I have had similar problems with hot dogs (and sausage as well) with my
severely milk-allergic daughter. We only use Kosher hot dogs now. My
understanding with the others is that they use lactose as a filler and for
some reason do not need to list it in the ingredients. I don't understand why
that is. Fortunately we learned about it  before my daughter had anaphylactic
reactions. Now unless a product is labeled as milk free, we do not use it
because it is too risky.

Ann Eves