Cat and dog owners often complain about their pet’s bad breath. Mostly they assume this is because of bad dental health. Oftentimes it is. As calculus grows to tartar, the bacteria that live on the gums have an environment favouring overgrowth leading to soft tissue damage and eventually tooth rot. Halitosis may also come from mouth/throat infections or growths not involved with the teeth.
But often, there is nothing wrong in the mouth and that funny smell enters the breath deep inside the lungs. In mammalian lungs, where oxygen and carbon dioxide pass between the air and blood, the microscopic air sacs at the end of the airway “tree” are called alveoli. They say that if you opened out and placed together all the linings of the alveoli of an adult human’s lungs, it would cover a tennis court. We can’t smell oxygen or carbon dioxide but we can smell some volatile molecules that end up in the blood. Let those molecules perfuse across a fine membrane, many square metres in area, and package that up into your animal’s exhaled breath and .. hey presto, bad breath!
If your pet’s breath smells like urine or ammonia that will be urea and the levels in their blood will be way above normal, healthy levels. This most likely will be from kidney failure, a well-documented phenomenon. Some say “foul/sweet/musty” breath can come with liver disease. I often smell cattle breath to aid diagnosis of bacterial pneumonia.
It is probable, too, that foul smelling breath can come from poor digestion or malabsorption. If enough undigested food makes its way into a carnivore’s large intestine some of the bacteria living there, normally in small amounts, can thrive. One of their waste products can be volatile gasses that can be absorbed into the rich blood supply around the alimentary tract. So if your cat or dog seems healthy, but has bad breath, one option is to try a different diet or oral probiotics to see if that helps.
If an animal has to break down their fat reserves aggressively, maybe because it cannot eat enough to meet its current energy demands or if it has uncontrolled diabetes mellitis, and glucose from carbohydrates in the diet can’t get to the “cellular engine rooms” where it is needed to be an energy source, then the levels in the blood of volatile molecules called ketones will rise. One of these is acetone. I can smell this “sweet/nail polish remover” smell on the breath of cattle with ketosis or ewes in late pregnancy with “sleepy sickness”.
I suspect a lot of the “doggie smell” complained about on some of our canines comes from itch/scratch causing skin bacterial and fungal overgrowth, sometimes before there is visible damage. If your small dog sometimes gives off a rotten-fish smell and scoots along on its bottom this is probably from anal sac trouble. Your vet will be thrilled to explain what the go is with them.