Vampires are depicted differently in every work of fiction, but one thing mainly stays the same: they need blood to survive. Human blood, to be precise. Some stories add in the rule that they can also drink blood from animals and usually put vampires who make the choice of only drinking animal blood on a morally higher ground than the savages who harm humans.
In my setting I want to take this a step further. Vampires are offered two choices to get their blood-fix: either drink from humans, or eat 'blood-oranges', the vampire variant of the snozzcumber. This fruit looks nothing like an actual blood orange and tastes absolutely horrible to both human and vampire, making it tempting to take a bite from a human every now and then. Even when it is common knowledge to them that biting humans will result in either the victim dying of blood-loss or turning into vampires themselves.
To keep these two choices absolute, I want to rule out any other source of blood. For some reason vampires need human blood, and don't consider animal blood an option. They can't drink from other vampires either. Vampires are already vampires, it completely takes away the risk of turning them like with humans.
How can I explain that vampires only consider humans as possible targets? I'm looking for an answer that suspends disbelief, so it's alright if a solution delves into pseudo-science/occult territory.