Alopecia is a common reason for veterinary visits. This article offers a practical guide for veterinarians to understand and manage alopecia in dogs, focusing on a systematic approach to identify the underlying causes. Gathering a comprehensive signalment and detailed history, along with performing thorough physical and dermatologic examinations, are crucial in guiding diagnostic investigations. A structured diagnostic process and accurate identification of the cause are essential for effective therapeutic management. This article outlines the key diagnostic steps and includes 2 algorithms to facilitate efficient and accurate diagnosis.<\/p>\n
Take-Home Points<\/strong><\/p>\n <\/div><\/div><\/p>\n In veterinary medicine, the terms \u201chypotrichosis\u201d and \u201calopecia\u201d are used to describe hair loss. Hypotrichosis refers to the thinning of hair and alopecia refers to complete hair loss in regions of the body or the entire body.1<\/sup> This article uses the term \u201calopecia,\u201d but the same information applies to hypotrichosis.<\/span><\/p>\n Alopecia is usually a major concern for dog owners. If it is not associated with a systemic disease, then usually it is considered a cosmetic disease for dogs. Nevertheless, alopecia is a common reason for owners to seek veterinary advice. To identify and address underlying causes of alopecia, a systematic approach is necessary. This article classifies the causes of alopecia and highlights key characteristics of some diseases that may pose diagnostic challenges in primary care.<\/span><\/p>\n Alopecia can be classified as inflammatory or noninflammatory. Various mechanisms can lead to each type of alopecia.<\/span><\/p>\n Inflammatory alopecia is defined by destruction of hair follicles or hair shafts.1<\/sup> Destruction may be due to inflammation targeting the hair follicle or other components of the adnexa (e.g., sebaceous glands) or self-trauma (e.g., licking, biting, scratching). Clinically, this type of alopecia has an asymmetric distribution pattern (focal or multifocal).<\/span><\/p>\n Noninflammatory alopecia may be the result of abnormally formed or developed hair shafts or hair follicles or of an impaired hair cycle.<\/i> Hair growing from abnormal hair follicles or hair shafts may break due to poor quality, or hair growth from these structures may cease. The resulting alopecia is partially to symmetrically distributed (generalized\/bilateral) and may occur early in life or when the patient is a young adult. If the hair cycle is impaired, the production of new hairs may be disrupted while old hairs shed, or many hair follicles may synchronize and shed at the same time. This type of alopecia is symmetrically distributed (generalized\/bilateral). <\/span><\/p>\n Because focal (multifocal\/asymmetric) alopecia is usually inflammatory while generalized (bilateral\/symmetric) alopecia is noninflammatory, classifying canine alopecia as focal or generalized is a crucial first step in identifying the cause (<\/span>FIGURE 1<\/b><\/span>).<\/span><\/p>\n Notably, many causes of alopecia overlap between focal and generalized inflammatory or noninflammatory cases. For cases of inflammatory alopecia, it is helpful to use an etiologic classification based on the underlying origin (e.g., inflammatory-infectious, inflammatory-noninfectious, neoplasia). While other classification methods exist, FIGURE<\/b><\/span><\/span>\u00a01 <\/b><\/span>organizes the causes of noninflammatory alopecia based on age at onset. Age at onset serves as a key clinical indicator in guiding the diagnostic workup of noninflammatory alopecia.<\/span><\/p>\n The most common causes of focal inflammatory alopecia are folliculitis due to infection (bacteria, yeast, dermatophytes<\/a> [<\/span>FIGURE 2<\/b><\/span>], mites), noninfectious conditions (immune-mediated inflammation, dermal inflammation and pruritus<\/a> due to hypersensitivity reactions), trauma, and neoplasia. Most of these causes are also pruritic.<\/span><\/p>\n Figure 2. Focal inflammatory alopecia due to dermatophytosis on the face of a dog. Note the circular areas of hair loss that typify folliculitis.