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If you notice behavioral changes in your cat, make an appointment with your veterinarian. You want to make sure there isn't an underlying medical issue causing your cat's depression that might require specific treatment.  Tell your vet about any changes you've noticed in your cat, such as changes in appetite, sleeping habits, and personality. Your veterinarian will perform a routine physical exam, listening to your cat's heart beat, examining her eyes and ears, and taking her temperature. If your vet feels it is necessary based on the behavioral changes you've outlined, she might call for blood work, x-rays, or other additional testing. Some test results will be ready within the appointment period while others may take a few days to come back. If there is no medical issue, your cat may be suffering from feline depression. Feline depression is linked to a variety of causal factors. Consider your current situation and try to figure out if there have been any big changes recently that might have triggered a depressive episode.  Have you moved recently? Changing residencies is one of the biggest causes of feline depression. Many cats have trouble transitioning from apartment to apartment or house to house and may fall into a temporary depression while adjusting to the move.  Has there been a death in your household recently? Whether it was a person or a pet, death does affect cats. They do not perceive or understand death the same way humans do, but will notice the absence of a person or animal. This can lead to feelings of depression.  Have you recently become more busy? Whether it's due to work, your social life, or a new romantic partner if you've found yourself spending less time with your feline friend this could easily cause feline depression. Cats, especially breeds like the siamese, are social creatures and may become depressed if they feel they're being neglected. Seasonal Affective Disorder (SAD) is not just an affliction of people. Cats can be affected by the time of year and may become depressed during winter months.  Winter months result in shorter days, which means less sunlight. Lack of sun can cause depression in cats, which results in changes to their behavior. If your cat's personality seems to change with the seasons, they may be experiencing seasonal depression.  Sunlight affects levels of melatonin and serotonin. This can lead to fatigue, anxiety, and sadness in both humans and cats. Outdoor cats are especially susceptible to depression in the winter as so much of their time is spend outdoors.
Get your cat examined by your veterinarian. Take stock of any recent changes. Note the time of year.