What is your diagnosis? Rectal scraping from a dog with diarrhea.

A 3-year-old, spayed female, mixed-breed dog was evaluated for chronic diarrhea, vomiting, and weight loss. A marked inflammatory leukogram, mild regenerative anemia, and marked hypoalbuminemia were noted. Cytologic evaluation of a rectal scraping revealed numerous round to ovoid protozoal cysts, 5-25 microm in diameter, with small to moderate amounts of pale blue cytoplasm and round eosinophilic nuclei. A distinct, variably sized, round to oval vacuole was often seen within the cytoplasm and frequently displaced the nucleus. The cysts were morphologically similar to Blastocystis sp., an amoeba-like protozoal parasite found in both diseased and asymptomatic humans and animals. Histologic findings in endoscopic biopsies from the stomach, duodenum, ileum, and colon were unremarkable and protozoal organisms were not observed. The dog was diagnosed with exocrine pancreatic insufficiency based on markedly decreased serum levels of trypsin-like immunoreactivity. Alteration of gastrointestinal flora secondary to the underlying pancreatic disease likely allowed overgrowth of the protozoa, which were considered an incidental finding. Their identification was important in avoiding an incorrect diagnosis and unwarranted treatment.