<\/p><\/div>\n Dermatomyositis is an inherited disease that causes hair loss and scarring on the face and legs, along with muscle wasting. Collies and shelties are predisposed.2,3 <\/sup><\/span><\/p>\n Sebaceous adenitis<\/a> is an immune system disorder that destroys the sebaceous glands.4<\/sup> Breeds like vizslas, Akitas, Samoyeds, standard poodles, Havanese, and springer spaniels are commonly affected.4 <\/sup> In short-haired dogs, it often appears as circular, scaly bald patches, while in long-haired dogs, a more widespread, diffuse hair loss is typical.<\/span><\/p>\n Granulomatous mural folliculitis is a rare cause of scarring alopecia in dogs. It is characterized by inflammation of the hair follicle walls, primarily involving lymphocytes, with histiocytes, macrophages, and giant cells also present. Little is known about its pathogenesis and long-term management.5<\/sup><\/span><\/p>\n Common causes of focal noninflammatory alopecia include follicular dysplasia in young dogs and alopecia due to impaired hair cycle (also called hair cycle arrest) such as postclipping alopecia in adult dogs. These conditions are nonpruritic. Some dogs can manifest focal noninflammatory alopecia due to alopecia areata, postinjection vasculitis, or traction alopecia (FIGURE<\/b><\/span><\/span>\u00a03<\/b><\/span>). These diseases have an inflammatory origin, but clinically the alopecic skin seems noninflammatory.1<\/sup><\/span><\/p>\n Figure 3. Focal noninflammatory traction alopecia due to hair clippers on the head of a dog.<\/p><\/div>\n Alopecia areata is an immune-mediated condition targeting follicular wall antigens in the hair bulb, leading to localized hair loss. It presents as nonpruritic, well-demarcated patches of alopecia, commonly affecting the head, neck, and legs, with possible pigmentation changes and leukotrichia.6,7<\/sup><\/span><\/p>\n Traction alopecia is caused by tension from rubber bands, leading to damage of hair follicles (FIGURE<\/b><\/span><\/span>\u00a03<\/b><\/span>).1<\/sup><\/span><\/p>\n Postinjection alopecia occurs at the site of subcutaneous administration of vaccines or medications (most commonly rabies vaccines<\/a>). Alopecia can appear 2 to 4\u00a0months postinjection as a localized, well-demarcated area of alopecia, often with atrophic, thin, or hypopigmented skin. In cases of vasculitis the lesions can be multifocal.3<\/sup><\/span><\/p>\n Generalized distribution of inflammatory alopecia is not very common. The causes are similar to those of focal inflammatory alopecia (FIGURE<\/b><\/span><\/span>\u00a01<\/b><\/span>).<\/span><\/p>\n Generalized noninflammatory alopecia is the most common distribution pattern of noninflammatory alopecia. The causes are either congenital or acquired and may or may not be inherited (FIGURE<\/b><\/span><\/span>\u00a01<\/b><\/span>). The first step in the diagnostic approach is determining the age at onset. Patient breed and coat color can further refine the differential diagnosis.<\/span><\/p>\n Congenital<\/b><\/p>\n In these dogs, hair loss is present at birth. The differential diagnoses include:<\/span><\/p>\n Postnatal Onset<\/b><\/p>\n These dogs have abnormal (thin\/sparse) hair at birth that is lost soon after birth. In American hairless terriers (hairless rat terriers), this alopecia is an inherited desired phenotype of the breed, and a workup is usually not wished by the owners.<\/span>9<\/sup> Born with a thin coat, these puppies gradually lose it, starting at the nose and receding until completely hairless by around 6\u00a0weeks old.9<\/sup><\/span><\/p>\n Scottish deerhound puppies with inherited alopecia lose their sparse hair within 2 months of birth.1<\/sup> These dogs are healthy, but this phenotype is not desired; thus, owners might seek veterinary advice to rule out other differentials and\/or do genetic testing for breeding purposes.1<\/sup><\/span><\/p>\n Juvenile Onset<\/b><\/p>\n Affected dogs lose their hair prior to adulthood. Juvenile-onset alopecia can be caused by dysplasia of hair follicles or hair shafts or be linked to certain coat colors.