Cyprinid herpesvirus-3 (CyHV-3) disturbs osmotic balance in carp (Cyprinus carpio L.)—A potential cause of mortality

•CyHV-3 infected carp experience hypo-osmotic stress and prominent renal electrolyte losses.•Osmotic stress in CyHV-3 infected carp is associated with gill and kidney deterioration.•Osmotic stress in carp under CyHV-3 infection may be responsible for mortality. Cyprinid herpesvirus-3 (CyHV-3) causes a fatal disease in carp (Cyprinus carpio) and its ornamental koi varieties which seriously affects production and trade of this fish species globally. Up to now, the pathophysiology of this disease remains unclear. Affected individuals develop most prominent lesions in gills, skin and kidney, in tissues which are involved in the osmotic regulation of freshwater teleosts. Therefore, here serum and urine electrolyte levels were examined during the course of an experimental infection of carp with CyHV-3. In infected carp an interstitial nephritis with a progressive deterioration of nephric tubules developed, which was paralleled by elevated electrolyte losses, mainly Na+ in the urine. The urine/plasma ratio for Na+ increased from 0.03 in uninfected carp to 0.43–0.83 in carp under CyHV-3 infection, while concentration of divalent ions were not significantly changed. These electrolyte losses could not be compensated since plasma osmolality and Na+ concentration dropped significantly in CyHV-3 infected carp. This was most probably caused by the progressive deterioration of the branchial epithelium, which in teleosts plays a prominent role in osmoregulation, and which was seen concomitantly with decreasing electrolyte levels in the serum of carp under CyHV-3 infection. Immediately after infection with CyHV-3, by day 2 post exposure, affected carp showed severe anaemia and prominent leucocytosis indicating the development of an acute inflammation, which could intensify the observed hydro-mineral imbalances. The data presented here show that an infection with CyHV-3 induces an acute inflammation and a severe dysfunction of osmoregulation in affected carp or koi, which may lead to death in particular in the case of acute disease progression.