Dog and cat hair loss is the abnormal thinning or complete loss of hair, and the medical word for it is alopecia. Hair loss is so common that it is one of the main reasons pets visit their veterinarians. Hair loss always indicates a medical problem, and fortunately the most common cause is an allergic reaction to fleas. Hair loss caused by fleas is easily remedied with topical flea treatments and removal of fleas from the environment. The opposite of hair loss is hair growth or hirsutism, which is also a medical problem.
For both dogs and cats, hair loss and hair growth can all be influenced by:
Parasitic fleas, lice, and mites cause hair loss because they cause itching. The pet scratches or bites vigorously and the hair is chewed or broken off. The moist, chewed skin is ripe for skin infections with yeast, ringworm, or bacteria. The yeast, ringworm, and bacteria also cause itching, and even more hair is chewed off.
Where the dog and cat hair loss occurs suggests which parasite is involved. Fleas attack the back over hips; lice often attack the back and back legs; mites focus on eyes, ears, mouth, and elbows.
When humans are allergic, our eyes water, our noses run, and we itch. With pets, allergies are expressed in the skin and ears rather than the eyes and nose. So, allergic pets have itchy skin, and in response they scratch or chew out their hair. Pets can be allergic to:
Common food allergies are caused by grains (wheat, corn, soy), meats and fish (beef, lamb, pork, salmon), milk and yeast. Common inhaled allergens are pollens, cigarette smoke, and perfumes–especially those added to cat litter. Pets can experience contact allergies when they walk through grass, across chemically treated decks, or on carpets with chemicals in the carpet or underpad.
Hair requires a constant supply of nutrients to remain anchored in the skin. Nutrients that support healthy hair are the same as those that support healthy skin: vitamins, minerals, antioxidants, and Omega 3 and Omega 6 fatty acids. Hair that doesn't receive a balanced supply of nutrients becomes dull, loosens, and falls out. For example, pets on starvation diets have thin, dull coats. Hair loss due to poor nutrition often involves the whole pet, but may be most obvious over areas that are easily worn and over the back and hips where hair follicles have shorter growth cycles and longer inactive periods. Some Northern dog breeds, such as the Siberian Husky, have a genetic tendency to zinc deficiency that leads to skin and coat problems. The problem is in the pet's inability to absorb zinc, which is usually present in adequate amounts in the diet. The medical term is Alopecia X of the Northern breeds.
Because the kidneys, liver, intestines, and other organs regulate the nutrients in the blood, diseases and drugs affecting these organs directly influence hair loss. For example, pets with inflammatory bowel disease, cancer or on chemotherapy often have dull, thinning hair throughout. Pets with kidney failure often have bedraggled, dull coats and may have a strong smell of urine from their skin. Pets with liver failure have orange-yellow skin (jaundice), and nails that grow long but are weak and flaky.
Hair is a living element anchored in the follicle and nourished by blood for most of its cycle. When the blood doesn't circulate, hair will not grow well. Pets with weak hearts, low blood pressure, and chronic anemia may have cool skin and dull coats. The thinning coat is from hair being lost from the follicle rather than being licked and broken off.
Hormones are chemical messengers made in one part of the body and used in other parts. Some hormones travel throughout the body, and other hormones travel only as far as the cells around them. Many hormones influence hair growth, including testosterone, estrogen, melatonin, growth hormone, thyroxin, and cortisol. Abnormal levels of these hormones cause hair to be too thin or to be too thick. Examples of how thyroxin and cortisol affect the hair follow.
The thyroid gland in the neck makes the hormone thyroxin which travels through the blood and influences almost every cell in the body. Thyroxin increases the rate that cells grow and multiply because it stimulates the cell's nuclear machinery. Hair follicles and skin cells are as strongly influenced by thyroxin. With normal thyroxin levels, hair growth is normal. With insufficient thyroxin, which usually occurs in dogs (hypothyroidism), hair growth is thin, especially over the back. With excess thyroxin, which usually occurs in cats (hyperthyroidism), the coat is poorly groomed and matted over oily clumps of skin cells.
