Is your cat out of sorts lately? Is he less interested in his daily routine? Not eating? Not playing? He could be depressed.
As with humans, emotional state can have a very significant effect on one's health. Happy cats, like happy people, are generally healthy and active and like to play, cuddle, eat great food, and have no trouble sleeping well. Depression, on the other hand, can lead to physical problems that may cause one's health to deteriorate.
At my shelter, we have taken in more cats than dogs when families had to move, as they didn't feel as strong a connection as they did with their dogs. Sadly, since cats can have feelings that run more deeply than a dog's, some of those cats pined away and died, no matter how hard we tried to help them.
Why would a cat become depressed? As with the moving example, cats suffer deeply at the loss of their bonds with others in their lives. They may bond with another pet, or with certain individuals in your family. Since you can't explain these things to them, all they know is that they are in a new place and the people they depend on are not there.
This disorients them and they become emotionally lost. If they refuse to eat (a common reaction to this kind of stress), they not only will quickly lose weight, but the liver will begin to break down, leading to hepatic lipidosis, or, fatty liver. It's fatal if not headed off soon enough.
If your cat is still at home, however, there is something else going on... possibly some disease process, or perhaps some changes in the dynamics of your home that are causing your cat to feel down. This could be the loss of a family member, or the addition of one. Or the addition or loss of another pet.
Try offering him something he likes, such as catnip or his favorite foods; spend more time with him, playing, cuddling or just talking to him. Cats can feel left out and ignored, too. Also try a home health examination to see if you can spot a health problem.
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If these things don't help, it's definitely time for a veterinary health checkup. And don't wait too long.