Introduction
Alopecia is defined as a complete or partial lack of hair in areas where hair normally is present. It is quite common in dogs as either a primary problem or as a secondary result of another underlying condition. There are distinct differences between cases of alopecia in which grown hair fibers are lost, those in which stumps of hair fibers remain, and those in which the hair root itself is shed from the hair follicle. Regardless of cause, alopecia reflects some disruption in the hair follicle from trauma, infection, immunologic/allergic attack, mechanical forces, endocrine/hormonal imbalances or abnormalities of the cellular receptor sites that control the hair growth cycle.
Causes of Alopecia in Dogs
There is no particular age or sex predisposition to the development of alopecia, although certain dog breeds do seem at increased risk for certain kinds of alopecia. Doberman pinschers and many other breeds are prone to developing color dilution alopecia. Recurrent seasonal flank alopecia tends to occur in Boxers, Bulldogs, and Airedale terriers. Alopecia X (also called adrenal reproductive hormone imbalance and sometimes called “black skin disease”) occurs more commonly in plush-coated breeds such as the Pomeranian, Chow chow, Keeshond, and Miniature poodle, and also in Arctic breeds (Samoyed, Siberian husky, Alaskan malamute).
Some of the more common causes of alopecia in dogs are described below.
Parasites
External parasites are notorious for causing alopecia in dogs. Fleas, ticks, lice and mites can all cause intense itching and scratching which leads to hair loss. Parasites can also physically damage the hair follicles, and allergic reactions to the parasites can cause hair loss as well. Demodectic mange (caused by various species of mites that live within the hair follicles) causes localized to generalized hair loss with redness and mild scaling.
Fungal Infections
Fungal infections of the skin (called “dermatophytosis”) can cause partial to complete alopecia with scaling and with or without associated redness. Some fungal infections are zoonotic, which means that they have the potential to cause skin lesions in people.
Bacterial Infections
Bacterial skin infections – especially those caused by Staphylococcal species – can cause alopecia with redness, crusting and circular patterns of hair loss. Bacterial folliculitis is the most common cause of multifocal alopecia in dogs.
Hormonal/Endocrine Imbalances
Several different endocrine disorders commonly contribute to alopecia. Hyperadrenocorticism (Cushing’s disease, or iatrogenic hyperadrenocorticism caused by veterinary corticosteroid administration), hypoadrenocorticism (Addison’s disease), hypothyroidism, hyperthyroidism, hyperestrogenism (in females) and hypoandrogenism can all result in some form of alopecia if the underlying condition is not controlled. Many dogs have focal hair loss around the genital and flank regions following spay or neuter procedures. Testicular neoplasia (cancer) can also cause hormonally-based alopecia.
Allergies
Immune-mediated reactions to certain foods, chemicals, pollens, grasses and many other potential allergens are among the most common causes of hair loss in dogs.
Inherited Conditions
Some types of alopecia have a genetic basis. These include: acanthosis nigricans, Alopecia X (also called adrenal reproductive hormone imbalance and sometimes simply “black skin disease”), follicular dysplasia, color dilution mutant alopecia, congenital hypotrichosis, pattern baldness and pituitary dwarfism, among others.
Reactions to Medication or Injections
Drug reactions can lead to hair loss. For example, chemotherapeutic treatments for cancer commonly cause alopecia. Hair loss often occurs at injection sites, usually caused by an inflammatory reaction to the substance that was injected. Rabies vaccines have been known to cause patchy hair loss around the injection site 2 to 3 months after being given.
Stress and Nutritional Deficiencies
Nutritional deficiencies and other causes of stress can cause alopecia. Bitches that are nursing puppies often “blow their coat” – a type of alopecia probably caused by the nutritional, physical. and emotional strain associated with whelping and lactation. Dogs that are especially anxious or high-strung, have psychological or behavioral disorders (separation anxiety, etc.), or who have abusive backgrounds may also lose hair as a result of stress.