Cytology, the study of cells, involves the collection and examination of cells and fluid from organs, tissues, and body cavities. Cells naturally shed from the surfaces of mucous membranes, organs, and skin lesions, and they can be collected for examination. Veterinarians can use washes to collect fluid and cells from difficult-to-reach tissues such as the respiratory tract, the guttural pouch, and the reproductive tract. Laboratory examination helps characterize the cell types and evaluate the fluid present in an effort to determine health or disease status.

Cytology exams provide a good source of information in a relatively short time. Many in-house veterinary laboratories can perform microscopic examination of a sample, and they can follow up with additional exams if warranted. Microbiological culture and antimicrobial sensitivity tests, histopathology (gross examination of biopsies), or special staining can be performed to obtain a more thorough diagnosis or prognosis.

Taking the Samples

The method of sample collection is determined by the site involved. Sampling from the skin or an exposed lesion might involve scraping or an impression smear of the lesion (obtained by pressing a slide against the lesion to pick up cells). Samples from mucous membranes lining the digestive, respiratory, or reproductive tracts can be collected using a sterile swab for a direct collection, or using a wash technique.

A wash, such as a tracheal wash of the respiratory tract, can be collected using a catheter (a tubular, flexible surgical instrument for withdrawing fluids from the body) or fiberoptic endoscope to instill a small quantity of sterile saline into the desired area and re-aspirate the saline that has had contact with the affected tissue, thereby picking up cells or microorganisms with the saline.

Needle aspiration of cells and/or fluid from a cavity, joint, or lesion allows collection of samples from a variety of sites with a standard needle and syringe. Technicians in the laboratory then preserve and prepare the samples for microscopic examination.

Fluid samples are examined to determine if they are transudates (typically clear and watery with low cellularity and protein arising from osmotic imbalances) or exudates (thick and cloudy with high cellularity and protein arising from inflammation or infection). Differentiating a fluid sample as a transudate versus an exudate helps determine the cause of the fluid accumulation, allows for initial treatment, and indicates the need for additional procedures.

An individual examines cells microscopically to determine the type and numbers involved in a disease process. Cell types might include red and white blood cells (the latter indicate inflammation), normal or abnormal tissues cells, or neoplastic (new and abnormal, indicating a benign or cancerous growth) cells. Fluid samples are commonly examined for bacteria and other microorganisms, such as yeast or fungi.

Fine needle aspirate is a common technique used to collect a small sample from a mass in the skin or other superficial organ. The individual at the microscope evaluates the type and amount of cells. The characteristics of any cell present--such as blood cells, tissue cells, microorganisms, and atypical or neoplastic cells--are evaluated. This is a useful technique to quickly differentiate an infectious or inflammatory process versus a cancerous process.

When a solid mass is evaluated, a needle aspirate for cytology examination has limitations--you're not able to examine the cells in question relative to the ones surrounding them, and there can be distortion due to the process of obtaining the aspirate. Suction is applied to the syringe during aspiration, allowing for removal of a small sample of cells from their normal architectural matrix. Because of the rapid results, it is fairly common for a needle aspirate to be performed as a screening procedure prior to recommending a biopsy.

Information from a biopsy leads to a more accurate diagnosis of a mass, allowing the clinician to recommend the optimum treatment plan and provide a prognosis. A biopsy is the surgical collection of a solid piece of tissue by the removal of a mass, or a core or wedge sample, leaving the bulk of the mass in place. The tissue sample is preserved and sent to a pathologist for preparation (on slides), staining, and evaluation. The advantage of the pathologist receiving a solid tissue sample is the ability to examine the normal or abnormal cells in their actual relationship to the cells around them. There is no distortion, and the internal cell structures can be analyzed. Various stains can be applied to examine cell health and function and the presence of microorganisms. Tumor cells can be classified, graded (a determination of malignancy or aggressiveness), and linked to prognosis. Surgical margins can be determined by noting the end of abnormal cells and the start of normal cells on the edges of a sample.

Take-Home Message

Laboratory cytology tests provide valuable information about your horse's health. Lumps, bumps, swellings, or drainage can be evaluated with minimal invasiveness or down time for your equine athlete. Be sure to discuss these testing options with your veterinarian.

About the Author

Kimberly Peterson, DVM

Kimberly Peterson, DVM, is an AAEP member and assistant professor in the Department of Veterinary Technology at Morehead State University in Morehead, Ky. Her husband, Eric, is an equine practitioner, and their family lives in Lexington.

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