12
Cytology of Cutaneous and Subcutaneous Lesions

Amy C. Valenciano¹, Andrew Burton², Angela Borchers³, and Rick L. Cowell⁴

¹ IDEXX Laboratories, Inc., Dallas, TX, USA

² IDEXX Laboratories, Inc., North Grafton, MA, USA

³ School of Veterinary Medicine, University of California, Davis, CA, USA

⁴ Stillwater, OK, USA

12.1 Introduction

Cytological examination can be very useful when a cutaneous or subcutaneous lesion is not easily diagnosed by clinical evaluation, and when there is lack of response to initial therapy. Cutaneous and subcutaneous lesions in the equine are easily accessible, and there are no significant contraindications for aspiration, scraping, or impression smear collection. Tranquilization and/or anesthesia is seldom needed. The cytological specimen can be collected, prepared, stained, and microscopically evaluated in minutes, often providing a diagnosis, prognosis, indication of appropriate therapy, and/or indication of the next diagnostic procedure.

12.2 Collection Techniques

Lesions may be swabbed, imprinted, scraped, and/or aspirated. Ulcerated or eroded masses should be imprinted initially to characterize the superficial pathology, then cleaned and dried followed by scraping or deep aspiration, to characterize the deeper pathology. Nonulcerated masses require only aspiration. If the mass is large or complex, aspiration from various angles or depths should be performed. If the lesion is cystic, aspiration from both the cystic contents and the solid portion should be performed. This is important, as often, cystic components are nonspecific.

Swabs are collected only when imprints, scrapings, and aspirates cannot be made, as this method often results in low cell yield. If the lesion is dry, a cotton swab can be moistened with sterile isotonic fluid, such as 0.9% NaCl, to minimize cell damage during collection and slide preparation, but is unnecessary if the lesion is moist or exudative. After sample collection, gently roll the swab along the flat surface of the slide. Do not rub the swab across the slide surface, as this will result in excessive cell damage. Imprints are made by removing any scab covering the lesion and then touching the surface of a clean glass slide to the surface of the lesion. If Dermatophilus congolensis infection is suspected, the underside of the scab is imprinted as well. The lesion can then be cleaned with a nonirritating antiseptic, wiped dry with a sterile gauze sponge, and reimprinted.

High‐quality cytological smears can also be made by imprinting biopsy specimens. The biopsied tissue surface is gently blotted on a clean gauze until dry and the tissue gently sticks to the gauze, then, the tissue is gently but firmly pressed onto a glass slide multiple times. Scrapings of cutaneous lesions are made by rubbing the edge of a blunt instrument, such as a glass slide or the back of a scalpel blade, across the lesion. This results in accumulation of cells along the edge of the blunt instrument. The cells can then be gently spread onto a clean, dry glass slide.

Aspirates are obtained by using a 20–25 gauge needle attached to a 3–20 mL syringe. If microbiological evaluation is to be performed, the area of aspiration should be surgically prepared. Otherwise, an alcohol swab can be used to clean the area. Hold the mass to be aspirated firmly to aid penetration of the skin and mass and to control the direction of the needle. Introduce the needle, attached to a syringe, into the center of the mass and apply strong negative pressure by withdrawing the plunger about one‐half to three‐fourths the volume of the syringe. If the lesion is large enough, redirect the needle to sample different angles and depths of the mass without removing the needle, and relieve the negative pressure first (apply negative pressure only when the needle is static). When aspiration is complete (often, high‐quality collections do not have sample showing in the syringe and sometimes not even in the hub of the needle), release negative pressure from the syringe and remove the needle from the mass. This step is important to avoid aspirating blood and surrounding normal tissue and to avoid the sample from being aspirated from the barrel and needle hub into the syringe itself, making expulsion of material onto slides difficult. Once the negative pressure has been fully released, remove the needle from the mass and skin. Then, remove the needle from the syringe and aspirate air into the syringe. Next, replace the needle onto the syringe and expel the material onto a glass slide by rapidly depressing the plunger.

