General Concepts

The horse’s disease is like an unsolved mystery in which the client is the witness to what has occurred and the veterinarian is the detective who must ascertain what the client observed. As this information is extracted, the veterinarian becomes the lawyer to determine whether the client is a credible or qualified witness. Obtaining a thorough history and being attentive to clues from the client are skills that must be practiced and developed by the clinician in order to develop a tentative diagnosis. A comprehensive history in conjunction with a thorough dermatologic examination is helpful for another practical reason: this is often when the veterinarian initially establishes her or his credibility as a professional with the client. In veterinary dermatology, the client-veterinarian relationship is often important for a successful outcome. Because many chronic diseases necessitate lifelong control and can be frustrating for client and veterinarian alike, it is critical for the client-veterinarian relationship to start well. If the veterinarian is thorough and obtains the most information possible from the history and the dermatologic examination, the client is more likely to recognize the effort and expertise of the veterinarian. These clients are often more agreeable to pursuing diagnostic tests or trial therapies if the information from the initial exhaustive examination is not sufficient for a diagnosis. Cursory examinations leave a sense of insecurity in some clients, making them reluctant to follow recommendations based on such examinations.

Owner’s Chief Complaint

The owner’s chief complaint, or chief cause of concern, is often one of the major signs initially used in establishing a differential diagnosis. Addressing the client’s chief complaint is an important step in achieving satisfaction of clients and obtaining their confidence and often initiates a favorable client-veterinarian relationship. Other findings not directly related to the chief complaint may be uncovered. Although these additional findings are important to discuss, the client’s chief complaint must always be addressed.

Age

Some dermatologic disorders are age-related, so age is important in the dermatologic history. For example, hereditary disorders such as epidermolysis bullosa, cutaneous asthenia, hypotrichosis, aplasia cutis congenita, epidermal hamartoma, and follicular dysplasia may be apparent at birth or within the first 6 months of age. Allergies tend to appear in more mature animals, probably because repeated exposure to the antigen must occur and the immune response has to occur before clinical signs develop.

Breed

Breed predilection determines the incidence of some skin disorders (Table 2-2, Table 16-1).

TABLE 2-2 Possible Breed Predictions for Nonneoplastic Skin Diseases

Sex

Rarely, the sex of the patient is a key to diagnosis. Males are predisposed to amyloidosis and mast cell tumors. Females with ovarian neoplasms may be predisposed to cutaneous lymphoma/pseudolymphoma.

Color

The color of an animal may also be related to certain problems; most notable is the association of solar dermatitis, actinic keratosis, and squamous cell carcinoma in white-skinned, thinly haired regions. Coat color may also relate to disease, as in lethal white foal syndromes and coat color dilution lethal (“lavender foal syndrome”), and the frequency of melanocytic neoplasia in gray horses.

Medical History

The clinician should obtain a complete medical history in all cases. In practice, two levels of history are taken. The first level includes those questions that are always asked, and this level is often helped by having the client fill out a history questionnaire before the examination (see Fig. 2-2). Often, the answers on these forms will change if the client is questioned directly, so these form answers should be reviewed and not relied on as initially answered. The first level should initially include questions about previous illnesses and problems. The second level in history taking relates to more specific questions that relate to specific diseases. These more pertinent questions are usually asked once the practitioner has examined the animal and is developing a differential diagnosis or has at least established what some of the problems are. However, it is vital to use a systematic, detailed method of examination and history taking so that important information is not overlooked. A complete history takes a lot of time to obtain and usually is a major component of the initial examination period. In practice, many veterinarians do not take the time to collect a thorough history for the problem. Learning which historical questions are most important for specific problems and being certain to collect this information becomes critical. Often, the history is completed during the examination and once the initial differential diagnosis is determined.

The clinician who can draw out a complete history in an unbiased form has a valuable skill. It is important that the questions presented to the client do not suggest answers or tend to shut off discussion. Some owners purposely or unconsciously withhold pertinent facts, especially about neglect, diet, previous medication, or other procedures they think may not be well received by the examining veterinarian. Other pertinent information may be left out because some owners are not aware of what is normal. Therefore, they may not supply valuable information because they do not perceive the information to be significant or abnormal. In other cases, they may recognize something as abnormal but attribute the observation to some other, unrelated cause.

