Introduction

The goal of this overview is to provide some basic knowledge of hair structure and hair cycles in dogs, cats and rabbits in an effort to help pet professionals and pet parents better understand shedding in pets. Shedding should not be viewed as sudden hair loss, but should be considered as the best indicator of the vitality of the skin and hair. Coat gloss, hair softness, a greasy or dry feeling to the coat and the presence of dandruff are all indicators of the hair's condition. However, all of these factors are judged subjectively. With a good understanding of the hair structure and characteristics, these indicators will become more objective. This is important for quickly detecting abnormalities.

Skin and Hair… a shedding point of view

"The largest and heaviest organ of the body"

Skin

• Composition

  • The skin is a soft outer covering of the pet. Skin thickness and turnover varies with species, breed, age and location on the body (Linda A., 2002). The skin is the thickest on the back and neck, thinning toward the abdomen and thinnest in the groin area. (Verena K. Affloter, 1994.) Rubbing and pressure can cause thickening of the outer layer and form protective calluses in some locations. The skin is the largest organ made up of multiple layers. In dogs and cats, skin pH (power of hydrogen) is acid and responsible for electrical conductivity (Matousek, JL., 2002). Because it has more hydrogen ions and a positive charge, it attracts free electrons. Skin pH differs among breeds and gender. Male dogs show a greater change in skin pH than that of female dogs (Linda, A., 2002).

  • Layers

    • Epidermis: outermost layer of cells. The top layer of the epidermis slowly loses its moisture and transforms into a cheesy material, epithelial keratin. When the cells are almost completely filled with this keratin, metabolism ceases and cells die. New layers of strong cells replace them. Basically, the epidermal cells are glued together like bricks of a wall by a layered substance composed of free fatty acids, cholesterol, cholesterol ester and ceramides.
    • Ceramides: essential in structuring and maintaining the water permeability barrier properties of the skin. They limit loss of water and solutes while preventing harmful substances from entering the skin. With ageing, ceramid levels are reduced which leads to loss of hydration, poorer skin turnover and dryer, more damaged skin.
    • Dermis: network of connective tissue containing nerves, blood vessels, hair follicles, sweat and oil glands. This layer is located directly under the epidermis.
    • Basal Cell: responsible for skin and hair pigmentation. Melanocytes located in the basal cell layer are responsible for the production of skin and hair pigment (melanin). Pigment production occurs under hormonal and genetic control.
    • Hypodermis: used mainly for fat storage. The hypodermis consists primarily of loose connective tissue and lobules of fat. It contains larger blood vessels and nerves than those found in the dermis.

• Function

  The main functions of the skin are to insulate, regulate body temperature and sensation. Severely damaged skin will try to heal by forming scar tissue. The skin sheds through a continuous hardening process called keratinization.

Hair

• Composition

  A hair in and of itself is dead material, but living cells produce it. Hair is made of the protein exokeratin, an extremely strong protein, ensuring skin impermeability. The structure and composition of exokeratin differs from the epidermal keratin.

• Structure

  • Parts

    • Hair Shaft: above the skin surface
    • Hair Root: imbedded in the skin surface
    • Bulb: the enlarged, hollow portion at the base of the root
    • Hair Papilla: projection of dermis into center of the bulb
    • Follicle: the skin indentation protecting the root

  • Layers

    • Cuticle: the outermost layer of the hair is a single layer of flattened, keratinized cells overlapping like shingles that can be crown-like, petal-like or flat.
    • Cortex: inside the cuticle is the thickest and intermediate area. The dead keratinized cells are very densely packed. These cells contain pigment granules, providing the hair color.
    • Medulla: a central core of loosely packed cells. Hairs can be hollow, increasing the insulation value of the coat. The total diameter of individual hairs decreases as the number of hairs per follicle increases. The shape of the hair is determined by the shape of the follicle. Straight follicles produce straight hair and curly follicles produce curly hair.

    The total diameter of individual hair decreases as the number of hairs per follicle increases. The shape of the hair is determined by the shape of the follicle. Straight follicles produce straight hair and curly follicles produce curly hair. No new hair follicles are formed after birth. Follicle density decreases with age, but follicle size increases!