Alternatively, a nonaspiration or tattoo technique may be used. This method is useful in very hemorrhagic lesions when adequate numbers of cells are difficult to retrieve without contaminating the sample with peripheral blood or when aspiration yield is low, which can happen with very firm and solid lesions. The nonaspiration technique collects cells within the needle by capillary action after shearing cells from the tissue with small, repeated movements of the needle tip. Using just the needle itself or the needle attached to a syringe filled with air and no negative pressure applied, rapidly and repeatedly advance the needle through most of the thickness of the mass, using an action like a sewing machine. Next, withdraw the needle from the mass and expel the material onto a glass slide as described above.

12.3 Slide Preparation

Gently spray the aspirated material onto the center of a glass slide, and place a second glass slide on top. Let the material spread between the two slides with only the weight of the second glass slide. Next, gently pull the slides apart from each other horizontally using a smooth technique with no downward pressure. If excessive cellular lysis is noted from cursory review of a stained slide, use a coverslip to spread the material instead of another glass slide.

For fluid‐filled and cystic lesions, make a few direct smears of the fluid component, using a blood smear technique with a small drop of fluid at one end of the slide and a second slide at a 45° angle to spread the fluid into a monolayer containing a feathered edge. A small drop of fluid can also be placed onto the middle of a slide, with a second slide on top and gently pulling the slides apart horizontally. If the fluid is clear and poorly cellular then, in addition to a direct smear, centrifugation and slide preparation from the sediment may also be performed. Fluid can also be sent to the laboratory for cytospin concentrated slides to be made. It is important to also aspirate the wall or solid portion of the mass, which can be sent in together with the fluid slides and fluid specimen.

12.4 General Evaluation of Cytological Smears

The first step in cytological evaluation of a smear is to determine whether sufficient numbers of intact cells are present and whether the sample is spread and stained adequately. If repeated collection attempts fail to yield sufficient numbers of cells, biopsy may be necessary.

Once a suitable cytological preparation has been produced, the smears are evaluated for evidence of inflammation and/or neoplasia. If all the cells from a solid mass are tissue cells (i.e., no inflammatory cells are present), the lesion is caused by neoplasia or hyperplasia. If all the cells are inflammatory cells, an inflammatory process is most likely the primary cause, but an inflamed, poorly exfoliative neoplasm cannot be ruled out. An admixture of inflammatory cells and tissue cells can be caused by inflammation with secondary tissue cell hyperplasia or dysplasia, or could be consistent with an inflamed neoplasm. Therefore, caution must be exercised in diagnosing neoplasia if evidence of inflammation is detected, as in the face of inflammation, tissue cells can become dysplastic and be mistaken for neoplasia.

12.5 Evaluation of Inflammatory Cell Populations

Figure 12.1 provides an algorithm to aid evaluation of the inflammatory cell component of cutaneous and subcutaneous lesions. Table 12.1 gives some general considerations for inflammatory responses.

If most of the inflammatory cells are neutrophils (Figures 12.2 and 12.3) but no bacteria are found, a covert infection may be present or the neutrophilic inflammatory response may be due to one of the conditions listed under Marked predominance of neutrophils in Table 12.1. The lesion should be cultured to identify a covert infection. If culture results reveal an infectious agent, appropriate therapy can be instituted. If culture results do not reveal an infectious agent or if therapy for the infectious agent identified by culture is not effective, cytological evaluation can be repeated or a biopsy can be submitted for histopathological evaluation.

When >15% of the inflammatory cells are macrophages (Figure 12.4) and/or giant inflammatory cells, fungal infection or foreign body granuloma should be considered. The slide should be carefully perused for organisms or signs of foreign material, such as refractile debris or eosinophilic material typical of adjuvant. Also, historical information concerning possible introduction of foreign material should be sought. If no organisms or foreign materials are found and there is no historical information indicating introduction of a foreign substance into the area, the tissue can be cultured or a biopsy can be submitted for histopathological examination.

