Eating

• Persistent vomiting in dogs is often triggered by eating something, or by certain types of eating behavior. The Second Chance veterinary website says that dogs often eat toxic plants, garbage or other things they find outside. They may also eat items found in the household, like shoes or rags. These toxic or foreign objects can cause vomiting. Gulping food too quickly or eating large quantities of unhealthy foods including table scraps can also lead to vomiting.
  
  

Blockage

• When there is a blockage inside the dog's stomach, it will vomit persistently because its body is trying to get rid of the obstruction. Second Chance says that most internal obstructions are caused by ingesting something large, like a tennis ball or bone. It can be something the dog was given to chew or play with that it accidentally swallows.
  
  

Parasites

• A dog will often vomit persistently if it has become infested with a severe load of intestinal worms or other parasites. This problem mostly commonly occurs in puppies, which commonly become infected by roundworm or hookworm infestations. According to Second Change, the roundworms lead to persistent vomiting by blocking the dog's intestines. Hookworms make the animal vomit because they lead to inflammation inside of the small intestine.
  
  

Motion Sickness

• Some dogs vomit persistently when they ride in a car. Like humans, they can be prone to motion sickness. This is especially common if the dog eats right before you take it in the car. Over time, some dogs may get over their tendency to vomit on car trips.
  
  

Stomach Inflammation

• Second Chance says that some dogs can develop stomach inflammation--also known as chronic gastritis--which causes them to vomit persistently. It can be triggered by eating irritating items including tree bark, sticks or dirt. However, its most common cause is stress, and it's mostly commonly seen in very high-strung or nervous dogs.
  
  

Tumors

• Elderly dogs may vomit persistently because they have a tumor growing in their stomach or inside the small intestines. The vomiting is triggered by chronic irritation caused by the tumor, This irritation comes from the tumor's position and growth, and it can increase as the size of the tumor increases.