The clinician must become skillful at extracting all the relevant history and observations, regardless of the client’s perception of their importance. The skillful clinician is ever tuned to listen for side comments by the client or by the children. These may be veritable “pearls” of information in a mass of trivia. They also help to establish the client’s accuracy and credibility.

Next, the following information about the skin lesions should be obtained from the owner: the date and age at onset, the original locations, the initial appearance, the tendency to progression or regression, factors affecting the course, and previous treatment (home or proprietary remedies used, and veterinary treatment). In addition, treatments of other problems should be determined and recorded. The relationships between all treatments and the onset of, or changes in, the disease should be recorded and a possible drug reaction considered. Cutaneous adverse drug reactions are diagnosed only when they are suspected, and because they may mimic any disease, the history aids in arriving at this diagnosis.

Almost all animals with skin disorders have been bathed, dipped, sprayed, or treated with one or more medications, and the owner may be reluctant or unable to disclose a complete list of previous treatments. It is important that the types of medication and the dates of application are completely divulged, because a modification of pertinent signs may have resulted.

Although the patient cannot relate subjective findings (symptoms such as itching, burning sensation, and pain), it is possible to determine the degree of pruritus reasonably well. The presence and severity of pain may also be evaluated in some cases by the patient’s response to stimuli: exhibiting shyness, pulling away, and responding with aggressiveness may be manifested when pain is provoked.

Pruritus is one of the most common presenting complaints and, in many cases, is a hallmark of allergy. The presence, location, and degree of itching are important criteria in the differential diagnosis of many skin diseases. The owner’s idea of the intensity of itching, however, may vary considerably from that of the veterinarian. Consequently, it is helpful to ask questions, such as “How many times daily do you see your horse rub, scratch, kick, or chew?” “Does it itch in many sites, or just a few?” “Does it shake its head?” “Does it stomp its feet?” “Does it lick the front legs or other areas?” “Does it rub parts of its body against things?” Because many diseases associated with pruritus are chronic, it may be helpful to have the client initially grade the itching on a scale of 0-10, with 0 being no itching at all, 1 being very mild, and 10 being as bad as the horse has ever itched. This level can be recorded and asked again as therapies or diagnostic procedures are tried to help determine how the horse’s problem changes over time. Also, by obtaining answers to the previous questions about the pruritus, the practitioner establishes a record of the severity of the problem and the symptoms seen when the owner indicates a level of itching. Over time, the client’s perceptions may change, and this approach allows any change to be determined and documented. It is also very helpful to determine, when possible, whether the pruritus involves initially normal-appearing skin (an itch that rashes, typical of allergy) or whether skin lesions precede or appear at the same time as the pruritus (a rash that itches, which may be seen with many disorders).

The same types of specific questions are helpful when discussing diets, because the owner often states the typical feedings and leaves out treats and supplements. Specifically, the clinician asks whether any vitamins, supplements, or treats are given. The clinician also asks about changes in food sources throughout the year.

Because contact irritants or allergens are important contributors to or causes of skin disease, it is necessary to inquire about the animal’s environment. Details about stables, paddocks, fields, bedding, grooming protocols, and so forth may be very important. Do the symptoms improve or resolve when the horse leaves its usual surroundings (travel, show, competition, boarding, and so forth)? Symptoms that resolve in a different environment are highly suggestive of a reaction to an environmental allergen or irritant. Other more organ-specific questions may also be asked, depending on the clinical suspicions of possible diseases.

In determining contagion, one should inquire about the skin health of other animals on the premises, and where the horse has been in the last 6 weeks. The presence of skin disease in the people living with the patient may also be highly significant in some disorders (e.g., dermatophytosis, sarcoptic mange).

At this point, the clinician usually has a general idea of the problem and is ready to proceed with a careful physical examination. In some cases, the clinician may want to come back to the medical history if further examinations indicate a more serious or underlying systemic disease.