Image described by caption. — **Figure 12.1** Algorithm to aid in evaluation of aspirates containing a preponderance of inflammatory cells.

If the proportion of eosinophils exceeds 10% (Figures 12.5 and 12.6), an allergic, parasitic, foreign body reaction or fungal infection should be considered. Again, the slide should be carefully searched for organisms or signs of foreign material. If none are found, the lesion can be cultured for bacteria and/or fungal organisms and a biopsy can be submitted.

12.6 Cytological Characteristics of Select Infectious Agents

Lesions characterized by >85% neutrophils (Figures 12.7 and 12.8), which are often degenerate, with fewer macrophages, are often associated with bacterial infection. Fungal infections tend to produce lesions containing prominent numbers of macrophages but often with a predominance of neutrophils. With certain fungal infections, eosinophils can be numerous. Bacterial and fungal infections can also contain lesser numbers of mature lymphocytes and plasma cells. Fibroblasts (spindle‐shaped tissue cells) can also be present and consistent with a reactive and reparative component. The cellular composition is also influenced by the type of infectious agent, location of the lesion, duration of the lesion and immune status of the patient.

12.6.1 Bacterial Cocci

Most pathogenic bacterial cocci are gram positive and of the genus Staphylococcus or Streptococcus (Figures 12.9 and 12.10). Staphylococci usually occur in clusters of 4–12 bacteria, whilst streptococci tend to occur in short or long chains. When cocci are identified in cytological preparations, aerobic and anaerobic culture and sensitivity should be performed to identify the organism and appropriate antibacterial therapy. As most pathogenic cocci are gram positive, antibacterial therapy effective against gram‐positive organisms should be used when it is necessary to initiate therapy before culture and sensitivity results are received.

12.6.2 Dermatophilus congolensis

Dermatophilus congolensis is a facultative anaerobic actinomycete that infects the superficial epidermis, causing exudative, crusty lesions. Infection occurs in humid, tropical, and subtropical locations, after significantly prolonged rainfall and concurrent with other skin diseases. Removal of crusts reveals eroded to ulcerated skin underneath. Cytological preparations from the undersurface of scabs are most rewarding in demonstrating organisms. These preparations usually contain mature epithelial cells, keratin bars, debris, and organisms. A few neutrophils may also be found. If the undersurface of the scab is dry and does not yield adequate cytological preparations, crusts and scabs may be minced in saline and smears made for cytological evaluation. D. congolensis replicates by transverse and longitudinal division, producing chains of cocci arranged in 2–8 parallel rows resembling small, blue railroad tracks (Figure 12.11). Also, many individual coccoid cells may be seen cytologically. Dermatophilosis can be acute, subacute, and chronic. Organisms may not be seen on cytology in some chronic cases, or in cases in which cytology is negative and infection is clinically suspected. In these instances, other testing such as RT‐qPCR may be necessary [1]. Diagnosis can also be obtained by biopsy and histopathology and culture. Dermatophilosis is a zoonotic agent and caution in handling infected animals and skin lesions/crusts is warranted.

Table 12.1 Some conditions suggested by certain proportions of inflammatory cells.