• Follicle Pattern

  The hair follicle is a unique composite organ, composed of epithelial and dermal compartments interacting with each other in a surprisingly autonomous way. Each of the follicular compartments is endowed with a specific differentiation and even the shape of the hair shaft is intrinsically programmed from the bulb (Bernard BA. 2003).

  • Parts

    • Primary Follicle: a large diameter usually associated with a sebaceous gland. Each primary hair follicle has a sebaceous gland, which is a kind of sweat gland to keep skin and hair smooth and elastic. Associated with each primary follicle is a band smooth muscle. Each primary hair has fine nerve fibers that serve as tactile sensors.
    • Secondary Follicle: smaller in diameter than primary follicles.

    (Differences between the primary and secondary follicles exist in the arrangement of the hair follicles related to the animal species.)

    • Simple Follicles: individual hair follicles.
    • Compound Follicles: clusters of hair follicles.
    • Sebum: responsible for hair "sheen". It acts as a lubricant for the skin and prevents dehydration of the skin. Sebum composition and sebum production differs in dogs, cats and rabbits and even among breeds.
    • Glassy Membrane: separates hair follicles from the hypodermis. Chemical signals managing hair growth diffuse through this membrane.
    • The follicle's outer layers are continuous with the surface epidermis:
      • Dogs have a compound hair follicle pattern. One primary hair and 2-15 secondary hairs emerge through the same pore. Primary and secondary hairs are found in groups of up to 20. These hairs are separated into groupings of three hairs through sebaceous glands and circular connective tissue in the dermis. The length, thickness, density and color vary between individuals and especially between breeds. Contrarily to coat color, genetic determination of hair growth is managed by only a few genes (Cadieu, 2009).
      • Organization is in clusters, each containing one large guard hair surrounded by many small down-hairs (William E. Staile, 2005).
      • The hair of the rabbit grows from a compound follicle that contains one large outer coat hair, a few smaller outer coat hairs of intermediate morphology and numerous fine undercoat hairs. As the hair shafts reach the surface, the compound follicle is subdivided by fibrous septa into smaller groups that surround the large outercoat hair. Each of these smaller groups of undercoat hairs penetrate the skin through a common follicle opening, whereas the outer coat hair has a separate opening (H. J. Whiteley, 1958).

• Growth

  • The hair grows from the deepest part of the active hair follicle. Hairs don't grow "through" the skin. They grow from the skin and they're really part of the keratinizing system of the epidermal layer.
  • Hair grows in cycles. When it reaches a certain length that is determined by the individual's genetic profile, growing stops. Each cycle consists of four phases: anagen, catagen, telogen and exogen. The duration of each phase and the rate of hair growth determine the length and amount of coat on an animal.

  • Anagen or Growth Phase

    • The anagen phase is the first phase of new hair growth. Pets that do not tend to shed heavily have a longer anagen phase. Pets that continuously shed have shorter anagen phases. The amount of time the hair follicle stays in the anagen phase is genetically determined. At the end of the anagen phase, an unknown signal causes the follicle to go into the catagen phase.

  • Catagen or Regressing Phase

    • The catagen phase is the transition phase. The catagen phase begins when the cell creation signals to stop. Hair stops growing during this phase as the outer root sheath attaches to the hair.

  • Telogen or Rest Phase

    • Telogen is a rest period between the catagen and anagen phases.

  • Exogen or Shedding Phase

    • The final phase, exogen, is the shedding phase. This phase occurs when the hair falls out and the follicle moves back into the anagen phase. The length of this phase depends on the season.
  • At any given moment, some hair follicles are in anagen, some in catagen and some in telogen. The timing and the ratio of the different cycles determine the shedding frequency, the length of the hair and presence or absence of undercoat. Some animals shed poorly because almost all of their follicles are in anagen almost all the time, so their hair just gets longer and longer. Some animals have most of their follicles in telogen and they may be almost completely hairless.
  • In dogs: hair growth rate varies between individual dogs and even between the different regions in the same dog. The growth rate is most rapid in the shoulder region, followed by the flank and the forehead regions (Parameswaran Gunuratnam PG, 1983). Each follicle grows independent of the others.
  • In domestic short hair cats, the hair growth rate is highest in the summer and lowest in the winter.