General Observations

A good examination necessitates adequate lighting. Normal daylight without glare is best, but any artificial light of adequate candlepower is sufficient if it produces bright, non-color-changing, uniform lighting. The lamp should be adjustable to illuminate all body areas. A combination loupe and light provides magnification of the field and good illumination. Before the clinician concentrates on the individual lesions, the entire animal should always be observed from a distance of a couple of meters to obtain a general impression of abnormalities and to observe distribution patterns. Does the horse appear to be in good health? Is it fat or thin, unkempt or well-groomed? Is the problem generalized or localized? What is the distribution and configuration of the lesions? Are they bilaterally symmetric or unilaterally irregular? Is the hair coat shiny or dull, and if it is dull, what is the pattern of those changes? Is it an appropriate color and pattern of colors for its breed? Are coat changes in quality or color lifelong, or did they develop before or after the symptoms for which the horse is presented?

To answer some of these questions, the clinician must examine the patient more closely. The dorsal aspect of the body should be inspected by viewing it from the rear, as elevated hairs and patchy alopecia may be more obvious from that angle. Then, the head, the lateral trunk, and the extremities should be observed. Next, the clinician should complete a thorough dermatologic examination.

Dermatologic Examination

After an impression is obtained from a distance, the skin should be examined more closely and palpated. It is important to examine every square centimeter of skin and visible mucous membranes. Many subtle clues are located where the client is unaware of problems.

Many observations need to be made. What is the texture of the hair? Is it coarse or fine, dry or oily? Does it epilate easily? A change in the amount of hair is often a dramatic finding, although subtle thinning of the hair coat should also be noted. Alopecia is a complete lack of hair in areas where it is normally present. Hypotrichosis implies partial hair loss and is a form of alopecia. Is the thinning diffuse, or are there numerous small focal areas of alopecia (the latter being often seen with folliculitis)? Hypertrichosis is excessive hair, and it usually has hormonal causes.

The texture, elasticity, and thickness of the skin should be determined and impressions of heat or coolness recorded. There is variation in an animal’s coat density in different body areas, with skin lesions often being discerned more readily in sparsely haired regions. Therefore, the clinician must part or clip the hair in heavily haired areas to observe and palpate lesions that are present but obscured.

When abnormalities are discovered, it is important to establish their morphologic features, configuration, and general distribution. The clinician should try to appreciate the different lesions and their patterns. Together, they often represent the natural history of the skin disease.

Morphology of Skin Lesions

The morphologic characteristics of skin lesions, together with their history, are an essential feature of dermatologic diagnosis. Morphologic features and the medical and dermatologic history are often the only guidelines if laboratory procedures cannot be performed or do not yield useful information. The clinician must learn to recognize primary and secondary lesions. A primary lesion is the initial abnormality that develops spontaneously as a direct reflection of underlying disease. Secondary lesions evolve from primary lesions or are artifacts induced by the patients or by external factors such as trauma and medications. Primary lesions (pustules, vesicles, papules) may appear quickly and then disappear rapidly. However, they may leave behind secondary lesions (such as focal alopecia, epidermal collarettes, scaling, hyperpigmentation, and crusts), which may be more chronic and give clues about the presence of previous primary lesions. Therefore, the identification and the characterization of both primary and secondary lesions are important. Careful inspection of the diseased skin frequently reveals a primary lesion, which may suggest a limited differential diagnosis.

A primary lesion may vary slightly from its initial appearance to its full development. Later, through regression, degeneration, or traumatization, it may change in form and become a secondary lesion. Although classic descriptions and textbooks refer to lesions as primary or secondary, some lesions can be either (e.g., alopecia can be primary [from hereditary disorders] or secondary [from chewing because of pruritus]). Scales and pigment changes may also be primary or secondary.

Secondary lesions may also be informative. A ring of scaling usually follows a point source of inflammation, either a papule or a pustule. This is also true of small focal circular areas of alopecia. Yellow- to honey-colored crusts usually follow the rupture and drying of pustules. In many cases, however, the significant lesion must be differentiated from the mass of secondary debris. Variations of lesions and their configurations are common, because early and advanced stages coexist in most skin diseases. The ability to discover a characteristic lesion and understand its significance is an important aspect of mastering dermatologic diagnoses.

The following illustrations can help the clinician identify primary and secondary lesions. Also, the character of the lesions may vary, implying a different pathogenesis or cause. The definitions and examples in Figs. 2-3–2-27 explain the relationship of skin lesions to equine dermatoses.

