Cats are very good at hiding illness. This is a protective instinct from the wild. They do this to prevent potential predators from knowing that they a in pain, weakened, or ill. Unfortunately this means that a pet owner may not realize there is a problem until an illness is fairly advanced.
Other factors can compound this. Many felines are a part of multiple cat households; and many felines live outdoors for part of the time. This can make it very difficult to detect decreases in appetite, the presence of vomiting and/or diarrhea, as well as changes in water consumption and/or urination. Outdoor felines are also at risk for contact with poison, getting attacked by other cats or dogs, and being hit by a car.
Cats are also great hiders. It can be very normal not to see them for long periods of time. When they are not feeling well, cats may hide to conserve energy or avoid pain. Be aware that the living arrangements that you provide for your feline can make it challenging to detect illness.
Bring your cat in for a veterinary exam at the first sign that you identify a health issue; and realize that your feline may have been ill for much longer than you think. It is well supported that bringing your beloved feline in for care at the first sign of a medical problem increases the likelihood that treatment will be successful.
Expect the veterinarian to run at least a blood panel, and possibly a urinalysis. Depending on symptoms and weight loss, X-rays may be recommended. Often a feline leukemia and feline immunodeficiency test will be suggested, especially if your cat ventures outdoors. Treatment will depend on symptoms and diagnostic findings. Be aware that hospitalization on intravenous fluids and nasogastric tube feeding may be recommended if your cherished companion hasn't been eating well for more than a day.
There are several steps that you can take to make it easier to identify sickness in your cat, even in multiple cat households. Try to feed cats separately. This gives you the best idea of how much each pet is eating. It is also a good way to feed a specialized diet or adjust calories for weight problems if needed.
Look at each pet often, ideally daily to see if he/she has normal energy. Weigh your feline monthly. You can do this fairly easily with a bathroom scale. Simply weigh yourself with and without your pet and subtract your weight. Even if this value is not identical to a weight measured at a veterinary clinic, it will help you to identify changes early on.
If your cat goes outdoors, check your yard regularly for signs of vomit or diarrhea. This is also a good practice for fly and parasite control. Ideally feces should be removed on a daily basis. Sometimes this can be difficult because cats can be very secretive about where they poop. If you cat uses a letterbox, watch for changes in how wet the litter is, and how often you are needing to clean it out.
Cheers to good health for your felines in the New Year!