Effects of a parasite-induced nephritis on osmoregulation in the common carp Cyprinus carpio

Carp Cyprinus carpio infected with the haemoflagellate Trypanoplasma borreli undergo progressive nephritis associated with a destruction of approx. 40% of the nephric tubules. In an attempt to analyse the effect of the nephritis on the osmoregulation of affected carp, the clinical chemical properties of plasma and urine samples were analysed. Parasitised carp excreted greater amounts of electrolytes in their urine than uninfected carp which excreted highly diluted urine with an osmolality of about 10% of plasma osmolality. During the course of the infection, urine osmolality increased up to 26% of plasma osmolality by Day 21 post-infection (p.i.). The plasma:urine ratio of Na+ also increased, while concomitant losses of Mg2+, Ca2+, K+ and inorganic phosphate were less pronounced. Infected carp were able to maintain a normal solute balance in their plasma. Plasma hydration (indicated by decreased protein contents) occurred on Day 21 p.i. Our data indicate that in T. borreli-infected carp, reabsorption processes of the distal renal tubule were disturbed, while secretory and absorption processes in the proximal tubule appeared to be less affected. In addition, infected carp were able to compensate their increased ion losses, probably by (energy-consuming) active absorption processes. The energy budget of infected carp was additionally affected by a substantial direct consumption of plasma glucose by the parasite.