Inflammatory cell population	First considerations	Second considerations
Marked predominance (85%) of neutrophils
Many degenerate neutrophils	Gram‐negative bacteria Gram‐positive bacteria	Abscess secondary to neoplasia, foreign bodies, etc.
Few degenerate neutrophils	Gram‐positive bacteria Gram‐negative bacteria Higher bacteria (Nocardia, Actinomyces, etc.)	Fungi Protozoa Foreign body Immune mediated Chemical or traumatic injury Abscess secondary to neoplasia
No degenerate neutrophils	Gram‐positive bacteria Higher bacteria (Nocardia, Actinomyces, etc.) Chemical or traumatic injury Panniculitis	Abscess secondary to neoplasia Fungi Foreign body
Admixture of inflammatory cells
15–40% macrophages	Higher bacteria (Nocardia, Actinomyces, etc.) Fungi Protozoa Neoplasia Foreign body Panniculitis Any resolving inflammatory lesions	Nonfilamentous gram‐positive bacteria Parasites Chronic allergic inflammation
>40% macrophages	Fungi Foreign body Protozoa Neoplasia Panniculitis Any resolving inflammatory lesions	Parasites Chronic allergic inflammation
Giant inflammatory cells present	Fungi Foreign body Protozoa Collagen necrosis Panniculitis Parasites (if eosinophils are present)
>10% eosinophils	Allergic inflammation Parasites Collagen necrosis Mast cell tumor	Neoplasia Foreign body Hyphating fungi

12.6.3 Small Bacterial Rods

Most small bacterial rods are gram negative but some, such as Corynebacterium spp., are gram positive. Some gram‐negative rods can be recognized cytologically as bipolar (Figures 12.12 and 12.13). All pathogenic bipolar bacterial rods are gram negative. Rod bacterial infections are usually associated with a marked neutrophilic inflammatory response. When small bacterial rods are recognized in cytological preparations, the lesion should be cultured to identify the organism and sensitivity tests performed to determine appropriate antibacterial therapy. If it is necessary to institute antibacterial therapy before culture and sensitivity results are received, therapy employed should be effective against gram‐negative organisms since most pathogenic small rods are gram negative.

12.6.4 Filamentous Rods

Filamentous rods that can cause cutaneous infections include Nocardia spp., Actinomyces spp., Mycobacterium spp., and certain anaerobes such as Fusobacterium spp. Because these organisms are often refractory to common antibacterial therapy and reliable culture of these organisms requires special conditions, cytological evaluation is very useful in indicating the need for special cultures. Rarely, the pathogenic filamentous rods of Nocardia spp. and Actinomyces spp. (Figure 12.14) may cause cutaneous or subcutaneous lesions (abscesses, ulcers, draining tracts, masses) in horses. Cutaneous infection with these agents is uncommon and usually occurs secondary to contamination of an existing wound. Mycobacterium spp. and some anaerobes, such as Fusobacterium, rarely may be filamentous. Nocardia and Actinomyces generally have a distinctive morphology in cytological preparations stained with Romanowsky‐type stains. They are characterized by long, slender (filamentous) strands that stain pale blue and have intermittent, small, pink to purple areas, giving the rods a beaded appearance. This morphology is characteristic of both Nocardia and Actinomyces spp. and the filamentous form of Fusobacterium spp. When these features are recognized cytologically, cultures should be performed specifically for Nocardia, Actinomyces, and anaerobes.

Image described by caption and surrounding text. — **Figure 12.11** Imprint from underside of scab secondary to *Dermatophilus congolensis* infection. There is a background of squamous debris and numerous chains of bacterial doublets. Scattered individual bacteria are also present. Wright’s stain, original magnification 250×.

Mycobacterium spp. (atypical mycobacterial infections and cutaneous tuberculosis), on the other hand, often do not stain with Romanowsky‐type stains. As a result, negative images (Figures 12.15 and 12.16) may be observed in the cytoplasm of macrophages and free in the extracellular space. Mycobacterium spp. stain bright pink with acid‐fast stains (Figure 12.17). Therefore, when negative images are encountered or when the character of the lesion suggests mycobacteria infection, an acid‐fast stain can be performed to demonstrate the organism and/or cultures for Mycobacterium spp. can be performed to identify the organism.

12.6.5 Large Bacterial Rods

Large bacterial rods found in cytological preparations may be pathogenic or nonpathogenic. Those that are pathogenic and sometimes infect cutaneous and subcutaneous tissues include Clostridium spp. and, infrequently, Bacillus spp. When large bacterial rods are thought to be pathogenic, both aerobic and anaerobic cultures should be performed. Also, the smears should be inspected for large bacterial rods that contain spores.

