This is a DNA based virus, which was probably imported to western Europe and North America, from breeders in either Japan or Israel. Once a fish is infected it will either die, or at the minimum carry the infection for life. If an outbreak is suspected then the antibodies that infected fish produce, can possibly be detected by ELISA methods, this method but is not yet widely available. All age groups are susceptible to KHV, however in some experimental investigations small fish of 2.5-6 grms were more likely to be infected. In other varieties of Carp there also appears to be size or age related susceptibility. The severity of an outbreak is often related to water temperature, as well as the factors just mentioned. The concentration of fish in any given pond or cage, will influence the outcome, as stress plays a large part, in this infection as well as so many others.
In most cases the infection will spread with terrifying speed, and at 24 C one can anticipate losses of more than 90%. As mentioned below, it has been found in some cases that increasing the Temperature to 30 C or a little more, if done slowly one can kill off the virus. It very much depends however on how early one makes the diagnosis, and if the fish have had time to develop some immune response to the virus.
The virus outside hosts fish can remain active for up to 24 hours, at 23-25 C, though it is usually shorter than this.
Kazuaki Matsui et al.