Cortisol is a hormone released from the adrenals that is carried by the blood and influences most cells in the body. When cortisol levels are too high–due to Cushing's disease or cortisol medication overload–hair thins over the back all down the tail, leaving tuft of hairs at the very end (rat tail). If hair is clipped anywhere on the body, it grows back very slowly. When cortisol levels are too low–hypocortisolemia or Addison's disease–hair loss may also occur.
Several oral, topical and injected medications cause hair loss. For example, high doses or long-term use of oral, topical, or injected steroids can cause hair follicles to shrink and hair to fall out. Hair regrowth is delayed until follicles are no longer influenced by high steroid levels. Some topically applied flea medications cause hair loss at the area of application. Injected vaccines cause hair loss at the injection site and in some pets there is further widespread hair loss over the next few months. Fortunately, this is rare. With all these medications, hair is lost from the follicle rather than bitten off.
Pets can develop hair loss because they have behavioral problems. For example, dogs with a separation anxiety behavior disorder lick patches of hair off their legs. This is called acral lick dermatitis. With acral lick dermatitis, dogs can lick so unrelentingly that their skin breaks down and becomes infected. What began as anxiety turns into a bacterial and yeast infection that is difficult to cure because the pet licks it whenever left alone. The areas normally involved are the front legs just above the wrist (carpus) and back legs near the ankle (tarsus). Cats can also over groom or barber themselves until bald spots spread over abdomen and thighs. When over grooming is causing bald spots, the hairs will be broken or chewed off and the only involved areas will be where the pet can reach so the back, head and neck are never involved.
All pets are susceptible to hair loss, but the following breeds often have their own particular problems. For example, Dachshunds have a breed predilection for thinning over the ears, abdomen, and neck. Hair loss due to hypothyroid disease affects the Afghan hound, Airedale, Boxer, Chow Chow, Cocker Spaniel, Dachshund, Doberman Pinscher, English Bulldog, Golden Retriever, Great Dane, Irish Setter, Irish wolfhound, Miniature and Giant Schnauzer, Newfoundland, Poodle, Scottish Deerhound, and Shetland Sheepdogs.
Hair loss due to hyperthyroid disease affects many breeds of cats, but is uncommon in Siamese and Himalayan cats.
Hair loss due to hyperadrenocorticism (Cushing's disease) affects these dog breeds: Beagle, Boston Terrier, Boxer, Dachshund, and Poodle.
These breeds grow hair very slowly after being clipped: Siberian Huskies, Alaskan Malamutes, Chow Chows and mixed breed dogs with Northern genetics. These breeds evolved in cold, snowy climates and shed and regrow their hair once a year or once every two years.
Hair begins growth within a little pocket called a follicle where it is nourished by proteins and other materials in the blood. Blood also carries hormones, such as epidermal growth factor, that determine the growing, resting, regressing, and shedding phases.
Hormones that control hair growth are influenced by sunlight and temperature, so that many pets have major growing and shedding cycles each spring and fall. With more pets kept indoors in a controlled climate, marked growing and shedding cycles are less frequent, and many pets appear to shed more evenly throughout the year.
There are some dogs that do not shed very much at all. These dogs have hair that grows constantly and can become very long. Poodles and Shih Tzus are good examples of dogs with constantly growing hair. Although these pets don't shed, their coats require care, usually clipping every 4-6 weeks.
Hair growth can be compared to leaf growth. Hair grows in cycles or phases, which is easy to understand when hair is compared to leaves on trees. Trees have new leaf growth in the spring, then during the summer leaves stay active but are no longer increasing in number. In the fall, leaves die. In late fall, they fall from the tree. For hair, the phases are similar:
Hairs over the body don't move through the phases at the same rate. For example, hair over the back and hips grows more slowly than hair on the chest. The rate at which hair grows is influenced by hormones and by blood supply. A good blood supply brings nutrients and a normal concentration of hormones so that the area experiences healthy hair growth. Areas with a poor blood supply do not have healthy hair growth.
Unlike humans who have one of a few hairs growing in each follicle, most dogs and cats have many hairs in each follicle. With so much hair in a follicle, when follicles go through the shedding phase there is lots of hair to clean up.