This phenomenon can be local or general, temporary or permanently.

Alopecia is a scientific term which is used in veterinary science for hair losing.Sometimes alopecia  is occurred with changing color of the skin.Hyperpigmentation  is used when  the color of the skin is changed to darkness and hypopigmentation is a vice versa situation.

Melanin  is a dark pigment which is in the skin.The more melanin skin has, the darker it is. Pruritus is a term which is used to explain scratching feeling.Now and then alopecia has pruritus.

Reasons: there are so many reason for alopecia including:

Genetic, skin parasites like fungal infection or flea, bacterial infection, after severe fever ,deficiency like zinc deficiency in husky  breed, vaccination , skin contact with irritant materials, some poisonings, skin autoimmune diseases, certain tumors, allergic reaction to certain medications, sunburn, food allergy especially in cats, self trauma, psychological  alopecia, endocrine diseases like cushing disease

Diagnosis:  on the basis of symptoms and clinical signs, skin scraping test, blood test , special endocrinology tests, radiography, sonography, pathology and sometimes trial treatment , diagnosis is done.

Prognosis: in most often with accurate diagnosis and treatment hairs will be regrow.For some certain diseases using medication during life is necessary.Needless to say sometimes diagnosis is expensive and it takes time.