• Hair Types

  • Guard Hairs (primary hairs) form the topcoat, outermost layer of hair that covers most of the skin surface. They are thick, long and stiff. The guard hairs are regularly arranged in broad tracts that follow the contour of the body and give the animal's coat its smooth appearance. Growing one hair per primary follicle, the guard hair provides a waterproof protective top layer covering the undercoat. Guard hairs are most abundant on the back of the animal.
  • Under-Hairs form the undercoat (secondary hairs). They are thin, short and soft. They give the coat its softness and provide insulation. Up to 15 secondary hairs can grow from the secondary follicle.
  • Tactile Hair (sinus hairs) are very large hairs modified to act as mechanical sensors. Thicker than topcoat or undercoat hair, they have the same structure. However, the base of the tactile hair resides in a blood filled sinus that greatly amplifies the motion of the hair and increases its sensitivity. Merkel cells, specialized sensory cells associated with the tactile hairs, detect the slightest touch. Tactile hairs are concentrated on the sides, top and bottom of the head (whiskers, eyebrows and chin hairs) and are strongly associated with sight and sense of hearing. Tactile hairs shed on a regular basis, but at a much slower rate than other hair types.

References: 1. Linda A.MO .Age, Breed, Sex and Period Effects on Skin Biophysical Parameters for Dogs Fed Canned Dog Food The American Society for Nutritional Sciences J. Nutr. 2002,132:1695S-1697S. 2. Verena K. Affolter, Peter F. Moore Histology features of normal canine and feline skin. Clinics in Dermatology 1994. Volume 12, Issue 4, 491-497. 3. Bernard BA. Hair shape of curly hair. J Am Acad Dermatol. 2003; 48 (6 Suppl): S120-6. 4. Matousek JL, Campbell KL. A comparative review of cutaneous pH. Vet Dermatol 2002; 13: 293-300. 5. Cadieu E, Neff MW, Quignon P, Walsh K, Chase K, Parker HG, Vonholdt BM, Rhue A, Boyko A, Byers A, Wong A, Mosher DS, Elkahloun AG, Spady TC, André C, Lark KG, Cargill M, Bustamante CD, Wayne RK, Ostrander EA.Coat variation in the domestic dog is governed by variants in three genes. Science. 2009, 2; 326(5949): 150-3. 6. William E. Staile. Sensory hair follicles in mammalian skin: The tylotrich follicle. American Journal of Anatomy. 2005, Volume 106 Issue 2, 133-147, 7. H. J. Whiteley. Giant Compound Hair Follicles in the Skin of the Rabbit. Nature 1958,181; 850. 8. Parameswaran Gunuratnam PG., T. Wilkinson. A study of normal hair growth in the dog Journal of Small Animal Practice 1983, Volume 24; Issue 7: 445-453, 9. Wouter H. Hendriks, Michael F. Tarttelina, and Paul J. Moughana. Seasonal Hair Growth in the Adult Domestic Cat (Felis catus). Comparative Biochemistry and Physiology Part A: Physiology 1997, Volume 116, Issue 1; 29-35.

Dogs

• Based on hair length, dog breeds can be divided into three types: hairless, short-haired and long-haired breeds. Short hair is dominant to long hair. The length of the hair is variable within individuals and across the breeds. Most animals grow their hair to a specific length and then the hair growth stops. The longer the anagen period, the longer the hair grows. Coat variation in dogs (length and texture) is governed by variants in three genes (Wayne RK, 2009, Cadieu E, 2009). Hereditary hair length variability in dogs is caused by mutations within the fibroblast growth factor (FGF5) gene.

• Hairless Breeds

  • Hairless breeds often have little to no hair. Some hairless breeds have noticeable patches of hair on their heads. Great care should be taken to protect and moisturize their skin.