<\/span><\/p>\n Hair follicle or hair shaft dysplasia<\/b> is commonly seen in specific breeds, such as:<\/span><\/p>\n Color dilution alopecia<\/b> can occur in almost all dog breeds with dilute coat colors (blue, gray, fawn, reddish); however, not every dog with dilute coat color will develop color dilution alopecia. The blue Doberman pinscher is most commonly affected.1,11<\/sup> Color dilution alopecia is caused by a defective transfer of melanin to the hair follicle, resulting in melanin clumps in the hair shafts and hair follicles, which are weak spots resulting in broken hairs.1,11<\/sup> The lateral aspects of the trunk are affected bilaterally and symmetrically.<\/span><\/p>\n Black hair follicular dysplasia<\/b> is an inherited disease that affects the black-haired areas of the head, ears, neck, and back within the first year of life (FIGURE<\/b><\/span><\/span>\u00a04<\/b><\/span>). Large melanin aggregates are present in the hair shafts. The black coat is dull and brittle, and the hair breaks easily, resulting in complete alopecia.1,11,16<\/sup><\/span><\/p>\n Figure 4. Young dog with focal noninflammatory alopecia of the black-haired skin, due to black hair follicular dysplasia. Only the black patch of hair is affected on this mainly white-coated dog. Courtesy Dermatology LSU<\/p><\/div>\n Pont-Audemer spaniels may have alopecia only of the brown-haired areas of the trunk and ears.1,17<\/sup><\/span><\/p>\n Adult Onset<\/b><\/p>\n In most of these dogs, the hair cycle is impaired by an underlying condition. In some cases, ischemic dermatopathy causes atrophic hair follicles due to impaired vascular perfusion of the adnexa associated with vasculopathy or vasculitis. Parson Russell terriers, Yorkshire terriers, Maltese, and Chihuahuas are overrepresented in cases of alopecia caused by ischemic dermatopathy.3<\/sup><\/span><\/p>\n Endocrinopathies<\/b><\/p>\n The hair cycle is influenced by various factors, including hormones, which can either stimulate hair growth (e.g., canine thyroid-stimulating hormone, thyroxine) or inhibit it (e.g., glucocorticoids, sex hormones).18<\/sup> Areas commonly affected by alopecia include the neck, shoulders, flanks, and caudal thighs, with the remaining coat often appearing dull and occasionally lighter in color. <\/span><\/p>\n Additional cutaneous and systemic signs can suggest specific endocrinopathies; for instance, cool skin or a tragic facial expression may indicate hypothyroidism<\/a>.19<\/sup> In cases of hyperadrenocorticism<\/a>, thin, inelastic skin on the abdomen, with comedones and sometimes prominent subcutaneous blood vessels, may be observed.19<\/sup><\/span><\/p>\n Alopecia related to sex hormone imbalances is noted in cases of testicular tumors, ovarian cysts, or exogenous estrogen treatment.19<\/sup> Hyperpigmentation may also be present. Additionally, female dogs may have enlarged vulvas and irregular estrus cycles, while male dogs can experience gynecomastia and a pendulous prepuce with linear erythema.19<\/sup><\/span><\/p>\n Alopecia X<\/b><\/p>\n Alopecia X typically occurs in Pomeranians (or other dogs belonging to the European spitz breed) and involves abnormal cutaneous steroid metabolism and steroidogenesis, leading to hair cycle arrest and subsequent alopecia (bilateral symmetrical, sparing the head, tail, and distal legs). A similar condition has been described in other Nordic dog breeds, miniature poodles, and schipperkes.1<\/sup> Recently, it has been proposed to name this entity alopecia X\u2013like for all the non-European spitz dogs, for genetic reasons.1,18<\/sup><\/span><\/p>\n Canine Recurrent Flank Alopecia<\/b><\/p>\n Canine recurrent flank alopecia can affect many breeds, but there is a strong predisposition in boxers, Rhodesian ridgebacks, Airedale terriers, French bulldogs, English bulldogs, and schnauzers.20 <\/sup> It is an inherited disease, and the pathomechanism remains unclear, although there is an association with photoperiod and climatic variations.20 <\/sup> Affected dogs have bilaterally symmetric, well-demarcated alopecia in the flank region. Unilateral (focal) alopecia in the flank area is also possible but not as common.1,20<\/sup><\/span><\/p>\n