12.6.6 Sporothrix schenckii

Sporothrix schenckii infection (sporotrichosis) most commonly occurs in a cutaneolymphatic form; however, a primary cutaneous form with no lymphatic involvement is seen occasionally. In the cutaneolymphatic form, hard subcutaneous nodules develop along lymphatics and the lymphatics may become corded. The nodules may ulcerate. In horses, the organisms are scarce and cytological preparations must be perused carefully. If organisms are not found, the lesion should be cultured and a biopsy of the lesion should be submitted for histopathological evaluation. In cytological preparations stained with Romanowsky stains, S. schenckii organisms are round to oval to fusiform (cigar shaped). They are 3–9 μ long and 1–3 μ wide and stain pale to medium blue with a slightly eccentric pink to purple nucleus (Figure 12.18). They may be confused with Histoplasma capsulatum if only a few organisms are found and the classic fusiform (cigar shape) is not seen.

12.6.7 Histoplasma capsulatum, Blastomyces dermatitidis, Cryptococcus neoformans, and Coccidioides immitis

Cutaneous lesions secondary to infection with Histoplasma, Blastomyces, Cryptococcus, and Coccidioides organisms are rare in the equine. Infection of the skin can be secondary to hematogenous and/or lymphatic spread from primary pulmonary infection. However, these organisms may rarely produce a primary cutaneous lesion from direct trauma and inoculation.

Characteristics of these organisms in cytological preparations stained with Romanowsky‐type stains are as follows: H. capsulatum organisms (Figure 12.19) are round to slightly oval but are not fusiform or cigar shaped. They are 2–4 μ in diameter (about half the size of a RBC), stain pale to medium blue, and contain an eccentric pink to purple nucleus. A thin, clear capsule surrounds the yeast. Accompanying inflammation is pyogranulomatous. B. dermatitidis organisms are blue, spherical, 8–20 μ in diameter, and thick walled. Budding is occasionally seen and is broad based (Figures 12.20–12.23). Inflammation is primarily pyogranulomatous with few to many organisms. C. neoformans organisms are spherical and usually have a thick mucoid capsule; occasionally, poorly encapsulated (rough) forms are found. Poorly encapsulated forms measure 4–8 μ in diameter and capsulated forms are 8–40 μ in diameter. The internal structures stain light pink to blue‐purple and may be slightly granular (Figures 12.24–12.26). The capsule usually is clear and homogeneous, but it may stain light to medium pink. Cryptococcal infection usually evokes a minor granulomatous response of epithelioid macrophages and/or inflammatory giant cells. In some cytological preparations, Cryptococcus organisms may outnumber inflammatory and tissue cells. Poorly encapsulated forms tend to elicit a greater inflammatory response than heavily encapsulated forms. C. immitis organisms are large, 10–100 μ in diameter, contoured, blue to blue‐green spheres (Figures 12.27–12.30). Budding is not seen. The spherules are filled with numerous endospores measuring 2–5 μ in diameter, which may be seen in larger organisms, and if the yeast is broken open, endospores may be seen spilling out. Inflammation is primarily neutrophilic with a lesser granulomatous component. Coccidioides organisms are often scarce in cytological preparations. The tremendous variation in size, presence of endospores, and pale blue/green tint to the organism differentiate C. immitis from nonbudding B. dermatitidis.

