Maybe it's interesting for to know how an importer is working in the south of France, surely not the only solution but one that seems to work.
When he import koï, he is not allowed to sell them and to mix them with others koï until an official vet came and make some test like basic skin test and maybe skv (I will contact the vet and i will confirm you that later).
The importer must have quarantene tank for all fish with a listing of all koi in the different tanks. During 2 weeks he can have a visit surprise of a vet who control the listing of all tank and control that no koï seems to be hill. If there is a quarantene problem, the importer lose his selling license; if there is a sickness problem he must treat the problem and other test will be done before he will be allowed to sell all the koï imported.
There are only a few importer that import directly from Japan, most of them buy the koi in Nederland, Germany or Belgium, so they don't have this problem.
I don't know if this system is really official or if it's an agreement between that importer and the vet service of south of France but it seems that it works.
Once I wanted to buy him a koï and he refuse because the two weeks were not past and he wasn't allowed to sell them, so he seems to respect the rules.
Otherway, I know that he loose a lot of his own biggest koi show last year, was it a water contamination with the imported koï ?, I have tryied to know but that was a tabu question ...
Marco