Nearly all diseases of the skin have the potential to cause alopecia. It is important to remember that it is normal to lose hair. Hair follicles are continually losing and developing new hairs. Alopecia can be separated into pruritic (itchy skin diseases) and non-pruritic.Pruritic Causes of Alopecia Fleas and flea allergy. Dogs with flea allergy tend to lose hair over the rump, on the back, hind legs, tail and belly.
Atopy. An allergy to airborne allergens such as pollens and molds, and food allergies tend to cause alopecia of the face, ears, feet, and other areas.
Scabies. Infestation with the Sarcoptes mange mite is an intensely itchy skin problem that causes alopecia of the ears, elbows, hocks, and other areas.
Other parasites such as Cheyletiella or lice cause pruritus and alopecia.
Pyoderma. An infection of the skin by bacteria or other infectious agents causes pustules. Pyodermas that involve hair follicles cause alopecia. Pyoderma may or may not be pruritic.
Dermatophyte fungi. Ringworm can cause either a non-pruritic or pruritic alopecia by damaging the hair shaft. The hair then breaks off and is shed into the animal's environment where it can contact other animals and transmit the disease.
Non-pruritic Causes of Alopecia
Lack of production of thyroid hormone by the thyroid gland. Dogs with hypothyroidism tend to have sparse hair coats or patchy hair loss especially over the back and along the tail.
Hyperadrenocorticism. Cushing's disease is an increase in the production of a hormone called cortisol by the adrenal glands. These dogs tend to lose hair in a symmetrical pattern and their skin will have a thin appearance. In addition, they have suppressed immune systems and are prone to pyoderma.
Other endocrine (hormonal) diseases such as imbalances in sex hormones produced by the adrenal glands can cause alopecia.
Autoimmune skin diseases are disorders where the body's immune system attacks a component of the skin. These diseases can cause alopecia and may or may not be pruritic.
There are a number of other skin diseases that can feature alopecia, such as congenital hair follicle diseases that cause alopecia in the young puppy, and acquired hair follicle diseases that occur later in life. These are rare and may require the help of a veterinary dermatologist for diagnosis.