Inflammatory lesions can occur due to infectious or non-infectious causes. Compared to tissue cells, inflammatory cells are
readily collected by aspirates and scrapings. Inflammation is characterized by the predominant cell type which helps to predict
the underlying etiology.
Neutrophils indicate suppurative or purulent inflammation. Non-infectious causes include immune-mediated disorders, chemical
irritation, neoplasia, and acute trauma. Infectious causes include most bacterial infections and some fungal organisms. Degenerative
neutrophils (swollen, karyolitic nuclei) are commonly associated with the toxic environment induced by bacterial infections
however degenerative changes also occur as cells become exposed on mucosal or external surfaces.
Macrophages may be seen in mixed, granulomatous, and pyogranulomatous inflammatory processes. Causes include yeast and fungal
agents, foreign body reactions, certain bacteria (Mycobacteria spp.), hemorrhage, and chronic inflammatory conditions. Epithelioid
macrophages have a wispy to epithelial appearance. These, and multinucleated cells, and are typical of granulomas making granulomatous
inflammation difficult to distinguish from histiocytic neoplasia.
Lymphocytes and plasma cells are associated with allergic or immune reactions, some viral diseases, feline cholangitis, feline
stomatitis and gingivitis, gastroenteritis, and vaccine reactions. Their presence can usually be differentiated from neoplasia
by the heterogeneity of the lymphocyte population.
Eosinophils are associated with tissue parasites, neoplasia (lymphoma, mast cell tumor, melanoma, others), fungal organisms,
protozoal organisms, eosinophilic granulomas, collagen necrosis, and hypereosinophilic syndrome.
Mast cells in low numbers are often seen in inflammatory lesions.
Infectious inflammatory conditions
Bacterial agents may be found in abscesses, contaminated surgical sites, and secondary to numerous underlying metabolic, allergic,
and neoplastic disorders. Specific agents that may be recognized include Actinomyces/Nocardia (filamentous), Mycobacterium (negative staining), and Yersinia (bipolar 'safety pin'). Scrutiny of diplococci has increased recently, with additional concerns over MRSAs. Bacterial infection
primarily induce a suppurative response, however the filamentous bacteria and Mycobacterium sp may induce a mixed to pyogranulomatous response.
Fungal diseases can be present as either cutaneous or subcutaneous lesions. The cutaneous diseases include those induced by
dermatophytes (Microsporium, Tricophyton, or Epidermophyton), Candida, and Malassezia. Dermatophytosis typically results in hairless lesions on the head, paws, ant occasionally the tail. The organism are best
detected through examination of skin scrapings taken from the periphery of the lesion as they may are often seen in association
with hair shafts. Dermatophytes induce a mixed inflammatory response. Cutaneous or mucocutaneous candidiasis is uncommon,
but when seen is detected as septate branching hyphae that induce a pyogranulomatous to granulomatous response. Malassezia can be detected by scraping, swab imprints, or impression touch prints of the lesion. As Malassezia can be found on normal dogs and cats and dermatitis is often seen as a secondary disease (e.g. to atopy, pyoderma, hypothyroidism).
Subcutaneous mycoses include sporotrichosis, histoplasmosis, cryptococcus, blastomycosis, and coccidioides (see Infectious
Although Prototheca is often mentioned with the fungi and the organisms have several morphologic features, it is actually an algae (see Infectious
Disease section for detailed description). Protothecosis typically produces a strong macrophagic to granulomatous inflammation.
Leishmania is the most commonly seen protozoal organism in skin and subcutaneous lesions (see Infectious Disease section for