The patterns of hair growth vary with species and anatomic location on the body. For some animals, seasonal alterations are noted. In dogs and cats, a mosaic exists and coordinated shedding cycles occur.

In some breeds the active or growing part of the hair cycle is prolonged, as in Yorkshire terriers, like it is in people. These breeds are more sensitive to drug therapy, like chemotherapy, that interfere with the hair cycle, and they develop alopecia as an adverse effect.

Numerous factors affect hair growth. They include:

  • Hormones. Some will stimulate hair growth while others will delay it.

  • Androgens. They cause courser hair with lengthened resting phase of hair follicles.

  • Progesterone. This produces courser hair with lengthened resting phase of hair follicles and decreased growth rate.

  • Estrogen. This results in fine sparse hairs and lengthened resting phase of hair follicles.

  • Thyroxine. This initiates hair growth and increases rate of new growth. A deficiency in thyroid hormone (hypothyroidism) usually results in poor hair growth and thinning of the hair coat.

  • Corticosteroids. These drugs retard hair growth by inhibiting new hair growth, thus alopecia or thinning of hair occurs as a consequence of this type of therapy.

  • Growth hormone. The lack of growth hormone results in retention of the juvenile coat or alopecia in the adult.

  • Insulin. This hormone is responsible for normal growth, although diabetics may have alopecia.

  • Nutrition. Poor nutrition can result in a loss of hair.

  • Protein. Cystine and methionine are requirements for hair growth. Protein-calorie malnutrition is characterized by dry, brittle and sparse hairs.

  • B vitamins. These vitamins, especially pantothenic acid, (for copper utilization) are important for proper hair growth.

  • Copper. This is important for hair production and a deficiency will result in a poor hair coat.

  • Some breeds, like the chow-chow, may have an arrest in the hair growth after clipping. This resolves spontaneously after several months of a lack of hair re-growth.

  • Excessive numbers of bacteria in the hair follicle (bacterial pyoderma) may cause circular areas of alopecia or generalized excessive shedding. These signs resolve with a few weeks (3 to 4 weeks) of systemic antibiotics. This presentation is very common in dogs with allergies.

  • Ringworm (dermatophytosis) can also cause the hair to fall out in spots. Ringworm can be diagnosed or ruled out by submitting hair samples for fungal culture. 

  • Mange can also cause patchy hair loss. Demodicosis is a type of mange that is non-contagious and can be diagnosed by performing skin scrapings. There are other types of mange that can also lead to alopecia.

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