Vomiting in cats may be a sign of a very serious health problem. It's important to consult your veterinarian if your cat is constantly vomiting. Cats may vomit a foamy substance for several reasons.
Chronic Renal Failure
Chronic renal failure is a serious kidney disease. A symptom of chronic renal failure in cats is when they vomit clear or white foam.
Feline parvovirus is also known as FPV or Feline Infectious Enteritis, FIE. Symptoms include foamy vomit, fever and, in more serious cases, sudden death. A cat with feline parvovirus will try to eat or drink, but will be unsuccessful. A stool sample will be needed for your veterinarian to make an accurate diagnosis.
Cats are constantly looking for new things to explore, including plants. If a cat vomits a substance of a foamy, green consistency, this could be an indicator that the cat may have eaten a toxic plant.
Cats may vomit a clear, foamy liquid if they are nauseous and have an empty stomach.
In some cats, vomiting a foamy substance is a normal reaction to stress. According to Veterinary Associates, "Most healthy cats will, on occasion, vomit whole or partially digested food, hairballs, or foamy, clear liquid."