Baz,
the Weaver/Bridgewater/T&M fish kill last year was not KHV. If you remember the CEFAS labs couldn't find any trace of known viruses in the bodies they examined. Infections, be they viral or bacterial do not just disappear when the host dies, some trace is always left. With bacteria, they will continue to feed and multiply on the dead flesh. Viruses need living cells to replicate, however since they are not strictly living, they cannot die when the host does, hence there would be a significant viral titre in the carcass which would be detectable by ELISA, viral culture or even nowadays Mass Spectroscopy.