Superficial or deep pyoderma and bacterial abscesses

bacterial infection of the skin may present as surface or superficial disease or deep pyoderma. Infection may be caused by primary skin disease but may also occur secondary to bite or puncture wounds. In addition, cystic lesions and tumors may become secondarily infected, particularly if ulcerated. When stained with Romanowsky-type stain, most bacteria will stain blue to purple, similar to chromatin. Bacterial organisms tend to be uniform in size and shape, which is helpful in distinguishing them from particulate debris, which tends to be irregular. Further characterization of bacterial organisms may be done with a Gram stain, but definitive identification requires culture. Both aerobic and anaerobic culture should be considered for subcutaneous infections.

Surface or superficial pyoderma presents with skin surface abnormalities, including pustules, papules, crusts, and erythema. Bacteria may be identified with impression smears or scrapings, but these techniques potentially could miss other underlying disease. Deeper pyoderma results from infection extending deeper into follicular tissue, resulting in nodules and abscesses. The most common bacterial pathogen for superficial and deep pyoderma is Staphylococcus intermedius (Figure 5-5, A); however, other secondary invaders may also exist, and in chronic cases, bacterial culture and sensitivity may be advisable.³ Bacterial infection may also occur as a consequence of a bite or other penetrating wound. These often result in a mixed infection with both aerobic and anaerobic organisms (see Figure 5-5, B).

Mycobacterium Spp

Mycobacterial infection manifests in several different forms, including slow-growing tuberculous; lepromatous, cutaneous nodular; slow-growing, nontuberculous, cutaneous; and fast-growing, cutaneous and subcutaneous. All of these forms may result in cutaneous lesions. Disseminated infection may also occur particularly with tuberculous disease. Cutaneous lesions include draining nodules, ulceration, and nodules.³ Definitive identification of organisms requires specialized culture for both rapid growers and slow growers. In some cases, polymerase chain reaction (PCR) testing of tissue samples may be helpful.

In cytologic samples, Mycobacterium spp. on rare occasions may be seen as filamentous organisms but are more commonly found as short rods. Mycobacterial organisms do not stain with Romanowsky stains because of their thick lipid walls. They still may be readily identifiable on Romanowsky-stained slides as nonstaining short rods, often noted in macrophages (Figure 5-6). These organisms will stain positive with an acid-fast stain.

Actinomyces Spp. and Nocardia Spp

These organisms are often associated with chronic draining or nodular lesions. Infection is typically secondary to a bite wound or penetrating foreign body. These filamentous bacteria are often branching and may be found in thick mats on smear preparations. Actinomyces and Nocardia spp. sometimes stain poorly with Romanowsky stains and may only be noted as a faint outline, often best visualized in the cytoplasm of epithelioid macrophages (Figure 5-7, A). When stained, they are light blue in color with an intermittent pink beading appearance. Nocardia spp. also may be partially acid-fast positive, which may help distinguish them from Actinomyces spp. (see Figure 5-7, B). Culture is required to definitively identify these organisms.

Malassezia pachydermatitis

Overgrowth of these organisms typically occurs on the skin surface and usually is not associated with a significant exudative process. These yeast are small (3-8 micrometers [µm] diameter) and typically shaped like a “footprint” (Figure 5-8). They are considered normal flora of skin, so just finding a few does not necessarily indicate abnormality. Greater than two organisms per 100× field has been suggested as indicative of overgrowth.³ It is not unusual to find large numbers of organisms as well as large numbers of anucleate squames but minimal to no evidence for inflammation. Overgrowth of these organisms is usually thought to be secondary to other diseases such as skin allergies, seborrhea, bacterial pyoderma, and skin conformation problems or secondary to antibiotic use.

Dermatophytes

Dermatophyte infections may manifest in varying ways. In dogs, infection typically manifests as expanding annular areas of alopecia, scales, and crusts. Infection may be localized but may also occasionally be seen with extensive skin involvement. On occasion, infection may result in a solid, raised, nodular lesion, referred to as a kerion. In cats, infection usually causes one or more areas of annular to irregular alopecia with or without scales. Skin impressions or scrapings from the edge of annular lesions typically will demonstrate cellular debris and mixed inflammation, including neutrophils, macrophages, lymphocytes, and eosinophils. Accumulations of hair follicle structures and, particularly around these areas, small fungal spores may be found, sometimes in dense aggregates (Figure 5-9). On rare occasions, septate hyphae may be found. Similar findings may be noted with aspiration of kerion lesions. The causative agents for dermatophytosis are most commonly Microsporum canis, M. gyseum, and Trichophyton mentagrophytes. Fungal culture is required to definitively identify the organism.

Histoplasma capsulatum

Infection with this organism typically causes systemic disease, and animals may present with a variety of clinical signs, including fever, weight loss, anorexia, coughing, and gastrointestinal signs. Infection may also cause cutaneous lesions that are characterized by papules, nodules, ulcers, and draining tracts. Impression smears from oozing tracts or fine-needle aspirate of nodular lesions reveal mixed inflammation with variable numbers of small (2-4 µm diameter) round yeast, with a thin nonstaining capsule and a basophilic nucleus that often is crescent shaped (Figure 5-10). These organisms may be noted free in the background and may be phagocytized by macrophages as well as neutrophils.