Figure 2-3 Macule—a circumscribed, nonpalpable spot up to 1 cm in diameter and caused by a change in the color of the skin. Patch—a macule larger than 1 cm in diameter. The discoloration can result from several processes: depigmentation, hyperpigmentation, erythema, or hemorrhage. Examples include postinflammatory hyperpigmentation and vitiligo (the photograph illustrates macules and patches of depigmentation due to vitiligo). Types of hemorrhagic macules and patches are as follows: purpura—bleeding into the skin (these are usually dark red but change to purple); petechiae—pinpoint macules that are much smaller than 1 cm in diameter; ecchymoses—patches of hemorrhage.

Figure 2-4 Papule—a small firm, palpable mass in the skin that is up to 1 cm in diameter. Papules may be elevated, intradermal, or subcutaneous. Most papules are caused by inflammatory infiltrates, and may or may not involve hair follicles. Most papules are caused by infectious agents (such as Staphylococcus, Dermatophilus, or dermatophyte) or ectoparasites. Papules caused by infectious agents typically involve hair follicles and begin as tufted papules

(photograph illustrates early staphylococcal folliculitis over thorax).

Figure 2-5 Papules, which proceed to oozing, crusting, and alopecia

(photograph illustrates bacterial pseudomycetoma).

Figure 2-6 Plaque—a larger, flat-topped elevation formed by the extension or coalescence of papules

(photograph illustrates an ulcerated plaque on leg caused by Rhodococcus equi).

Figure 2-7 Pustule—a small, circumscribed, elevated lesion that is filled with pus. Pustules may be intraepidermal, follicular, or subepidermal in location. Their color is usually yellowish to whitish. Pustules are uncommonly seen in the horse, and are usually caused by bacterial infection or pemphigus foliaceus

(photograph illustrates pustules [arrow] caused by pemphigus foliaceus).

Figure 2-8 Multiple pustules on medial surface of hind leg caused by a cutaneous adverse drug reaction (ivermectin).

Figure 2-9 Abscess—a demarcated, fluctuant lesion resulting from the dermal or subcutaneous accumulation of pus. The pus is not visible on the surface of the skin until it drains to the surface. Abscesses are larger and deeper than pustules

(photograph illustrates streptococcal abscess).

Figure 2-10 Vesicle—a sharply circumscribed, elevated lesion that is filled with clear fluid and is up to 1 cm in diameter. Vesicles (arrow) are uncommonly seen in the horse and are caused by autoimmune dermatoses (the photograph illustrates pemphigus foliaceus), epidermolysis bullosa, viral infections, cutaneous adverse drug reactions, photodermatitis, and burns (chemical, thermal). Bulla—a vesicle that is over 1 cm in diameter.

Figure 2-11 Wheal—a sharply circumscribed, raised lesion consisting of edema that usually appears and disappears within hours. Wheals usually produce no changes in the appearance of the overlying skin and hair coat, and disappear without leaving a trace. Wheals are very common on the horse, and are most commonly seen with hypersensitivity reactions (atopic, food allergy, contact reactions). Wheals may be annular and papular to flat-topped

(photograph illustrates wheals caused by intradermal injections).

Figure 2-12 Wheal—may also be polycyclic (ringlike, arciform, serpiginous) in appearance

(photograph illustrates urticaria in an allergic horse).

Figure 2-13 Nodule—a circumscribed, firm mass in the skin that is over 1 cm in diameter. Nodules may be elevated, intradermal, or subcutaneous in location. Nodules are very common in the horse, and are usually caused by granulomatous inflammation (infectious or sterile) or neoplasia

(photograph illustrates an alopecic, ulcerated equine sarcoid on leg).

Figure 2-14 Tumor—a large mass that may arise from any cutaneous cell-type. Tumors are very common in the horse. Most tumors are neoplastic or granulomatous in origin (photograph illustrates bacterial pseudomycetoma). Cyst—an epithelium-lined cavity containing fluid or solid material. It is a smooth, well-circumscribed, fluctuant to solid mass. Cutaneous cysts are usually lined by adnexal epithelium (hair follicle, sebaceous gland, epitrichial sweat gland) and filled with keratin, hair, and/or glandular secretion.

Figure 2-15 Alopecia—a loss of hair that may vary from partial (hypotrichosis) to complete. Alopecia is common in the horse and is most often associated with inflammation of hair follicles (infectious or sterile) or self-trauma in response to pruritus

(photograph illustrates contact dermatitis over thorax).

