Molecular comparison of isolates of an emerging fish pathogen, koi herpesvirus, and the effect of water temperature on mortality of experimentally infected koi.

Koi herpesvirus (KHV) has been associated with devastating losses of common carp (Cyprinus carpio carpio) and koi (Cyprinus carpio koi) in North America, Europe, Israel and Asia. A comparison of virion polypeptides and genomic restriction fragments of seven geographically diverse isolates of KHV indicated that with one exception they represented a homogeneous group. A principal environmental factor influencing the onset and severity of disease is water temperature. Optimal growth of KHV in a koi fin cell line occurred at temperatures from 15-25 degrees C. There was no growth or minimal growth at 4, 10, 30 or 37 degrees C. Experimental infections of koi with KHV at a water temperature of 23 degrees C resulted in a cumulative mortality of 95.2 %. Disease progressed rapidly but with lower mortality (89.4-95.2 %) at 28 degrees C. Mortality (85.0 %) also occurred at 18 degrees C but not at 13 degrees C. Shifting virus-exposed fish from 13-23 degrees C resulted in the rapid onset of mortality.