It's similar to SCID in Arabians or LWOS in Pintos/Paints. If a horse is a carrier.. they are just like any other horse, just have 1 gene that you need to be careful when selecting a mate for them. I know of a few SCID carrier stallions that require mare owners supply a negative test when applying to get a breeding, and I'm sure most stallion owners with a Herda postive stallion would require that as well.
It's really, not that big of a deal, unless you don't want to test your mare. And if that is the case, I recommend not breeding at all.