• Single-Coated Breeds

  • Single-coated breeds have a topcoat, but lack an undercoat. Single-coated breeds are often referred to as non-shedding breeds because their shedding is so minimal, it is often unnoticed.

• Short-Haired Breeds

  • The hair lies close to the body. Quite often, short-haired dogs have the highest density of follicles, resulting in more topcoat and less undercoat. Short-haired breeds are classified as dogs having hair shorter than 2 inches.

Functions

The skin and hair combined are the animal's largest and heaviest (12-24% of body weight) sensory organ; monitoring the environment and influencing body temperature.

Protection

• Hair is the first defense against environmental influences. Hair protects against invasion of microorganisms and colonization by parasites. Hair between the paw pads protects the pads from ice and snow. Hair inside the ears limits penetration of foreign particles.
• The skin acts as an important barrier between the animals' external and internal environmental factors. It prevents against dehydration forming an impermeable protection to the loss of fluids, electrolytes, minerals, nutrients and water. The oily excretion from skin glands waterproof and lubricate the skin.

Insulation

• The hair represents an insulating layer between the skin and the external environment. The hair regulates changes in body temperature, providing insulation from the winter's cold and summer's heat. Erection of the hair starts when a stimulus of cold causes a discharge from the nervous system that triggers contraction of the little muscles. Contraction of these muscles elevates the hair follicles above the rest of the skin so the hair seems to "stand on end". Lofting the hair increases the air spaces and increases the insulating value of the hair.

Communication

• The hair is the receptor for touch, pressure, vibration, heat, cold and pain. Tactile hairs gather information about the world around. Besides the senses of smell, taste, sight and hearing, the sense of touch is one of the animal's most important senses. This sense is made possible by various cells and nerve endings in the skin, which send impulses to the central nervous system.
• Hair participates in body communication. When animals are faced with an unfamiliar situation (strange persons, noise, aggression), adrenaline is released, causing contraction of the little muscle linked to the hair follicle. This makes the animal's hair stand on end. This process is called piloerection. It generally starts from the neck area down to the tail and is most expressed at the shoulder and hip areas. The effect is more pronounced on the shoulder area because the hair in this area is much longer. In long-haired breeds, the effect is less impressive.
• Pheromones, chemical substances, are produced in some glands of the skin. These molecules send information to other animals via smell. Pheromones are released in response to stress, alarm, danger and sexual fertility.

References: Wayne RK, Ostrander EA.. Coat variation in the domestic dog is governed by variants in three genes. Science 2009; 326:150-3. Edouard Cadieu et al. Coat Variation in the Domestic Dog Is Governed by Variants in Three Genes. Science Express, 2009; Vol. 326. no. 5949: 150-153 C. Drögemüller, S. Rüfenacht , B. Wichert and T. Leeb. Mutations within the FGF5 gene are associated with hair length in cats. Animal Genetics, 2007; Volume 38 Issue 3: 218-221

How

• The shedding cycle is directly related to the hair growth cycle. During the shedding phase of the hair growth cycle, hair sheds from the hair follicle. As the growth stage begins, new hair comes in. Hairs that are in the resting stage are held in the follicle by friction and are not anchored in. They fall out very easily, especially during grooming. Dogs, cats and rabbits go through these stages continuously. Several factors affect the amount of hair the animal will shed and when shedding is more pronounced.

Grooming

If you do not remove it for them, they will release it on their own, but…

Why

• Shed hair can cause skin irritation and needs to be removed. Grooming is necessary for an animal's health and hygiene. The removal of shed hairs helps the hair in its renewal process. More importantly, grooming cleans the skin of dandruff, eliminates dry skin flakes and improves its insulation properties.
• Brushing stimulates the skin and increases blood circulation at the base of each hair. The secretions from these glands will keep the hair weatherproofed and promote a shiny gloss.
• All pets should be groomed regularly. Brushing and grooming can lead to the early detection of parasites or other skin issues (Eckstein RA, 2000). Regular grooming provides an excellent opportunity to give a quick overall checkup. Most pets enjoy grooming sessions which strengthens the bond.
• Regular grooming will remove the loose hair from the coat that the pet has shed. This significantly decreases the amount of hair that might be ingested causing hairballs.

