Thank you David,
A rotten odor from Nina's mouth is quite concerning. When we smell abnormal odors from a young cats mouth, we do have to consider a few issues. This includes issues based within the mouth, the stomach, and the lungs. Anal glands can cause oral odors when cats are licking their backside frequently, but this would be less likely here since they often have a pungent, fishy odor instead of rotten one.
Now the most common site of the above to lead to rotten breath is disease within the mouth. Specifically, we often see an unpleasant change in breath due to tarter build up, tooth decay, gingivitis, and rotting teeth (since these cats aren't brushing their teeth). In mild cases, sometimes we can help them at home with dental care (ie brushing with enzymatic kitty tooth pastes/brush) and oral treatments (ie Plaque-off,
Dental Diets like Hill's T/D,
Otherwise, another issue that can lead to oral based odors would be injuries or issues wth the rest of the mouth. In this vein, any foreign material lodged between the teeth or injuries to the tongue (via rodent bites, etc). can also cause these signs. As well, while your cat is fairly young, we can't rule out oral tumours. These can arise under the tongue or in oral mucosa. These can appear as ulcers and can have a secondary infection associated with them. As well, we can see infections arise and cause odor assoicated with polyps (which are benign) in the mouth or back of the throat.
As well, another consideration is that we can see these signs associated with feline herpes virus induced oral ulcers. Herpes is a virus that causes lifelong infection, and often these cats are carrying it since kittenhood. It lies latent but can pop up and cause signs of respiratory disease (ie cough, sneezing, and runny eyes) or oral disease (ie gingivitis, oral ulcers of the mouth and throat). In the latter, this could open the door for bacterial infection and odor.
Less commonly, but other concerns for your lass would be odors and ulcers associated with kidney disease. That said, I wouldn't expect this for a 3 year old cat who is otherwise well. Still, if her breath smells rotten and you have noticed an increase in her thirst/urination, then this may be something to rule out.
Looking to sites that will use the mouth as an outlet (and thus could cause the odor you are smelling), we do also have to consider the stomach and lungs. Now you have not noted any respiratory signs, so this will hopefully be less likely. But if you have seen any flu signs (ie coughing sneezing, snotting) or changes in her respiration then we'd have to consider pneumonia as it can cause rotten oral odors. More commonly though, we can see diets disagree with the gastrointestinal tract. Often this will cause gas in animals but it can also lead to noxious gas/burps being passed. In these cases, we sometimes need to make a diet change (sometimes soy levels or fiber levels can disagree with the GI bacteria and lowering can help them). Otherwise, we can combat this by treating with charcoal (to bind those odors). With this, there are preparations to add to food (example)
that might be worth trying here.
Overall, there are a number of sites and issues that could cause her foul breath. Ideally, since oral based disease is the most common source of these odors, you do want your vet to thoroughly examine her mouth. Depending on their findings, you will know what is to blame and be able to take steps to address it (ie antibiotics, removal of rotten teeth). At the same time, they can listen to her lungs if there is any concern that the lungs could be involved. Otherwise, you can consider one of the dental/oral aids above +/- consider ruling out GI based odors to see if you can settle this rotten breath for her.
I hope this information is helpful.
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All the best,
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