Sporothrix schenckii

Infection with this agent may cause cutaneous, cutaneolymphatic, or disseminated lesions. In dogs, cutaneous lesions often occur on the head or trunk and typically present as nodules. These may ulcerate with purulent discharge and crusts. In cats, draining tracts are typically noted on distal limbs, head, or tail base. Samples from these sites typically reveal neutrophilic to pyogranulomatous inflammation, with variable numbers of yeast organisms noted in the macrophages, in the neutrophils, and in the background. In particular, cats often will have large numbers of organisms in discharge material, and for this reason, cats with this infection need to be handled with caution to prevent zoonotic spread. The yeast organisms are pleomorphic in shape, varying from round to oval to cigar shape, 2 to 10 µm in length, and 1 to 3 µm wide (Figure 5-11). Although similar in size to Histoplasma, Sporothrix may be distinguished by its variety of shapes.

Blastomyces dermatitidis

Clinical signs associated with blastomycosis include anorexia, weight loss, cough, ocular disease, lameness, and skin lesions. Skin lesions may be found in up to 40% of cases.³ These lesions include firm papules, nodules, plaques, ulcers, draining tracts, and abscesses. Material for cytologic evaluation may be collected from draining tracts or aspirated from nodules or abscesses. These samples reveal neutrophilic to pyogranulomatous inflammation with variable numbers of organisms. These are dimorphic fungus, which are typically found in the yeast phase in infected animals, although on rare occasion may form hyphae in tissues.⁴ The yeast organisms are variably sized (5-20 µm in diameter), with a thick, blue wall and a thin, nonstaining capsule (Figure 5-12). Occasionally, broad-based budding will be seen, which is useful in distinguishing from Cryptococcus. Blastomyces are larger than Histoplasma and Sporothrix, but typically smaller than Coccidiodes spherules.

Cryptococcus Spp

Cryptococcosis is most commonly associated with nasal signs (sneezing, snuffling, nasal discharge) in cats and central nervous system or ocular signs in dogs. Both species may also have skin lesions, which, however, are more common in cats. The skin lesions may occur as solitary lesions or as part of disseminated disease. Lesions include papules, nodules, ulcers, abscesses, and draining tracts. Sampling from these lesions often reveals large numbers of organisms. These are yeast organisms that are round in shape (occasionally fusiform) and variably sized (2-20 µm in diameter). The organisms stain pale pink to purple and often have a granular appearance. They typically have a large nonstaining capsule, which may be up to 40 µm in diameter (Figure 5-13). Sometimes, only smaller yeast organisms with thinner capsules are found, and thus, the lack of a thick capsule should not be used to rule out Cryptococcus spp. infection. On occasion, narrow-based budding, which is a useful diagnostic feature, may be seen. Minimal associated inflammation may be present, in which case, it is typically granulomatous. The etiologic agent is typically Cryptococcus neoformans; however, Cryptococcus gattii has emerged as another pathogen occurring in the tropical and subtropical regions of Australia, South America, Southeast Asia, and Africa and also in the North American Pacific Northwest.⁵ Morphologic features do not distinguish these two species.

Coccidioides immitis

Infection with this organism usually causes lung disease but may disseminate systemically and cause skin involvement. Skin lesions may occur as subcutaneous nodules or draining tracts. Cytologic preparations from these lesions typically contain large numbers of inflammatory cells, including neutrophils and macrophages. The organisms are often scarce and not finding an organism does not rule out infection. When present, the organisms are usually found as spherules, which are variable in size but may be large (10-100 µm in diameter) (Figure 5-14). Because of the large size and low numbers of spherules in a sample, these are often easiest to find at low-power (4 to 10×) scans and may be missed if microscopic examination is only done at high power. The spherules have a thick, blue wall with a finely granular internal appearance (endospores). It is often necessary to focus up and down to see the internal detail because of the thickness of the spherules relative to the accompanying cells. On rare occasions, small (2-5 µm) endospores may be seen free in the background or phagocytized by cells. The smaller-sized spherules may be mistaken for Blastomyces, but Coccidiodes spherules do not show budding.

Opportunistic Fungi

Many different fungal organisms may infect animals and cause either localized or disseminated skin disease as well as systemic disease. The terminology applied to these infections is complex and beyond the scope of what is possible to diagnose via cytology alone. Fungal skin infections typically present with chronic draining lesions or nodules. Animals with these infections frequently are immunocompromised or have a history of immunosuppressive therapy. Cytologic samples from these lesions will typically reveal pyogranulomatous to granulomatous inflammation. Some cases will also have increased numbers of eosinophils. When organisms are present, usually they occur as hyphal segments (Figure 5-15), although on occasion conidial structures may be found. Categories of opportunistic fungal infection include phaeohyphomycosis (pigmented septate hyphae), hyalohyphomycosis (non-pigmented septate hyphae), mycetoma (localized nodular infections) and zygomycosis (broad, poorly septate hyphae). The structure of the hyphae (septate versus nonseptate; pigmented versus nonpigmented; hyphal width) may be helpful in suggesting a particular species of fungus, but as a rule, fungal culture is necessary to definitively identify the organism.

Pythium insidiosum

Pythium spp. are not true fungi but are water molds that infect dogs more commonly than cats. Infection results in gastrointestinal signs but may also cause cutaneous lesions. Infection usually is thought to occur from standing in or drinking infected water. Cutaneous lesions often involve the extremities and typically are ulcerated nodules with draining tracts. Cytologic samples from these lesions reveal pyogranulomatous inflammation, usually with an eosinophilic component. When organisms are present, they are found as poorly staining, broad, poorly septate hyphae ranging from 2 to 7 µm in diameter. The hyphae do not stain well with Romanowsky stains but are easily visualized with GMS stain (Figure 5-16). A related organism that also may cause skin infections, Lagenidium sp., has hyphae with broader (diameter ranging from 25 to 40 µm) and more irregular or bulbous hyphae than Pythium. Definitive identification of these organisms may require a combination of fungal culture, serology testing, and biopsy with immunohistochemical testing, other molecular diagnostics, or both.³