PKD: the proliferative kidney disease in fish

The present study gives an overview about the knowledge and the diagnostic possibilities of the proliferative kidney disease (PKD) in fishes. This disease is responsible for economically significant losses in farmed fish and severe reductions in wild fish populations in Europe and North America. PKD is affecting mainly salmonid species. In Swiss rivers the proliferative kidney disease pictures the highest percentage of diseases among brown trouts. The main symptom of PKD is a massive proliferation of the interstitial kidney tissue which named the disease. Other unspecific symptoms are anemia, ascites, exophthalmus and apathy. The causative agent of the disease is Tetracapsula bryosalmonae (Myxozoa). Many aspects of the life cycle of this parasite remain unclear so far but it is known that the appearance of this parasite is linked to the presence of a range of freshwater bryozoans. PKD appears mostly in summer during high water temperatures (> or = 15 degrees C) and can lead to high mortality rates. A diagnosis can be made by using light-microscopic, lectinimmunhistochemical as well as molecular biological methods. Before we can develop successful management strategies the knowledge gaps in understanding the disease cycle and the exact pathogenesis of PKD need to be closed.