The cardiac form of dog parvo is the least common form, and the most deadly. The virus directly attacks the muscles of the heart, then blood vessels, leading to hemorrhaging.
This form is not passed orally, but usually infect puppies in utero or shortly after birth. Unfortunately, it is hard to catch and sometimes there no symptoms at all before the puppy dies suddenly. The only thing to watch out for is difficulty breathing, but by then it might be too late. The obvious signs of the intestinal form are often missing with this type of infection.
Thankfully, widespread vaccination of breeding dogs has cut the occurrences of this form so it’s not something you should be worried about. You have a better chance of being struck by lightening. Better to be on the lookout for the symptoms of the intestinal strain.