Salmo gairdneri), which have recovered from proliferative kidney disease (PKD) were resistant to reinfection by natural exposure and to intraperitoneal (IP) injection of kidney homogenates which contained the parasite. The latter challenge method demonstrated that external defenses (skin, mucus components, gut enzymes, etc.) are not required for successful resistance to the parasite and suggests that an acquired immunity follows clinical PKD. In contrast, immunity did not develop in trout that had previous exposure to the infectious stage but that did not experience PKD.

In vitro incubation of PKX cells in serum from immune trout did not reduce experimental transmission suggesting that humoral factors alone do not confer resistance to infection.

The effect of subclinical PKD infection on blood parameters, disease resistance and immune defenses was examined in rainbow trout under laboratory conditions. Hematological and serological changes occurred primarily in those fish with severe renal lesions. Hematocrits decreased in the absence of signs of hemolytic anemia and leukocyte numbers, particularly phagocytes, increased in infected fish. Nephritis impaired divalent ion excretion and albumin retention. There was a rise in several globular protein fractions (including immunoglobulin) in the plasma of infected fish. Infection by the PKX parasite stimulated the non-specific defenses of the trout as evidenced by the higher survival rate following challenge with the bacterial pathogen Vibrio anguillarum among PKX infected trout compared to control trout. Greater numbers of phagocytic cells were also present in the kidneys of trout with PKD compared to control fish. Chemiluminescence and nitroblue tetrazolium assays demonstrated that PKX infection did not reduce oxidative metabolism of activated, anterior kidney cells. PKX infection did not effect the efficacy of vaccination with anguillarum bacterin. Similar or heightened immune responses to sheep red blood cells as determined by plaque forming cell assays were also observed in trout with PKD in comparison to controls. It appears that subclinical PKD alone is not a major predisposing factor to secondary disease and affected fish can recover without serious problems if other environmental and biological stressors are reduced.