Although the most common cause of foamy white mucus in dog vomit is an upset stomach, it can be symptomatic of more serious illnesses like bloat, kennel cough or rabies. In order to determine the severity of the dog's condition, VetInfo recommends switching the dog to a bland diet and contacting the vet if the dog continues to vomit after a couple days.
If the dog stops vomiting after a few meals consisting of rice and boiled chicken, the vomiting was most likely due to an upset stomach, says VetInfo. If the dog's stomach feels bloated and tight, however, and the dog appears anxious and unwilling to sit or lie down, it may have vomited due to bloat, which is common among large deep-chested dogs. The stomach swells and rotates making it impossible for stomach contents to exit. Bloat can be deadly and must be treated immediately.
VetInfo notes that kennel cough is an upper respiratory disease that manifests as a dry hacking cough and a combination of yellow mucus and white foam in the vomit. Although not life-threatening, it can be difficult to eliminate completely.
Foamy white vomit caused by rabies is very severe and only appears during the later stages of the disease as a result of nerve damage to the mouth and throat, according to VetInfo. Rabies can be easily prevented with a vaccination.