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However, it should also be noted that cytology might not provide a definitive diagnosis. This article will present guidelines for evaluating cytological samples obtained from various sources for avian patients.

Cytology of the conjunctiva and cornea Cytology samples may be collected from the conjunctiva and cornea using a swab moistened with saline or carefully scraping lesions with a metal or plastic spatula. Local ophthalmic anesthetic agents should be used judiciously when collecting these samples as they are toxic to the cells and may affect the results. Individual epithelial cells or sheets of cells with brown-black pigmented cytoplasmic granules is characteristic of normal conjunctival cytology. These granules should not be confused with bacteria since normal cytology of the conjunctiva may contain a few extracellular bacteria. Inflammatory responses can indicate a bacterial, parasitic, protozoan or mycotic conjunctivitis. Chronic, non-healing corneal lesions should be evaluated for the presence of inflammatory or infectious etiologies and foreign bodies.

Normal cytology of the skin consists of anucleated cornified squamous epithelial cells, nucleated partially cornified squamous cells, a variable amount of background debris and extracellular bacteria. Mixed-cell populations may indicate an inflammatory or neoplastic condition with secondary infection.

The presence of numerous heterophils and extracellular bacteria is indicative of bacterial infections. Gram's stains are commonly used in avian practice. However, some bacterial pathogens may require special stains other than Gram stains to clearly identify them. Fungal infections are characterized by mixed-cell (macrophages, lymphocytes, plasma cells and giant cells) inflammation with fungal organisms present in the sample. Cutaneous or subcutaneous foreign body lesions often produce mixed inflammatory cell populations consisting of macrophages, multinucleated giant cells and heterophils (See suggested reading). Parasitic infections, such as Knemidocoptic mange, usually are identified by hyperkeratosis of the skin around the cere, feet, legs, eyes or vent and the presence of the mites on skin scrapings. Viral infections, such as avian poxvirus, produce lesions on unfeathered areas of skin around the eyes, cere and feet, as well as lesions in the oropharynx or cornea. Diagnosis of poxvirus infections may be accomplished by scraping the raised lesions and examining them for the presence of large cytoplasmic vacuoles (Bollinger bodies), which contain smaller, round eosinophilic inclusions (Borell) on Wright's stain.

Dermatologic conditions, such as lipomas, xanthomas, lymphoid neoplasia, carcinomas or sarcomas may also be identified by cell type and presence or absence of inflammatory cells. Lipomas often are identified easily following aspiration by the presence of fat droplets and fat cells. Cytology of xanthomas should show macrophagic inflammation with multinucleated giant cells and cholesterol clefts.

Round-cell neoplasms, such as lymphosarcoma, usually yield few individual round cells when aspirated. Carcinomas or neoplasms of epithelial cell origin often yield sheets or clumps of cells of varying size, and nuclear/cytoplasmic ratio characteristic of the type of neoplasia. Neoplasias of mesenchymal cell origin (spindle-cell tumors) also tend to yield few cells that are found as individuals in contrast to epithelial or round-cell tumor types.

Cytology of the digestive tract Differential diagnoses for plaques, nodules and ulcers in the oropharynx include candidiasis, trichomoniasis, bacterial abscesses, squamous metaplasia due to hypovitaminosis A, papilloma, neoplasia, poxvirus, herpesvirus or physical trauma. These lesions may be assessed by scrapings, swabs, aspiration or imprints of excised tissue. Cytology samples of the esophagus and ingluvies may be obtained using saline-moistened swabs, flushes or washes.