Spring Viremia of Carp SVC
This virus is just about everywhere and is tough on the koi or carp immune system, opening them up to what “really kills them” which is a secondary bacterial infection. Still, the disease is reportable (mostly because it’s reportable in the UK) and the impact on a business or pond if it’s found to contain SVC infected fish is grave. Total depopulation.
This document details SVC quite thoroughly.