Figure 2-16 Scale—an accumulation of loose fragments of the stratum corneum. The corneocyte is the final product of epidermal keratinization. Normal loss occurs as individual cells or small clusters not visible to the eye. Abnormal scaling is caused by the shedding of larger clusters. Scales (flakes) vary in consistency from thin, nonadherent, and white to thick, adherent, waxy, and yellowish to grayish to brownish in color. Scales are common in the horse and are most often produced by inflammatory dermatoses

(photograph illustrates seborrhea sicca in association with chronic liver disease).

Figure 2-17 Crust—formed when dried exudate, serum, pus, blood, cells, scales, or medications adhere to the surface of the skin. Crusts are common in the horse and are usually annular and associated with infections (the photograph illustrates dermatophytosis) and ectoparasites.

Figure 2-18 Hair cast—an accumulation of keratin and follicular material that adheres to the hair shaft. Hair casts indicate abnormalities in follicular keratinization and may be seen in primary seborrhea, dermatophytoses, staphylococcal folliculitis, and demodicosis

(the photograph illustrates hair casts in association with contact dermatitis; hair shafts are encircled by keratinous and medication-derived material).

Figure 2-19 Abnormal pigmentation—usually caused by increased or decreased amounts of melanin. Increased melanization of skin and hair are referred to as melanoderma (hyperpigmentation) and melanotrichia, respectively

(photograph illustrates alopecia and melanoderma caused by pemphigus foliaceus).

Figure 2-20 Melanotrichia caused by insect-bite hypersensitivity.

Figure 2-21 Decreased melanization of skin and hair are referred to as leukoderma (depigmentation) and leukotrichia, respectively

(photograph illustrates leukotrichia caused by reticulated leukotrichia).

Figure 2-22 Epidermal collarette—a special type of scale arranged in an annular rim of separated (“peeling”) epithelium or loose scales. Collarettes usually represent the remnants of the roof of a pustule, vesicle, or bulla, and hence the prior existence of a fluid-filled lesion

(photograph illustrates epidermal collarettes in association with staphylococcal folliculitis).

Figure 2-23 Scar—an alopecic, often depigmented area of fibrous tissue that has replaced severely damaged skin

(photograph illustrates a chronic pressure sore with scar formation and central ulceration).

Figure 2-24 Erosion (arrow)—a shallow epithelial defect that does not breech the basement membrane zone and consequently heals without scarring. Erosions usually result from the progression of papular, pustular, and vesicular diseases or self-trauma

(photograph illustrates pemphigus foliaceus).

Figure 2-25 Ulcer—a deeper defect that breeches the basement membrane and exposes the underlying dermal and possibly subcutaneous tissues. Scars may be left when ulcers heal (photograph illustrates an ulcer resulting from a skin slough caused by clostridial cellulitis). Excoriation—self-induced erosions and ulcers caused by rubbing, biting, scratching, and so forth. Excoriations indicate the presence of pruritus and are often linear in nature.

Figure 2-26 Lichenification—a thickening and hardening of the skin with an exaggeration (wrinkled or rugose appearance) of the skin surface markings. The most common cause is chronic inflammation

(photograph illustrates insect-bite hypersensitivity).

Figure 2-27 Necrosis—the death of skin due to ischemia or toxins. Necrotic skin becomes bluish, purplish, and black, and eventually dry and firm (leathery). Common causes include clostridial infection (photograph illustrates well-delineated necrosis and ulceration due to clostridial cellulitis), vasculitis, and pressure.

Configuration of Lesions

The configuration of skin lesions may be helpful in establishing a differential diagnosis. Some diseases often have lesions present in certain configurations, and although exceptions exist, recognizing the pattern of lesions adds information for decision-making. Lesions may be single, such as the solitary dermatophytosis lesion, a foreign-body reaction, or a neoplasm. Multiple lesions are most commonly seen in skin diseases of horses.

When lesions are linear, external forces such as scratching, being scratched by something, and having something applied to the skin are often responsible. In other cases, linear lesions may reflect the involvement of a blood or lymphatic vessel, a dermatome, or a congenital malformation. Diffuse areas of involvement tend to suggest a metabolic or systemic reaction, such as endocrine disorders, keratinization, and immunologic or hypersensitivity disorders. Annular lesions are often associated with peripheral spreading of a disease. Common examples of annular lesions are bacterial folliculitis, dermatophilosis, and dermatophytosis. Coalescing lesions occur when multiple lesions are present and spread so that they overlap.

