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Causes of Hair Loss in Dogs and Cats

Causes & Diagnosis
Hair Loss
Causes & Diagnosis
What can cause pets to lose their hair?

Hair loss in dogs and cats can appear as areas without hair (bald patches or alopecia) or areas with thinning hair. There may also be changes in the skin with increased pigmentation, redness, swelling, crusting, flaking, or moist oozing and discharge. There may be an unpleasant odor. If hair loss is caused by malfunction of adrenals, liver, kidneys, ovaries, testicles, or thyroid, pets may also have these signs:

  • Behavior changes and irritability
  • Lethargy
  • Restlessness
  • Weight gain or weight loss
  • Increased water consumption
  • Increased urination (polyuria/polydipsia or PU/PD)
How is the cause of hair loss diagnosed in dogs and cats?

Hair loss in dogs and hair loss in cats is caused by so many different medical and behavioral problems that taking a thorough history is essential to steer the diagnosis toward problems caused by illness, infection, stress, surgery, behavior, pregnancy, parasites, vaccinations, or drugs. In addition to a thorough history, some of the following medical tests can be necessary to reach a diagnosis: combing for flea dirt, skin scrapings, skin biopsy, skin culture, hair culture for fungal growth, blood test, urinalysis, X-ray, and Woods lamp illumination.

Evaluating hair loss in dogs and cats

Before ordering medical tests, veterinarians evaluate:

  • The pattern of hair loss
  • Whether the hairs fell out or were chewed out
  • Whether there are signs of systemic illness
Patterns of hair loss in dogs and hair loss in cats

Among the hair loss in dogs and cats, patterns may include:

  • Thin all over
  • Thin over the back and sides
  • Thin over the ears
  • Thin along the neck
  • Thin under the belly
  • Complete loss along sides and down tail leaving the tip with hair (like a lion's tassel)
  • Circular patches of baldness
  • Bald abdomen
  • Bald patches on fore legs above the wrist (carpus)
  • Bald patches on back legs on the side above the ankle (tarsus)
  • Balding along the back of the hind limbs up over the scrotum

Generally, the pattern of balding or thinning suggests the cause of hair loss. For example, pets with chewed off hair and bald, patchy areas over the base of the tail, inside the back legs, and on the abdomen often have fleas. Pets with thin skin and hair loss along the back and sides, and down the tail, but not the tip of the tail often have high cortisol levels and Cushing's disease. Pets with thick skin, weight gain and dull, thinning hair over the back and sides often have low thyroid levels and hypothyroid disease.

Hair loss due to chewing and shedding

To evaluate whether hair was chewed off or fell out, hair is put under the microscope. Hair that falls out has a root with a little bulb. Hair that is chewed out does not end in a smooth bulb. Hair that falls out suggests illness, hormonal, or endocrine problems, but hair that is chewed suggests fleas, other external parasites, or behavioral problems.

Signs of an underlying disease or illness

Often pets that are ill hide symptoms and we first realize they might be ill because the hair falls out. For example, pets with Cushing's disease begin drinking more and urinating more, but we might not notice until we realize their hair is falling out on their sides. Pets with hypothyroid disease become lethargic and put on weight, but we might not realize this is a medical problem until their hair thins or fails to grow back in after they are clipped.

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