There’s no cure for the killer koi disease, but there may be a way to save them from dying.
Scientists have found that fish do regulate their busy temperature to fight off infections. This has been shown with Zebrafish research, where sick individuals seek warmer waters, almost to replicate what we experience as fevers.
Similarly, carp do the same, when faced with KHV. Speaking with my Israeli colleague, Danny Benjamin (who is a koi farmer), he says in the face of a KHV outbreak, crank up the water temperature, and hold it between 30-32 degrees Celsius for about a month. Your koi will gain immunity, and will not die from the viral disease.
Though, survivors are likely going to be carriers of the herpesvirus, and could spread to non-immune fish, or reactivation of the virus in times of stress (transport, change in seasons, spawning) could prove fatal. Like a ticking time bomb.
This also goes for vaccination of fish for the virus.
It’s imperative that you consult your fish veterinarian about the options you have.