Different Stages

A skin disease and its individual lesions progress from its earliest appearance to a fully developed state and, in many cases, to a chronic or resolved stage. The distribution, the configuration, and the histologic appearance of lesions change. The evolution of lesions should be determined either by obtaining the history or by finding different stages of lesions on the patient. Papules may develop into vesicles and pustules, which may rupture to leave erosions or ulcers and finally crusts. An understanding of these progressions helps in the diagnostic process.

As lesions develop in specific patterns, they also involute in characteristic ways. The lesions change, along with their histologic appearance. For example, a macule may develop into a papule and then a crust or crusted erosion. It may then spread peripherally, occurring as a ring of lesions; the lesion then appears as a circular patch with multiple papules or crusts on the margins and central alopecia. The fully developed lesions could appear as a large alopecic patch with a central area of hyperpigmentation and multifocal erythematous macules, papules, or crusts intermittently along the leading margin; this lesion could then appear arciform. Scaling may also occur at the leading margins of inflammation. The healing phase of a chronic lesion may appear as a patch of alopecia and hyperpigmentation with no other primary or secondary lesions because they have spontaneously resolved or responded to therapy.

Because diseases and their lesions are evolutionary, the clinician must attempt to learn about all of the stages. The recognition of the different stages and the lesions becomes important when selecting areas to sample for diagnostic tests.

Distribution Patterns of Skin Lesions

The areas of skin involved with lesions or affected by symptoms of the disease help in determining the differential diagnosis because most skin diseases have a typical distribution. It is important to emphasize that the accurate determination of the distribution necessitates detection of all changes in the hair coat or the skin, the location of symptoms related to the disease, and the location of all primary and secondary lesions. An adequate determination of the distribution pattern can be achieved only by a thorough history and dermatologic examination; cursory examinations are often incomplete.

The study of skin diseases involves understanding the primary lesions that occur and the typical distribution patterns. Diseases less commonly present with atypical patterns. The combination of the type of lesions present and their distribution is the basis for developing a differential diagnosis. The distribution pattern may be very helpful by allowing clinicians to establish the differential diagnosis based on the region involved when animals have lesions and symptoms confined to certain regions (Tables 2-3 and 2-4). In some instances, this regional pattern is such a major feature that it is a required aspect of the disease. Cutaneous reaction patterns are also a very useful prompter for establishing a differential diagnosis (Table 2-5).

TABLE 2-3 Regional Differential Diagnosis for Nonneoplastic Dermatoses^a

a Cutaneous adverse drug reactions can mimic ANY dermatitis (see Chapter 9) and contact dermatitis can affect ANY body region (see Chapters 8 and 13).

TABLE 2-4 Common Localizations of Neoplasms, Cysts, Hamartomas, and Keratoses^a

a See Chapter 16 for details.

TABLE 2-5 Differential Diagnosis for Some Cutaneous Reaction Patterns

These tables are useful in the differential diagnosis. However, the clinician must also be aware that other diseases may occur in these regions and that diseases that often affect a certain region can also occur elsewhere and not involve the typical region. Therefore, regional evaluations aid in ranking differential diagnostic possibilities: they do not determine the diagnosis.

Differential Diagnosis

A differential diagnosis is developed on the basis of a compilation of the preceding information. The possible diagnoses should be considered in their proposed likely order of occurrence. This point is important as the first step in developing a cost-effective plan.

Developing a Diagnostic or Therapeutic Plan

Laboratory tests or therapies can be recommended on the basis of tentative and differential diagnoses. If a strong tentative diagnosis is not determined from the history and the physical examination, the approach should be directed at the two or three most likely diagnoses. Client-veterinarian interaction is critical at this point. The client decides what is going to be done, but his or her decision is based on the clinician’s recommendations. Therefore, the client needs to know the tentative or possible diagnoses, the expected costs, and anticipated results of the diagnostic or therapeutic options proposed.