Pattern Baldness<\/b><\/p>\n Pattern baldness affects mostly dachshunds, Boston terriers, and Chihuahuas, with onset at early adulthood.1<\/sup> These dogs have bilaterally symmetric alopecia on the pinnae, ventral neck, chest, and abdomen due to impaired hair cycles leading to hair follicle miniaturization.1<\/sup><\/span><\/p>\n Telogen Effluvium and Anagen Effluvium<\/b><\/p>\n Telogen effluvium is a rare condition of hair follicle synchronization at the exogen (shedding) stage of the hair cycle that results in alopecia before the synchronized anagen (growing) hairs reach the epidermis. Thus, a trichogram at early stages will show telogen (resting) hairs; at later stages, it will show anagen hairs. Pregnancy, stressful events, and severe disease can cause this condition.1<\/sup><\/span><\/p>\n Anagen effluvium is a condition characterized by impaired anagen hair cycles due to interference with the proliferating anagen hair bulbs, resulting in abnormal hair follicles and hair shafts that break upon reaching the skin surface. Interference can occur from multiple factors, usually drugs. Initially, a trichogram will reveal anagen hairs with irregularly narrowed or pointed ends, while sampling during later stages will show only telogen hairs since all anagen hairs will have been lost.21<\/sup><\/span><\/p>\n Other<\/b><\/p>\n Alopecia of Swedish Lagotto Romagnolo dogs (on the trunk bilaterally) with seasonal recurrence and estrus-associated worsening or onset has been described.1,22<\/sup><\/span><\/p>\n Gathering a comprehensive signalment and detailed history, along with performing thorough physical and dermatologic examinations, are crucial in guiding the diagnostic investigation.<\/span><\/p>\n As described above, some diseases are more often seen in young dogs (e.g., demodicosis, dermatophytosis, congenital alopecia, juvenile-onset generalized alopecia), whereas others are more commonly seen in adult dogs (e.g., neoplasia, endocrinopathies).<\/span><\/p>\n If the patient does not belong to a breed described as having a predisposition to an alopecic disease, it is important to investigate whether any littermates or related dogs have similar signs.<\/span><\/p>\n Color dilution alopecia can occur in any dog with dilute coat color. Black hair follicular dysplasia has been reported in bicolored or tricolored dogs and affects only the black-haired body areas.11<\/sup><\/span><\/p>\n The patient\u2019s sex can affect the incidence of certain diseases. Neuter status is important. If the patient is an intact female, any relationship between the alopecia and the estrus onset or cycle should be investigated. Alopecia onset or worsening is associated with estrus in Lagotto Romagnolo dogs with alopecia.22<\/sup><\/span><\/p>\n Alopecia associated with gonadal disorders can occur in regularly cycling female dogs. Some affected individuals show alopecia characterized by hair loss in the collar region, rump, perineal area, and ventrum starting 4 to 6 weeks postestrus.19<\/sup> Alopecia is associated with clinical signs of overt pseudopregnancy. Hair typically regrows spontaneously following the resolution of pseudopregnancy, either with or without therapeutic intervention; however, recurrence is likely during subsequent estrous cycles. Persistent anestrus can be a valuable hint in intact female dogs with hyperadrenocorticism.<\/span><\/p>\n Intact female or male dogs (or dogs with cryptorchidism or ovarian remnants) can develop neoplasia of the gonads leading to alopecia due to increased hormonal production.19<\/sup><\/span><\/p>\n The following questions are helpful in obtaining a complete history for alopecic patients.