Why does foam come from animals mouths that have rabies?

Question

Why does foam come from animals mouths that have rabies?

Answer 1

In movies, it just looks like they are foaming but it's from an increase in saliva production. They can also drool.

Answer 2

The foam is simply saliva that cannot be swallowed.  One of the symptoms of rabies is paralysis of the mouth and jaw, another is the overproduction of saliva and tears, and saliva accumulates and spills out.  This is why rabies used to be called "hydrophobia"--its victims are panicky about needing water but being unable to swallow.

Answer 3

Exactly as Marie pointed out, the rabies virus can infect not only the nerves involved with saliva production but also salivary glands.  In combination with painful spasms of the muscles of the pharynx & larynx, the animal will manifest the typical "foaming at the mouth" with this constellation of problems.