12.6.8 Dermatophytes

The dermatophytes Trichophyton spp. and Microsporum spp. cause cutaneous lesions that may have a typical ringworm‐like appearance or appear as gray to yellow‐brown crusty lesions or as follicular papules. Scrapings from the edge of the lesion are most rewarding when searching for dermatophytes. Dermatophytes can be identified in cytological preparations using the standard 20% potassium hydroxide in wet mount preparations stained with new methylene blue, or in air‐dried preparations stained with Romanowsky‐type stains. Cytologically, very small, spherical conidia are found free within the smears as well as within hair shafts (endothrix invasion) or on the hair shaft surface (ectothrix invasion). With Romanowsky‐type stains, conidia stain medium to dark blue with a thin clear halo (Figure 12.31). An inflammatory reaction composed of an admixture of neutrophils, macrophages, lymphocytes, eosinophils, and plasma cells may be seen in cytological preparations from skin scrapings. Fungal culture of hair and crusts can also be helpful to diagnose dermatophytosis.

12.6.9 Fungi That Form Hyphae in Cutaneous and Subcutaneous Tissues

Many fungi can infect and form hyphae in cutaneous and subcutaneous tissues. They may cause single or multiple, small to very large lesions that range from nodules to ulcers to draining tracts. These fungi induce a granulomatous inflammatory response characterized by epithelioid macrophages and inflammatory giant cells. Neutrophil, lymphocyte, plasma cell, and eosinophil numbers are variable. Pheohyphomycosis refers to infections by pigmented fungi. Whilst most fungi stain well with Romanowsky‐type stains (Figure 12.32), some do not and are recognized as negative images (Figure 12.33). Fungal culture or histopathological examination with special immunohistochemical stains may be used to definitively classify the fungus.

12.6.10 Leishmania

Leishmania spp. are protozoans which can infect skin and subcutaneous tissues of horses, involving any part of the body but often the head, neck, and ears. Lesions range from small to very large, are thickened, nodular, crusty, and may be ulcerated or nonulcerated. Imprints, scrapings, and aspirates yield a variable inflammatory mixture of neutrophils, macrophages, lymphocytes, and plasma cells. Usually, numerous, small (2–4 μ), round to oval amastigotes with light blue cytoplasm, a red oval eccentric nucleus, and a small dark (red‐purple) kinetoplast at right angles to the nucleus are found within macrophages and free in the preparation [2] (Figures 12.34 and 12.35).

12.7 Noninfectious Inflammatory Lesions

Inflammatory lesions not caused by infectious agents include immune‐mediated diseases, allergic and hypersensitivity reactions, and sterile foreign body reactions. Cytological evaluation along with clinical evaluation may be helpful in diagnosing noninfectious inflammatory lesions.

12.7.1 Allergic/Hypersensitivity Inflammatory Reactions

Allergic or hypersensitivity reactions in horses can have many different etiologies and presentations. Hypersensitivity reactions may be due to exposure to allergens from food, inhalation, insect hypersensitivity, contact allergens, drug reactions or atopy. The clinical manifestation of these reactions results in many different gross presentations including formation of hives, nodules, wheals, and papules, which can progress to alopecia, crusting, and hypopigmentation (e.g., secondary to chronic irritation associated with insect bite hypersensitivity). Urticaria and angioedema are also common manifestations of hypersensitivity in horses.

Lesions secondary to allergic or hypersensitivity disease typically contain a predominance of eosinophils, particularly in type I hypersensitivity reactions such as those associated with contact allergens or arthropod bites, largely mediated through release of cytokines such as interleukin (IL)‐5 [3, 4]. Mast cells and basophils are variably present, and may predominate in type III hypersensitivity reactions that most commonly manifest with urticaria and angioedema. Neutrophils may be present, especially in ulcerated or crusted lesions. Additionally, macrophages, lymphocytes, and plasma cells may be present in lesser numbers in chronic lesions. A predominance of eosinophils may be seen in other lesions and correlation with other clinical findings and diagnostic tests is imperative.