• Hairballs

  • Regular grooming with a deShedding tool such as the FURminator^® decreases the amount of hair your pet ingests, decreasing the likelihood of a hairball. A true "hairball" is comprised of nearly 100% hair, cylindrical in shape and may be mixed with semi-digested food.
  • The dog's tongue is smooth and hair does not stick to it. Most of the ingested hair will pass through the digestive system and is normally excreted in small amounts in the stool. Although, dogs are less affected by hairball formation, long-haired breeds are susceptible. In few cases, digestive disorders can occur (N. G. Kock, 1974).
  • Cats spend about 10% of their waking hours self- grooming by licking their fur clean. The cat's tongue surface is rough, covered with conic hooks and is very prone to collecting hair. During self-grooming, cats can ingest up to two thirds of shed hair. This equates to about one ounce of hair daily (Hendriks WH., 1998).
  • Cats are used to digesting their own hair, as well as the hair of their prey. The majority of this hair passes through the digestive tract. However, some hair can stay in the stomach and form into a hairball. Rare stomach disorders are usually the cause of excessive hairball formation. Common hairball symptoms include hacking, gagging and retching. Usually, ingested hair in the esophagus will be regurgitated. Sometimes however, the hairball can form an obstruction. Stomach hairballs are vomited relatively quickly after ingestion.
  • While ingested hair will usually pass gradually through the intestines and be eliminated via stool, in some cases hairballs form in the intestine. This can cause constipation and even a blockage of the intestinal tract.
  • High fiber diets or laxatives can stimulate the moving of ingested hair through the intestinal tract for easier elimination of hair and hairballs through the stool.
  • Nowadays, in domestic indoor living cats, hairball formation is very common. Occasional hairball episodes occur weekly. Vomiting one hairball monthly in long-haired cats should be considered normal.
  • Long-hair cats and short-hair cats living together are more prone to suffer hairball formation due to social grooming. All of the factors which increase shedding (nutrition, behavior, drugs, hormonal status, breed, indoor lifestyle) also increase the amount of hair swallowed and therefore the prevalence of hairball formation.
  • Self grooming is a comfort behavior. Licking releases endorphins, natural substances released by the brain. All stress inducing factors can promote self grooming and eventually lead to over grooming. The behavior becomes compulsive if excessive self grooming is going on even when the stress inducing factors are no longer present. Some breeds are more prone to compulsive grooming and thus more prone to hairball formation. Excessive compulsive self grooming can be limited by adapting a cat's environment, especially for indoor living animals. The presence of toys and different living levels allows the cat to simulate the hunting behavior.
  • Formation of hairballs can only be minimized by limiting hair ingestion. When cat parents groom their cat regularly, the excess loose hair is removed before the cat is able to ingest the hair. Brushes tend to slide over the surface of the fur and don't get all the loose hair out. deShedding tools should be used on cats with double coats to help remove the excess hair.
  • Rabbits groom themselves constantly and swallow hair. Rabbits are not physically capable of vomiting the contents of their stomach. The presence of excess hair can lead to intestinal blockage that can be fatal. Diagnosis of  a "hairball" or "wool block" is commonly made in rabbits. During heavy shedding seasons hair ingestion is the main reason for hairball formation. Stress, poor diet and lack of exercise can also contribute to excessive shedding. Rabbits do not typically self groom excessively (Meredith A., 2006).

When

• Grooming frequency should depend on coat type, hair length and shedding seasons. Grooming sessions should be an integral part of pet education and start as soon as possible in young animals.

• Dogs

  • Grooming should be daily in double coated and long-haired dogs. For short-haired breeds, grooming should occur at least once a week.
  • Hair growth and shedding cycles are influenced by multiple factors. More than the dog's breed needs to be considered when deciding on a grooming routine.
  • Grooming should always be a positive experience for the pet, often after play, eating or exercising. Grooming stimulates blood circulation in and to the skin.

