Bad breath, or halitosis, is very common in dogs and cats; however, there are a wide range of possible causes. Some are simple to treat; others less so – but bad breath is almost always symptoms of an underlying problem.

There is one, harmless cause of halitosis – eating something rotten or smelly (much more common in dogs than cats)! Some dogs love eating faeces or rotting food; this may be habit, or greed – but in a small percentage of cases is due to a condition called pica. This is when the animal will eat pretty much anything, whether or not it is actually food-like, and may be due to mineral or vitamin deficiencies or certain brain diseases. In most cases, however, eating rotting or smelly things isn’t due to a disease condition (although it may well lead to a nasty episode of vomiting and diarrhoea!).

Metabolic diseases can also cause bad breath – especially diabetes and kidney failure. These conditions are both associated with changes in urination and drinking, and often weight loss. If untreated, both are potentially fatal. In diabetes, the breath may smell sweet (because of the excess sugar in the bloodstream); sour (because of increased bacterial growth, as the bacteria feed on the sugar); or musty (as yeasts grow in the mouth). In kidney failure, the breath may smell metallic (due to a build-up of toxins and waste products that the kidneys aren’t filtering).

Diseases of the respiratory tract such as sinusitis, nasal infections, and nasal tumours may also lead to bad breath. This is caused by the production of pus (dead, dying and decomposing white blood cells, bacteria and blood) in the nose, which trickles down into the back of the throat.

Some diseases of the gastrointestinal system can also cause halitosis, particularly megaoesophagus (where the gullet becomes swollen and dilated so food pools in it) or persistent vomiting (e.g. due to a blockage of the bowel, gastritis, kidney or liver disease). Infections of the mouth or the lip folds (e.g. in spaniels) may also cause it.


However, by far the most common cause of bad breath in dogs and cats is dental disease. Unless we regularly brush their teeth, most (although not all) dogs and cats will develop tartar and plaque on their teeth. This material is a mixture of salts from the saliva and masses of bacteria, living off the food in the mouth.

While this is on the ends of the teeth, it isn’t a major problem (although it may smell a little); however, once it reaches the gum line, it rapidly becomes dangerous. When these plaques of bacteria touch the gum, they cause inflammation and infection of the gum tissues (called gingivitis). If untreated, this will spread down into the sockets of the teeth (periodontal disease) and lead to damage to the ligaments of the teeth.

In some cases, infection may even penetrate the bone causing a tooth root abscess (which may burst through into a sinus causing sinusitis) or even osteomyelitis (infection of the bone). Even simple gingivitis is a risk factor for heart disease and kidney failure as the bacteria can easily enter the bloodstream – in severe cases, they may even suffer from blood poisoning and septic shock.

In addition, a dog with severe gingivitis or periodontal disease will be unwilling to eat, and may eventually starve to death. If untreated, gingivitis will almost inevitably progress (the speed is variable; the fact of progression is not). As you’ve found, in many cases by the time the problem is diagnosed, the affected teeth cannot be saved.

So how do we know what’s going on?

In bad breath cases, you really do need to find out what’s causing the problem! For that, you’ll need to get him seen by your vet. Kidney disease is easily detected on a blood test (elevated levels of urea and creatinine, two waste substances normally filtered by the kidneys); it can also be detected by certain tests on the urine (urine protein/creatinine ratio, or UPC; and specific gravity). Diabetes may be apparent on a single blood test (as a raised blood sugar level) – however, if the patient is very stressed (more of an issue in cats), you can get a false positive result. For a definitive diagnosis, it is often best to send away a blood sample for a fructosamine test (which will show the average blood sugar level over the last few weeks).


Respiratory disease is usually easy to recognise (snotty nose, sneezing, coughing, facial deformity in the case of some tumours or polyps), although actually working out what’s causing it often requires advanced imaging (X-rays and endoscopy). Similarly, it is very unusual for bad breath to be the only symptom of a dog or cat with a significant gastrointestinal problem – vomit or diarrhoea, or regurgitated food matter, is a more common finding. Lip fold dermatitis is easily recognised on examination, as when opened out, the lip folds are red, sore and often smell musty.

As I said above, dental disease is the most common cause. Often, a simple visual examination will reveal significant plaque and tartar; and gingivitis may be obvious just by looking at red or swollen gums. Occasionally, there is a tumour or other disorder of the gums, but again, this is usually clear to see. Your vet will be able to tell you what the chances are that dental problems are causing your pet’s bad breath.

So what can be done about it?

That, of course, depends what the underlying problem is… Diabetes cannot usually be cured (although some cats, if caught early enough, can go into full remission if treated appropriately and aggressively), but can be managed with appropriate, diet, insulin injections and good blood-sugar monitoring. The same applies to chronic kidney failure – this can be managed with appropriate diet, ad lib access to water, and sometimes medication (ACE inhibitors).

Respiratory and gastrointestinal disease does need diagnosis and treatment – if the underlying cause is treated, the halitosis will usually resolve at the same time.

If the dental disease is significant enough to cause bad breath, it does need treatment. It is important to remember, however, that old pets can, perfectly safely, undergo anaesthesia for a dental as long as there aren’t any underlying heath issues. Old age is not a risk factor for anaesthesia per se, it just means it’s more likely that they’ll have some medical problem that is! In older patients (cats and dogs) use of a fast, modern anaesthetic gas (e.g. sevofluorane), intravenous fluids and good, careful monitoring means that their risk isn’t that much greater than a young pet, assuming they are otherwise healthy (and if they aren’t, bad breath is the least of your worries).


Fortunately, however, in many cases the vet will be able to remove the worst of the tartar by hand without needing a full dental under anaesthesia. Often, the dental disease can also be controlled (not cured, but kept manageable) by regular and diligent tooth brushing and the use of appropriate mouth-washes.

David Harris BVSc MRCVS

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