12.7.2 Eosinophilic Granuloma with Collagen Degeneration

This condition, also termed nodular necrobiosis, nodular collagenolytic granuloma, collagenolytic granuloma, eosinophilic granuloma, or acute collagen necrosis, is the most common cause of nonneoplastic nodular disease in horses. Many cases of eosinophilic granuloma are considered to represent a form of hypersensitivity reaction, such as that secondary to arthropod bites or injection reactions [5]. Atopic dermatitis has also been proposed as a possible pathogenesis. Lesions are characterized by single or multiple nodules that are generally well circumscribed and firm. Predilection sites include the neck, withers, and girth. Lesions typically occur in warmer months, and males are more frequently affected [6]. Nodules are firm, ranging in size from 1 to 10 cm in diameter. They do not typically present with alopecia, ulceration, or pigmentation but some lesions may ulcerate or become cystic.

Collagen degeneration elicits an inflammatory response characterized by marked infiltration of eosinophils and monocytes, with development of epithelioid macrophages and inflammatory giant cells. As a result, cytological preparations from areas of collagen degeneration contain numerous eosinophils and variable numbers of macrophages, epithelioid macrophages, and inflammatory giant cells. Eosinophilic, amorphous debris representing necrosis may be found. Lymphocytes and plasma cells are scarce and no microorganisms are seen. Mineralized debris secondary to dystrophic mineralization may be present in chronic lesions.

History, clinical presentation, and cytological findings are highly suggestive for eosinophilic granulomas although histopathology is required for definitive diagnosis. These lesions are characterized histologically by a granulomatous reaction and flame figures comprising infiltrates of eosinophils and eosinophil granules around collagen bundles [7].

12.7.3 Multisystemic Eosinophilic Epitheliotropic Disease (MEED)

This is an uncommon, idiopathic disease in equids in which eosinophils infiltrate various tissues such as the skin, gastrointestinal tract, liver, and bile ducts, lymph nodes, pancreas and pancreatic duct, kidney, and respiratory tract. A peripheral eosinophilia may or may not be present. Most cases of MEED include skin lesions, which are pruritic and characterized by a generalized exfoliative dermatitis seen as raised papules and nodules which may be ulcerated. The inflammatory infiltrate consists of eosinophils, lymphocytes, and plasma cells [8, 9].

12.7.4 Parasite‐Induced Inflammatory Reactions

Parasite‐induced inflammatory reactions are characterized by numerous eosinophils and few to many neutrophils. Macrophages may be present in large numbers as well. Variable numbers of lymphocytes and plasma cells may be present; occasionally, the parasitic organism is found. Habronemiasis, also known as “summer sores,” results from larvae of Habronema majus, H. muscae, or Draschia megastoma. Flies frequently deposit these larvae in wounds or areas with increased moisture and predisposed areas include the eyes, penile sheath, and distal extremities [10]. Lesions secondary to habronemiasis present as chronic, exuding, nonhealing nodules and wounds. Grossly, material aspirated ranges from brown to yellow with gritty, calcified material. Microscopically, many inflammatory cells are seen, ranging from a predominance of macrophages, with multinucleated cells, to a predominance of eosinophils. Occasionally, parasitic organisms may be seen. Onchocerca spp. (especially O. cervicalis) cause a seasonal dermatitis, seen in summer months. Microfilaria in the skin cause lesions such as alopecia, swellings, nodules, and papules that may be exudative and calcified. They are associated with chronic eosinophilic dermatitis.

12.7.5 Immune‐Mediated Skin Lesions

Pemphigus foliaceus is the most common immune‐mediated skin disease in horses [11, 12]. The disease results from autoantibody production against cell adhesion proteins, particularly desmoglein 1 (DSG1), of stratified squamous epithelium, inciting acantholysis. Grossly, pemphigus lesions typically appear as alopecic crusts, erosions, and pustules. Lesions often affect the face and limbs initially prior to spread. Cytological samples from immune‐mediated lesions typically contain a large population of nondegenerative neutrophils. Whilst these lesions are sterile, ulceration with secondary infection may be present.

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Tags: Equine Hematology Cytology and Clinical Chemistry

Oct 30, 2022 | Posted by admin in EQUINE MEDICINE | Comments Off

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