Diagnostic tests and laboratory procedures are useful whenever a definitive diagnosis cannot be made from the case history and clinical examination alone. Laboratory procedures may confirm many clinical diagnoses and provide a logical basis for successful therapeutic management. They should be recommended on the basis of the most likely diagnosis and should not be randomly suggested or recommended just to be comprehensive. The cost-effectiveness of each test should also be considered. In practice, it is often unacceptable to recommend numerous tests to screen for the long list of possible diagnoses in any given case. Instead, the results of recommended tests should confirm or rule out the diagnoses that the clinician deems most likely.

Surface Sampling Plan

The lesions and pathologic changes of a skin disease are often readily available for study, and a variety of laboratory tests are based on this easy access to the skin’s surface. A great deal of information may be obtained by studying microscopically materials collected from the hair and skin. Skin scraping and obtaining an acetate tape impression are techniques used to find microscopic ectoparasites. Hairs may be removed, and exudates may be collected and examined microscopically. Most of these techniques may be done in general clinical practice and rapidly add valuable information to a case work-up. However, practice and study may be necessary to maximize the benefits of many tests. The effort to learn these techniques is well worth the time. The alternatives are not to obtain this information, to do other more expensive and time-consuming tests, or to send samples to a laboratory, which adds cost and time delays.

Acetate Tape Impression

This alternative to skin scraping has been recommended to find superficial ectoparasites such as Chorioptes mites, poultry mites, forage mites, chiggers, and pinworms. Clear, pressure-sensitive acetate tape (Scotch No. 602 [3M Co.] is a good type) is pressed to the hair surface and to the skin adjacent to parted hairs or in shaved areas. Superficial scales and debris are also collected. The tape is then placed adhesive side down over a drop or two of mineral oil on a glass slide. The mineral oil helps clear debris and facilitates the examination. A drop of mineral oil is then placed on top of the tape, a coverslip is applied, and the slide is examined.

Hair Examination

Plucking hairs from the skin and examining them under the microscope is referred to as trichography and is helpful for diagnosing self-inflicted alopecia, dermatophytosis, piedra, nutritional or congenital hair dysplasias, trichorrhexis nodosa, anagen defluxion, telogen defluxion, and pigmentary disturbances of hair growth. Trichography is performed by grasping a small number of hairs with the fingertips or rubber-covered hemostats, epilating them completely in the direction of their growth, laying them in the same orientation on a microscope slide with mineral oil, and examining them with the low-power objective of the microscope. Epilating hairs in other directions or with uncovered hemostats can produce confusing artifacts. If abnormalities are detected during scanning, closer examination will be necessary to categorize the defect.

Hairs will have either an anagen or telogen root (bulb). Anagen bulbs are rounded, smooth, shiny, glistening, and often pigmented and soft, so the root may bend (Fig. 2-28). Telogen bulbs are club- or spear-shaped, rough-surfaced, nonpigmented, and generally straight (Fig. 2-29). A normal hair shaft is uniform in diameter and tapers gently to the tip. All hairs should have a clearly discernible cuticle and a sharply demarcated cortex and medulla. Hair pigmentation depends on the coat color and breed of horse, but should not vary greatly from one hair to the next in regions where the coat color is the same.

Figure 2-28 Anagen hair. Note rounded, pigmented bulb, which is bent dorsally (arrow).

Figure 2-29 Telogen hair. Proximal end is rough-surfaced, nonpigmented, and pointed.

Normal adult animals have an admixture of anagen and telogen hairs, the ratio of which varies with season, management factors, and a variety of other influences (see Chapter 1). No normal animal should have all of its hairs in telogen (Fig. 2-30); therefore, this finding suggests a diagnosis of telogen defluxion or follicular arrest (see Chapter 10).

Figure 2-30 Telogen defluxion. All epilated hairs are in telogen.

Examination of the hair shaft follows bulbar evaluation. Hairs that are inappropriately curled, misshapen, and malformed suggest an underlying nutritional, metabolic, or hereditary disease (Fig. 2-31). Hairs with a normal shaft that are suddenly and cleanly broken (Fig. 2-32) or longitudinally split (trichoptilosis) indicate external trauma from excessive rubbing, chewing, licking or scratching, or too vigorous grooming. Breakage of hairs with abnormal shafts can be seen in congenitohereditary disorders (see Chapter 14), trichorrhexis nodosa (see Chapter 10), anagen defluxion (see Chapter 10), alopecia areata (see Chapter 9), and dermatophytosis (Figs. 2-33 and 2-34; see Chapter 5). Abnormalities in hair pigmentation have not been well-studied. When unusual pigmentation is observed, external sources (e.g., chemicals and topical medications, sun bleaching) or conditions that influence the transfer of pigment to the hair shaft (e.g., drugs, nutritional imbalances, endocrine disorders, idiopathic pigmentary disorders) must be considered. Hair casts are usually seen in diseases associated with follicular keratinization abnormalities, such as primary seborrhea and follicular dysplasia. Hair casts should not be confused with louse nits (Fig. 2-35).