<\/span><\/p>\n Is (or was) the patient on any medication? <\/b><\/p>\n Glucocorticoids may result in alopecia caused by iatrogenic hyperadrenocorticism. Exposure to estrogen, via therapy or from the owner\u2019s skin, can induce alopecia. Certain chemotherapeutic agents, such as doxorubicin, cyclophosphamide, and 5-fluorouracil, may cause alopecia (anagen effluvium), particularly in breeds with an anagen-dominant hair cycle.21,23,24<\/sup> Topical insecticides can lead to focal alopecia due to topical reaction.25<\/sup><\/span><\/p>\n It is crucial to avoid therapeutic diagnostic trials for endocrinopathies without a confirmed diagnosis. Notably, alopecia resolution following thyroxine therapy does not confirm hypothyroidism, as thyroid hormones stimulate the hair cycle, promoting regrowth without indicating a prior deficiency of thyroxine.26<\/sup><\/span><\/p>\n How much time does the dog spend outside? Does the patient hunt rodents?<\/b><\/p>\n Dogs (especially Parson Russell terriers) that hunt are prone to dermatophytosis caused by Trichophyton mentagrophytes<\/i>.27<\/sup><\/span><\/p>\n Where on the body did the alopecia start?<\/b><\/p>\n Generalized symmetric\/bilateral noninflammatory alopecia of the trunk is common in patients with hypothyroidism and hyperadrenocorticism.<\/span><\/p>\n Is there a seasonal change?<\/b><\/p>\n Canine recurrent flank alopecia often begins in the autumn\/winter and can spontaneously resolve in the spring\/summer.<\/span><\/p>\n Is the patient pruritic?<\/b><\/p>\n All diseases causing noninflammatory alopecia (focal or generalized) are nonpruritic, unless a secondary bacterial infection is present.<\/span><\/p>\n Does the patient exhibit systemic clinical signs? Any alteration in urination or food\/water intake?<\/b><\/p>\n Polyphagia, polyuria, and polydipsia could suggest hyperadrenocorticism, whereas significant weight gain and lethargy may indicate hypothyroidism.<\/span><\/p>\n Did a severe disease, stressful event, or pregnancy occur 1 to 4 months before the onset of alopecia?<\/b><\/p>\n Hair cycle synchronization induced by such events (telogen effluvium) can lead to sudden, marked, generalized alopecia.<\/span><\/p>\n The general examination may reveal signs that support diagnosis. For example, dogs with hyperadrenocorticism may have a pendulous abdomen. In cases of lethargy, hypothermia, and bradycardia, hypothyroidism should be considered. Enlarged nipples and testicular tumor (or cryptorchidism) can indicate hyperestrogenism in intact male dogs.<\/span><\/p>\n Identifying the distribution of alopecia and any other skin lesions, regardless of association with inflammation, is crucial during the diagnostic workup. This process helps establish a list of diagnostic differentials and guides the next diagnostic steps. The first step is to determine whether the alopecia is focal or multifocal (asymmetric) or generalized (symmetric\/bilateral) and then to assess whether it is inflammatory or noninflammatory (FIGURE<\/b><\/span><\/span>\u00a01<\/b><\/span>). This step can be challenging in cases of noninflammatory alopecia complicated by secondary inflammation due to pyoderma. In such instances, treating the pyoderma first and then reassessing is recommended. Typically, inflammatory alopecia is focal\/multifocal or asymmetric, whereas noninflammatory alopecia has a generalized or bilateral\/symmetric pattern.<\/span><\/p>\n FIGURES<\/span> 5 AND 6<\/b><\/span> illustrate the step-by-step clinical diagnostic testing algorithm for focal and generalized alopecia, respectively.<\/span><\/p>\n Cytology<\/b><\/p>\n Cytology is valuable in cases of inflammatory alopecia associated with other lesions. For noninflammatory alopecia, it is helpful only when there is a secondary bacterial or yeast infection. Samples should be obtained from pustules, papules, or exudative lesions. Cytology may reveal bacteria; yeast; fungal hyphae; or lymphocytes, as seen in epitheliotropic lymphoma.<\/span><\/p>\n Skin \u201cLumps and Bumps\u201d Cytology<\/a><\/p><\/blockquote>\n\n
Pathophysiology<\/h2>\n
Causes<\/h2>\n
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<\/a><\/p>\nFocal Inflammatory Alopecia<\/h3>\n
<\/a>Focal Noninflammatory Alopecia<\/h3>\n
<\/a>Generalized Inflammatory Alopecia<\/h3>\n
Generalized Noninflammatory Alopecia<\/h3>\n
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<\/a>Diagnostic Approach<\/h2>\n
Signalment<\/h3>\n
History<\/h3>\n
General Examination<\/h3>\n
Dermatologic Examination<\/h3>\n
Diagnostic Testing<\/h3>\n
<\/a><\/p>\n<\/a><\/p>\n
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