Foam coming from a dog's mouth might be caused by exercise, anxiety, nausea, ingestion of a bad-tasting substance, a dental disease, liver or kidney disease, difficulty swallowing, poisoning, a seizure, a side effect of medication or rabies. If the foaming is observed for any considerable length of time without a known cause, the dog should be taken to a veterinarian.
Rabies is generally the first concern when a dog exhibits excessive foam and drool from its mouth. However, in 2005 there were only 76 canine cases of rabies in the United States. The chance of a vaccinated dog actually becoming susceptible to the disease is very slim. It is more likely that the dog is exerting a lot of energy during exercise which is causing it to drool, pant heavily and foam at the mouth.
If foaming at the mouth is followed by a loss of consciousness, or if the animal's limbs are twitching or jerking, the dog may be having a seizure. Seizures can last anywhere from a few seconds to a few minutes. Dogs that are suffering from a seizure should be carefully moved away from any furniture on which they could hurt themselves and should be taken to the vet as soon as the seizure ends. Owners should also keep their hands away from the dog's mouth and head during the seizure.