Figure 2-31 Cutaneous adverse drug reaction (ivermectin). Anatomy of hair shaft is focally obscured, and shaft is weakened, resulting in breakage (arrow).

Figure 2-32 Insect-bite hypersensitivity. Distal end of hair is fractured (arrow) from rubbing.

Figure 2-33 Dermatophytosis. Hair shaft is focally thickened and fuzzy (arrow).

Figure 2-34 Dermatophytosis. Arthroconidia and hyphae are seen within the hair (arrows).

Figure 2-35 Louse nit (arrow) attached to hair.

Cytologic Examination

An enormous amount of vital diagnostic data can be obtained by microscopic examination of stained material, such as smears of tissues or fluids, during a clinical examination. It is possible to accomplish this with minimal equipment and in less than 5 min. The cost is much less than that for bacterial culture and susceptibility testing or biopsy. Although the same information as obtained by these more expensive tests is not really gathered, microscopic examination often supplies sufficient data to narrow a differential diagnosis and develop a diagnostic plan.

The type of inflammatory, neoplastic, or other cellular infiltrate; the relative amount of protein or mucin; and the presence of acantholytic keratinocytes, fungi, and bacteria can be determined by cytologic evaluation. It is the most common and most rewarding office test performed by the authors. The equipment includes a clean microscope slide, a coverslip, a stain, and a microscope.

Cytologic findings

Cytologic study is helpful to distinguish between bacterial skin infection and bacterial colonization, to determine the relative depth of infection, to conclude whether the pustule contains bacteria or is sterile, to discover yeasts and fungi, to identify various cutaneous neoplasms, or to find the acantholytic cells of the pemphigus diseases. Samples obtained by impression smears or swabs usually contain variable numbers of keratinocytes. Typically, the keratinocytes are anuclear corneocytes. Many corneocytes will be rolled-up, deeply basophilic, and spiculelike. Corneocytes contain variable amounts of melanin granules (brown or black in Diff Quik-stained specimens), reflecting the animal’s normal pigmentation. If the skin is parakeratotic, nucleated corneocytes will be present. Occasionally, cells from the stratum granulosum are recognized by their keratohyalin granules (blue or purple in Diff Quik-stained specimens). Keratohyalin granules vary in size and shape and should not be confused with bacteria.

Bacteria are a frequent finding in impression smears from skin and can be seen as basophilic-staining organisms in specimens stained with new methylene blue or Diff Quik. Although identification of the exact species of bacteria is not possible with a stain (as it is in a culture), it is possible to distinguish cocci from rods and often to institute appropriate and effective antibiotic therapy without performing a culture and antibiotic susceptibility testing. Generally, when cocci are seen, they are Staphylococcus pseudintermedius or S. aureus. If no bacteria are found in the stained fluid, the clinical condition is probably not a bacterial pyoderma. If neither granulocytes nor intracellular bacteria are seen, even large numbers of bacteria are not likely to be of etiologic significance.

It is also possible to obtain some clues as to the type of bacterial pyoderma or the underlying condition. In general, deep infections have fewer bacteria present, with the vast majority being intracellular. In addition, deep infections have a mixed cellular infiltrate with large numbers of neutrophils, macrophages, lymphocytes, and plasma cells. The presence of these cells suggests that longer-term antibiotic therapy is necessary. Large numbers of intracellular and extracellular cocci are seen more commonly in superficial bacterial infections (Figs. 2-36 and 2-37), and the inflammatory cells are almost exclusively neutrophils.

Figure 2-36 Staphylococcal folliculitis. Degenerate neutrophils and nuclear streaming (arrow).

Figure 2-37 Staphylococcal folliculitis. Degenerative neutrophils, nuclear streaming, and intracellular cocci (arrow).

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