Under a microscope, samples collected from the rectum can yield answers to gastrointestinal signs in patients
In evaluating patients with signs of gastrointestinal distress, rectal cytopathology often complements other diagnostic methods for screening causes of diarrhea and may also determine a diagnosis. In a session of the American College of Veterinary Internal Medicine (ACVIM) Forum 2023 in Philadelphia, Pennsylvania, attendees were provided with an overview of rectal cytopathology including sample collecting and processing, as well as potential findings and their meanings.1
Francisco O. Conrado, DVM, MSc, DACVP (Clinical), a clinical pathologist and an assistant professor at the Cummings School of Veterinary Medicine at Tufts University in North Grafton, Massachusetts, presented the session in a virtual format. “This might seem like a funny topic but it’s a very inexpensive, easy test that can help you diagnose your patients with gastrointestinal diseases,” he said. “I feel like it’s a topic that’s not talked about enough, and I would like clinicians to perform [it] more [often] because there are definitely some cool diagnostic findings that can be done by using this test.”
Although there are various fecal diagnostic tests, which Conrado briefly mentioned in his talk, the presentation focused on dry-mount fecal cytology. He discussed how to perform this type of test as well as findings that are considered normal or abnormal.1
“For fecal or rectal cytology, there are several methods described in literature,” he said. These methods of collection include a swab of direct fecal material, rectal saline lavage, and rectal scraping. There are differences in sensitivity for detecting abnormalities with each method of collection, [because] various parts of the rectum are sampled and they have unique cellular components, according to Conrado.1
Rectal scraping involves direct, gentle scraping of the rectal mucosa and is often used to identify deep infection. Common tools for sample collection are a cotton-tipped applicator moistened with saline, a blunt spatula, and a gloved fingertip. Conrado also noted that although lubricants should not be used when collecting samples, saline solution can used to provide lubrication.1 “If you’re using a lubricant, your sample is not going to be useful for cytology,” he said. “You should also be using samples that are fresh.”
Samples should ideally be less than 5 minutes old, Conrado noted. He recommended making a slide as quickly as possible following sample collection, and also staining the slide as quickly as possible.1 “You want to make sure you prepare a thin film and have a monolayer of material,” he said.
“You’re going to be using your microscope for this. To be able to look at bacteria, you want to use your oil immersion objectives. You will want to protect your objectives with a glass coverslip so that you don’t get fecal material on them,” he said.
Fecal material may detach from the slide during staining, and using a dipping technique for staining slides may contaminate stain solutions with bacteria, Conrado noted. He suggested changing the stain solution after staining cytology slides when a dipping technique is used to prevent contamination of subsequent samples, which may include blood smears or other materials.1
Evaluating a sample
Rectal and fecal samples are assessed for background flora, the presence of abnormal eukaryotic cells, and pathogenic microorganisms, according to Conrado. “As we say in clinical pathology, we don’t really care how you [evaluate a sample], so long as you have a systematic approach,” he said. “You want to start at a low magnification, and you want to end at high magnification. Making sure you don’t miss steps in between.”
In a normal fecal smear, there should be none or rare nucleated cells seen, but digesta and “a mixed population of bacteria should be present,” (Figure 1) according to Anne Barger, DVM, MS, DACVP, a professor and head of the Department of Veterinary Clinical Medicine at the University of Illinois College of Veterinary Medicine in Urbana, in a paper published by the Illinois State Veterinary Medical Association. Moreover, a rectal scrape sample sometimes shows clusters of rectal epitheliums as well as the mixed population of bacteria.2
Cocci is rare in a normal sample, but other microbial flora can be observed, according to Conrado. Cyniclomyces guttulatus, a yeast commonly found in rodents and lagomorphs, including rabbits and guinea pigs, may also be seen in canine fecal samples, including coprophagia. Other findings seen in a normal rectal or fecal sample include accellular elements such as digesta and plant matter, and cellular elements, which may include squamous or rectal columnar epithelial cells.1
With abnormal rectal and fecal samples, an overgrowth of flora or microorganisms may be observed (Figure 2). Other abnormal findings may include high numbers of fecal neutrophils, which suggest conditions such as bacterial enteritis, distal colitis or proctitis, whipworm infestation, inflammatory bowel disease, and distal intestinal tissue necrosis. Findings can also include fecal eosinophils that suggest eosinophilic enteritis or colitis as well as an inflammatory response to certain infections, nematode parasites, and foreign body reactions or neoplasms, according to Conrado.1
Inflammatory bowel disease may be detected with samples of fecal matter.1 “You can see occasional lymphocytes in healthy dogs, but if you see them in excess and if you see lots of plasma cells or other evidence of antigenic stimulation, that is typical of an abnormal finding,” said Conrado.
Macrophages in fecal matter is also an indication of a gastrointestinal problem. “They typically indicate chronic inflammation,” said Conrado. Fecal samples for patients with chronic inflammation may show intracellular microorganisms such as those that are fungal, oomycetal, or algal. Macrophages may also indicate granulomatous colitis, an inflammatory condition that is predominantly found in French bulldogs and young adult boxers, according to Conrado.1
Common rectal and anal tumors such as perianal gland tumors and apocrine anal sac adenocarcinoma are neoplasms with distinct features that make them “fairly easy to diagnose with cytology,” wrote Barger. With perianal gland tumors, the cells appear similar to hepatocytes, whereas apocrine anal sac cells are fragile and often ruptured with bare nuclei. Other tumors such as adenocarcinoma, lymphoma, and plasmacytoma “exfoliate well and can be [often] diagnosed with cytology,” Barger noted.2
There are benefits to using rectal cytopathology as a tool for diagnosing gastrointestinal disease. Rectal scraping and fecal cytology are inexpensive and easy to perform, according to Conrado. These testing methods will often aid other diagnostic techniques in determining a patient’s condition but may also lead to a definitive diagnosis on their own.1