Factors influencing hair shedding

Normal cycle of hair growth

The skin of cats and dogs is completely covered with hair made up of compound hair follicles except for the nose, footpads and mucocutaneous junctions of each body orifice. Three types of hair are present, the primary or guard hair, the secondary hair and the tactile or sinus hair. The coat comprises (except in hairless breeds) primary (guard) hair and secondary (lanugo) hair. Hair growth in cats and dogs is cyclic. Each cycle consists of an active growing phase, anagen; followed by a transitional phase, catagen, in which the hair bulb becomes pinched off from its dermal attachment; and a resting phase, telogen, in which the hair is retained in the follicle as a club hair and is eventually shed. The ratio of primary to secondary and hair density are important in determining coat type in dogs.

Factors influencing hair growth and shedding

Genetic factors: Rate of hair growth in dogs is dependent on the site of the hair and therefore also the ultimate hair length, with longer hair growing at a faster rate than shorter hair. Variations in the duration of the period of anagen are also likely to be partially responsible for this effect. There are also individual variations in rate of hair growth, even within members of the same breed.

Photoperiod: Photoperiod is responsible for the production of seasonal moults. Shedding occurs mainly in Spring and Autumn. It appears that increasing photoperiod is a stimulus for hair loss and replacement in spring. It appears that photoperiod is more important to shedding than environmental temperature. This effect is seen in indoor dogs and cats who are exposed to many hours of artificial light and moult continuously throughout the year.

Hormones: Hormones can affect the initiation of anagen and the rate of hair growth. Thyroxine and growth hormone are both important in the initiation of anagen and will increase hair growth rate; oestrogens inhibit anagen and rate of hair growth; androgens have a variable effect. Disease and stress can inhibit anagen and in some cases there may be a sudden, synchronised cessation of anagen which results in hair loss of some two to three months later. Ovarian hormones may exert an influence on the time for which club hair are retained in resting follicles, the duration of both the active and the resting phases of the follicular cycle, and the rate of hair growth. Further evidence of the role of gonadal hormones in hair growth and shedding arises from the observation that in both cats and dogs, symmetrical bilateral alopecia which occurs most frequently in neutered animals is responsive to ovarian hormone replacement therapy. Hormones influencing metabolism and growth have a non-specific action on hair growth and thus the rate of shedding.

Environmental temperature: Seasonal effects on the hair growth cycle unrelated to those caused by seasonal variations in hormone levels may be ascribed to environmental temperature.

Diet/Nutrition: Hair is composed of 90 percent protein and as a consequence up to 30 percent of the body’s daily protein requirement is utilised in the processes of hair growth and epidermal keratinisation. Various mineral and vitamin deficiencies have been observed to increase hair loss and/or cause alopecia in dog and cat. Nutritional dermatoses are rare when a well balanced, complete diet is fed, but may occur when the diet is poorly formulated or stored, when the animal’s intake is reduced (due to stress or disease), or when the animal is unable to digest, absorb or utilise the nutrient as a result of disease or genetic factors. Nutritional supplements are commonly used to improve coat condition, but they are rarely formulated specifically for dermatoses.

Age: Puppies appear to have a lower percentage of follicles in the telogen phase than older dogs and one might therefore expect older dogs to moult more copiously.

Body region: The speed of hair growth in the different regions of the body may be very marked in some animals (for example, Afghan Hounds who have very short hair on the face and long hair of varying lengths on the body and limbs). These local variations may be due to either variations in the duration of anagen, or the speed of hair growth, or both. The speed of hair growth appears related to the ultimate length of the hair.

Breed: No striking breed differences are identified in respect of the duration of moulting. The average length of moult is 43-53 days.

Husbandry: Living conditions may affect the predisposition of dogs to begin moulting or to display continuous moulting. Moulting is likely to begin sooner in dogs living indoors, particularly in long-haired breeds, and especially for the Spring moult. Of dogs housed indoors, 39.3 percent moult continuously, a figure almost twice as high as that in dogs living in the open.

Hair clipping: Clipping of the hair increases the rate of epidermal cell renewal. It is thought that hair clipping stimulates a more rapid epidermal cell turnover to produce a more protective surface to the epidermis. Surgical wounds further decrease epidermal cell renewal time.

Some facts about dog skin and hair coat

  • Dogs have compound hair follicle.
  • There are approximately 100-600 hair per sq cm of dog’s skin.
  • Dogs do not synthesise enough vitamin D in the skin.
  • Dog’s skin is thinner than human skin.
  • 30 percent of daily protein intake is taken up by hair production.
  • Newborn puppy’s skin and coat represent 24 percent of his bodyweight (12 percent adult dog).
  • pH of skin in dogs is different from human skin.
  • Dogs and Cats have sweat glands over the entire body surface. They are simply not confined to foot pad or planum nasale but it is not a major means of thermoregulation in dogs and cats.

Alopecia X

Alopecia X: The breeds most affected by Alopecia X are Pomeranians, Chow Chows, Miniature Poodles, Siberian Huskies, Keeshonds and Samoyeds.

Signs and Symptoms: Adult onset (typically nine months to three years) symmetrical alopecia which progresses to involve the neck, back, hindlegs, chest, rump and tail. It may or may not have hyperpigmentation in affected areas. Hair on head, tail and feet are usually not affected. It usually affects male dogs more than female. It may occur before or after neutering. The coat is dry and dull with loss of guard coat and hair may regrow at site of biopsy.

Post-clipping alopecia is a common condition found primarily in sled dogs (Samoyeds, Siberian Huskies and Alaskan Malamutes), Keeshonds and Chow Chows. After the hair is shaved closely, like when shaving the hair for venepuncture, surgery or wound management, re-growth is delayed for up to 6 to 12 months. Hair that eventually grows back may be darker in colour.

(Kallahalli Umesh is Waltham Regional Associate, South Asia)