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.
Lack of hair growth in dogs is different from hair loss, commonly referred to as alopecia.
In some dog 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 in dogs. 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.
Diagnosis of Lack of Hair Growth
If your dog has been clipped and the hair is failing to grow back, it may be due to a hormonal deficiency or imbalance. In order to diagnose this, blood samples are necessary to measure concentrations of various hormones.
If your dog is losing hair in spots then it may have a bacterial infection or ringworm or mange. Your veterinarian may need to do skin scrapings or pluck hairs for cultures.
Treatment of Lack of Hair Growth
Therapy of lack of hair growth will depend on the underlying cause.
- Make sure your dog is receiving a balanced diet with an appropriate level of proteins, minerals and vitamins.
- Depending on the presence and the type of skin infection, you may have to use medicated shampoos on your pet or administer oral medications to clear the infection.
- If you have an allergic dog, work with your veterinarian to manage the allergies without the use of steroids (e.g., prednisone) because these medications have serious systemic adverse effects and may also delay hair growth.
- If your dog is diagnosed with a hormonal deficiency you may have to supplement your pet with hormones. This treatment is usually life-long.
- Prevention and follow-up depends on the cause for the lack of hair growth. As a general rule, the use of steroids should be limited or avoided as they cause medical problems and arrest hair growth.