In WWZ, the zombies were explicitly toxic(in some probably-scientifically-problematic way). At least one section discusses military working dogs, which cannot safely bite the zombies; but are valuable in detecting, avoiding, and/or putting a bullet through the head of, them. However, zombie-toxins are otherwise never mentioned, and appear not to leach out into bodies of water where active zombies are hiding, affect humans who survive close combat encounters with zombies that would (in the case of an actual toxin) leave the place spattered with aerosolized whatever-it-is, and so on.
I suspect that, as with most things zombie, it comes down to the fact that zombies just don't make thermodynamic sense, and so are either 'rabies++' zombies, which are aggressive, fast-moving, and still very much alive(at least until their dubiously logical behavior causes them to starve or die) until they are just another dead mammal or they are supernatural necromancy zombies, who are indeed decaying like the dead meat they are(albeit possibly at a rate dictated more by narrative causality than strict adherence to real-world decay rates); but will simply become skeletons, animated by the same unnatural powers of undeath, once the meat is gone.
Anything not in one of those two camps, you are really just handwaving for the sake of having zombies; because that's an end in itself.