Vomiting is a complex action that involves modulation of the respiratory and swallowing centres. Vomiting can be initiated in a number of ways. Peripherally, there are visceral receptors in the abdomen that respond to stretch or inflammation of abdominal organs and the vestibular apparatus of the ear. Centrally, there is a region in the brain called the chemoreceptor trigger zone where circulating substances such as drugs or toxins can stimulate the vomiting pathway. These pathways all synapse at the vomiting centre in the brain and trigger vomiting. It is possible that higher centres in the brain also can stimulate vomiting. Thus, there can be a number of very serious causes of vomiting such as intestinal obstruction, pancreatitis, kidney failure, liver disease, brain disease, and diabetes ketoacidosis to name a few; and equally vomiting can be mild and self-limiting in nature.

There are a number of priorities that are important when evaluating an animal with vomiting:

1. Is the Animal Really Vomiting?

Owners will often describe their animal as "throwing up," but don't differentiate much more than that. Vomiting is an active process. It starts with production of excessive saliva in order to neutralise the stomach acid, which results in repeated swallowing attempts. Some animals will only display this sign, consistent with nausea and not been seen to vomit. The normal intestinal motility is then inhibited and there is an active expulsion of the vomitus by forceful contractions of the abdomen and diaphragm.

Regurgitation is a specific sign of oesophageal dysfunction. Regurgitation is a 'passive' process (i.e., there are no abdominal effort or preceding signs of nausea such as are seen in vomiting). Cats are the exception to this, and painful conditions may cause excessive swallowing attempts.

The development of regurgitation in a dog after a prolonged period of vomiting is not unexpected, as there is likely to be some reflux and inflammation of the stomach. However, if regurgitation is the initial presenting problem, then urgent investigation is warranted, as an oesophageal foreign body or other serious oesophageal condition could be present.

2. Do They Need Correction of Any Fluid or Electrolyte Abnormalities?

Vomiting results in loss of fluids. When first assessing the need for an animal to require treatment, it is important to establish how severely affected the animal is by vomiting. As with people, in many circumstances dogs or cats will recover without intervention from mild vomiting.

It is essential that animals that do require fluid therapy are identified and prompt veterinary attention is sought. Some indications that would suggest that this is the case are:

 The animal is either very young or very old (these animals are more likely to suffer effects of dehydration quickly).

 The animal has concurrent diarrhoea (the amount of fluid loss is much greater than with vomiting alone).

 The animal has a preexisting illness.

 The animal is vomiting large volumes, the vomitus has the appearance of ground coffee (indicating digested blood), or a faecal odour.

 The animal is not eating or drinking and has been vomiting despite having food withheld.

 The animal is weak, has injected, pale or yellow gum colour or appears to be uncomfortable.

The most effective way of replacing fluid lost by vomiting is by intravenous fluid therapy. The type and rate of the fluid will be determined by the underlying condition. In general, the more collapsed the animal, the more rapid the fluid replacement necessary. If a collapsed dog or cat is on the way to a veterinary clinic, then a safe fluid type to choose to have ready is Hartmann's (or lactated Ringer's).

If the vomiting is only mild, or the owners are unable to bring the animal in, then basic advice should be given. Food should be withheld for 12–24 hours. Water should be offered frequently, but only small amounts at a time. Frozen ice cubes can be placed in the animal's bowl to encourage little and often water intake. Rehydration solutions available for people can be offered to animals; however, the taste may be off-putting and result in decreased fluid intake. In general, if an owner feels they need to 'syringe' in water, then the animal is likely to be much better off treated with IV fluids. However, if the owner feels strongly about this, then a syringe should only be placed at the tip of the tongue and the animal encouraged to swallow small volumes at a time. Once the animal has stopped vomiting, small frequent meals can be given. Most pet foods fulfil the definition of 'bland,' but boiled rice or pasta and cooked chicken is a suitable short-term alternative if vomiting is the main presenting sign. If there has been concurrent diarrhoea, then additional fibre is necessary. Prescription diets are available that fulfil all these requirements. If a home-cooked diet is strongly desired, then added Metamucil or use of brown instead of white rice may assist the return of the gut to normal function.

3. Do They Have a Potentially Infectious Condition?

Vomiting is seldom the sole presenting sign of infectious disease, but may precede diarrhoea by a number of hours in conditions such as parvoviral enteritis. If an animal has a recent history of being in contact with a large number of dogs (kennels, show), is young or with a poor vaccination history, then infectious disease should be assumed until it is ruled out. These cases should not be brought through the waiting room if at all possible, and should enter the hospital through an alternative entrance. Ideally, the animal should be carried or trolleyed so that it is not in contact with the ground and kept in one area until its infectious status can be determined. Of course, should a condition such as parvoviral enteritis be confirmed, then disinfection of all in contact areas should be undertaken.

4. Should They Be Given Anti-Nausea or Anti-Vomiting Medication? What Other Supportive Management Should Be Done?

If an animal has severe enough vomiting to require fluid replacement, generally they would benefit from treatment with an antiemetic. However, no antiemetic is going to be effective at stopping all episodes of vomiting and it is not proven that stopping vomiting actually improves outcome in many conditions. Prior to a final diagnosis being made, it is best to avoid the use of antiemetics that increase gastrointestinal motility - as contraction against an obstruction may be detrimental.

 Maropitant (Cerenia®). This drug is effective against vomiting caused by peripheral and central disease. It can be given as an injection or as tablets for travel sickness. It does not affect gut motility. Currently, it is relatively expensive.

 Metoclopramide (Maxolon® and other generic brands). This drug has been used for decades in veterinary medicine. It is a moderately weak antiemetic, and also has effects on gut motility. It can be given as an injection, an IV infusion (most effective method of administration) and oral tablets. It is fairly inexpensive.

 Ondansetron (Zofran® and other generic brands). This is one example of a group of drugs developed to treat people for nausea following chemotherapy. It is very effective, and available as dissolvable wafers as well as in an IV solution. It is relatively expensive, but a useful add-on drug.

 Prochlorthiazine (Stemetil®) has also been used extensively in veterinary medicine. The major side effect of this drug is sedation.

Along with IV fluid therapy and control of vomiting, care should be made to carefully assess the animal for pain - as pain may exacerbate the vomiting and prolong return to normal. Currently, the recommendation in people and animals is to feed through diarrhoea, as food delivered directly into the intestine is vital for intestinal health. This premise is always a little more difficult to defend when the animal is vomiting. However, feeding should be considered early in the treatment of any animal with gastrointestinal disease. One or two breakthrough vomits should not be a reason to stop feeding.