• Cats

  • Daily grooming is recommended for long-haired cat. Short-haired cats can be groomed less often with a minimum of three times a month. Grooming routines should be adapted to the lifestyle and shedding cycle of the cat. Brushing is very important for ridding the cat of loose hair and limiting the amount of hair that can be ingested during self grooming.

• Rabbits

  • Because of their constant shedding, rabbits need to be brushed at least weekly to remove loose hair. As shedding can occur in some breeds in a couple of days, grooming during this time should be daily especially in satin and wool fur breeds.
  • Sessions should be extremely short, but lengthened progressively to make the animal feel comfortable and relaxed during grooming.

How

• Grooming sessions should always start with the most sensitive areas and end in a positive way on less sensitive areas. Grooming movements should always be in the direction of the hair growth.
• Brush with short strokes to penetrate through the hair and avoid excessive pressure that may cause brush burn or skin irritation, especially on pets with sensitive skin.
• In dogs and cats, skin pH (power of hydrogen) is acid and responsible for the electrical conductivity (Matousek JL., 2002). When the surrounding air is dry, electric conductivity decreases. Animal hair begins to accumulate a static charge when the relative humidity falls below 55-60%. The moisture content of hair keratin decreases. The cuticle becomes rougher, snagging and tangling occur and the hairs become more prone to static electricity.
• The use of anti-static sprays can be used, but the pet parent should read the labels and understand the product before using it as some products can affect the skin. Moistening the hair with a light mist of water before grooming sessions will help reduce static electricity.

Where

• If possible, grooming should be done outside or in a place that is easy to clean up. This allows for quick cleanup and reduces the chance that any parasites that may have been on the animal will spread.

Tools

• Grooming tools should be chosen based on coat type and length, as well as purpose of use. Most grooming tools vary in size depending on the size of pet. This should be considered when purchasing grooming tools to ensure proper tools are being used to avoid skin irritation.
• The design of most combs and brushes limits the ability to remove undercoat effectively. For combs, the number of blades should be inversely proportional to the hair length to limit traction on the follicle base. The lack of selective action on the undercoat is often partially compensated by strengthening pressure and traction during grooming increasing the risks of skin irritation and damaging the top coat.
• The use of deShedding tools is particularly indicated in double coated, short and long haired animals. The position of the edge allows the tool to reach the undercoat, without damaging guard hairs. It is important to choose a tool with coat length and animal size in mind. The use of deShedding tools allows the groomer to closely examine the animal's skin, which often mirrors the animal's general health status. In some breeds, however, hair structure doesn't allow the safe use of deShedding tools.

Grooming is essential to maintain proper hygiene and overall health in pets. To make the right breed choice, research should be done into the breed's coat type and grooming needs. The main benefit of grooming is skin health and beauty. Early detection of skin issues can occur during grooming sessions. Some skin issues may need to be discussed with the pet's veterinarian.

References: 1. R. A. Squires. Oesophageal obstruction by a hairball in a cat. Journal of Small Animal Practice. Volume 30 Issue 5:311-314 2. V R Barrsa, J A Beattya, P L C Tisdalla, G B Hunta, M Gunew, R G Nicolla and R Malika. Intestinal obstruction by trichobezoars in five cats. Journal of Feline Medicine & Surgery, Volume 1, Issue 4, December 1999,:199-207 3. Eckstein RA, Hart BL.Grooming and control of fleas in cats. Appl Anim Behav Sci. 2000 10;68(2):141-150. 4. N. G. Kock , S. Geroulanos, P. Hanloser, H. Schauwecker and H. Säuberli. Continent colostomy: An experimental study in dogs. Diseases of the Colon & Rectum. N° 17, Number 6, 1974 :727-734 5. Wouter H. Hendriks , Michael F. Tarttelina and Paul J. Moughana .Seasonal Hair Growth in the Adult Domestic Cat (Felis catus) Comparative Biochemistry and Physiology Part A: Physiology Volume 116, Issue 1, 1998: 29-35 6. Matousek JL, Campbell KL. A comparative review of cutaneous pH. Vet Dermatol 2002; 13: 293-300. 7. Meredith A. in Skin Diseases and reatment of Rabbits. Ed. Blackwell Science Ltd., 2006: 287-311