I recently found the following question in the Catster answers section:
I’ve just been given a cat, but he can’t meow, and I’ve noticed a rotten smell coming from his mouth. Any ideas?
I definitely have some ideas about this matter.
I am presuming that your cat is attempting to meow but is making no noise. A change in voice or an inability to vocalize may be linked to irregularities with the larynx (voice box), tongue, or mouth. Vocalizing — meowing, barking, or talking — occurs when airflow is manipulated by the voice box, tongue, and mouth in a specific fashion.
Bad breath, or halitosis, is often linked to infection (although metabolic problems such as diabetes or kidney disease also can cause it). Bacteria produce characteristically strong odors which become noticeable when they overproliferate, which is the definition of an infection.
When we put your cat’s two symptoms together, I become worried about an infection in the voice box or the mouth. Some potential causes include dental infections, infected foreign objects lodged in the mouth or throat, respiratory infections, polyps, or even an infected tumor.
Respiratory infections can become overwhelming or can spread to the lungs and become pneumonia. Dental infections are painful and can cause tooth loss. Foreign objects can cause serious tissue damage and erosion of nearby structures. Polyps can become a chronic nuisance and can compromise breathing, and tumors can grow or spread. There are no two ways about it: Your cat needs to see a vet.
It is possible that a simple physical examination will reveal the source of the problem, but your vet may recommend anesthesia to properly evaluate your cat (it isn’t possible to examine the throat or larynx without heavy sedation or general anesthesia). Either way, it sounds like a course of antibiotics is in order.
If your cat simply is not trying to meow, and my presumption that he is trying but failing to meow is incorrect, then the matter is slightly less urgent. Animals often will not vocalize when they are settling into a new home. Over time, your cat may start to find his voice — or not; some cats simply don’t make noise. However, the unpleasant odor from his mouth should be evaluated by a vet as soon as possible.
In fact, even if your cat isn’t showing any symptoms whatsoever I’d still recommend a trip to the vet. New-to-you cats may require vaccinations, testing for feline leukemia and FIV, and deworming. These procedures are especially important if you already have other cats that might be at risk of catching something brought into the house by the newcomer.