Watch for Changes in Behavior
For a cat that has gradually stopped good grooming habits or has a poor hair coat, one possible explanation is if the cat is overweight -- they simply can't reach everywhere like they used to. Poor nutrition and some hormonal or metabolic diseases can also cause poor condition of the skin and haircoat.
Time for a Vet Check
For a cat that has shown gradual changes in grooming habits paired with a malodorous drool, your veterinarian will want do do a complete oral examination to look for potential causes. Possibilities include, but are not limited to: diseased teeth and gums, an oral foreign body, or an oral tumor (more common in senior cats).
Take a Closer Look
A senior cat with a foul odor, drool, and poor grooming habits, after ruling out obvious dental/gum disease, would be suspect for an oral tumor. While dogs often have benign growths in the mouth, cats more commonly have malignancies. Many cats still seem hungry and interested in food, at least initially. The owner feeds the cat, sees the interest in food, but may not notice that the cat is only eating a small portion or not at all. Other pets may finish the food to further confuse the issue.
Oral Tumors in Cats
Common oral tumors for both dogs and cats are squamous cell carcinoma, fibrosarcomas, and malignant melanomas1. Oral tumors can be anywhere, but the ones under the base of the tongue can be hard to see at first. The tumor grows, ulcerates, and becomes infected. This is cause for the odor and in some cases, pain. These tumors themselves are generally not painful, but the infection is. While the tumor might not be readily apparent, it can interfere with normal swallowing, and this leads to drooling and inability to eat.
Treatment for Tumors
As with human medicine, treatments are available in the form of surgical excision, chemotherapy, and radiation. The patient's age, overall health, size and invasiveness of the tumor all come into play when deciding the best treatment option.
The Sooner the Better
Any time your pet exhibits a new behavior, clinical sign, or general change in attitude, please consult your veterinarian as soon as possible. Hopefully it is something that is easy to address. In the case of something more serious, the sooner the problem can be attended to, the better.
Ref 1: American Veterinary Dental Society - Oral Tumors