Proliferative kidney disease in brown trout – infection level , pathology and mortality under field 1 conditions

Proliferative kidney disease in brown trout – infection level, pathology and mortality under field 1 conditions 2 3 Heike Schmidt-Posthaus, Regula Hirschi, Ernst Schneider 4 5 Centre for Fish and Wildlife Health, Department of Infectious Diseases and Pathobiology, University 6 of Berne, Laenggassstrasse 122, P.O. Box 8466, 3001 Berne, Switzerland 7 Alte Landstrasse 156, 8800 Thalwil, Switzerland 8 9 Running title: PKD related mortality and pathology under field conditions 10 11 ABSTRACT: 12 Proliferative kidney disease (PKD) is an emerging disease threatening wild salmonid populations. In 13 temperature controlled aquaria, PKD can cause mortality rates of up to 85% in rainbow trout. So far, 14 no data about PKD related mortality in wild brown trout are available. The aim of this study was to 15 investigate mortality rates and pathology in brown trout kept in a cage within a natural river habitat 16 which is known to harbor Tetracapsuloides bryosalmonae. Young of the year (YOY) brown trout, free 17 of T. bryosalmonae, were exposed in the river Wutach, in the Northeast of Switzerland, during three 18 summer months. Samples of wild brown trout caught by electrofishing near the cage location were 19 examined in parallel. The incidence of PKD in cage exposed animals (69%) was not significantly 20 different to the disease prevalence of wild fish, 82 and 80%. The mortality in cage exposed animals, 21 however, was as low as 15%. At the termination of the exposure experiment, surviving fish showed 22 histological lesions typical for PKD regression, suggesting that many YOY brown trout survive the 23 initial infection. Our results at the river Wutach suggest that PKD in brown trout does not always 24 result in high mortality under natural conditions. 25 26