It is known that depression is caused by a chemical imbalance in the brain that is treated with selective serotonin reuptake inhibitors. It is not unlikely that this imbalance occurs in other species besides humans. In the past, depression in an animal was synonymous with lethargy, but today it is widely considered a mental illness and/or clinical condition.
Antidepressants are used in veterinary medicine, having been first introduced by Dr. Nicholas Dodman at the Cummings School of Veterinary Medicine at Tufts. They are mostly used however, to treat behavior issues, such as obsessive-compulsive disorders and separation anxiety. When Dr. Dodman started using them in treating dogs, it was so revolutionary that it made the front page of The Wall Street Journal. This sort of treatment has not yet become standard protocol for treating depression.
Stress is also a big factor in pets feeling uneasy, so what may be identified as depression, may not necessarily be a fully physical issue. For example, cats do not adjust well to a move or the addition of a new pet or person in the household. They in turn, often respond by avoiding the litter box, meowing incessantly or hiding. It could be that the pet may be experiencing stress and may not necessarily be clinically depressed.
More research clearly needs to be done in this area, and since our pets lift our spirits daily, it would be wonderful to be able to return the support and love.