A single case isn’t unheard of, it’s not like it’s an eliminated disease. Cases always crop up in the Gulf States and who knows how many asymptomatic carriers are living in backyards, going their entire lives untested.
Now if we are talking some sort of blood doping product, possibly a blood derivative where one of the donor horses was positive and that blood was subsequently used to create a few vials of whatever the hell was dreamed up in some race horse version of a BALCO lab… Well, it’s easy to see how one untested horse became so many positive horses. For one trainer.