Background: Macrophage-like (ML) cells are rarely observed on blood smear examinations, and the significance of these cells has been poorly described.
Objective: The objective of this study was to retrospectively describe selected hematologic and clinical characteristics of dogs with ML cells in peripheral blood.
Materials and methods: Complete blood count (CBC) reports with blood smear evaluations from the clinical pathology laboratory records at North Carolina University College of Veterinary Medicine were retrospectively reviewed. Data were collected over a 10-year-period. Dogs were defined as having circulating ML cells if three or more ML cells were present on a single blood smear. Hematologic and clinical data of dogs with circulating ML cells were compared with age-matched hospital-derived control dogs.
Results: Of 61,631 CBC records, 87 reports (0.14%) described the presence of ML cells. Thirty-nine dogs met the inclusion criteria. The hemogram of dogs with circulating ML cells was characterized by a pronounced inflammatory and stress leukogram, anemia, and thrombocytopenia. Of the 39 dogs, 19 (49%) had systemic or severe localized inflammatory/necrotic diseases. Eighteen (46%) dogs were diagnosed with neoplasia of histiocytic (5) and non-histiocytic origins (13). Dogs with circulating ML cells had a shorter median survival time (34 days) than the control dogs (595 days, P < .0001), with an increased occurrence of death/euthanasia within 1 month (3.89-fold). However, the presence of circulating ML cells was not found to be an independent prognostic factor in a multivariable model.
Conclusions: The hemograms and diagnoses of dogs with ML cells suggest that severe inflammatory conditions or histiocytic and non-histiocytic neoplasia are common causes for circulating ML cells.
Keywords: canine; hepatic lymphoma; histiocytes; histiocytic sarcoma; monocyte; pancytopenia.
© 2021 American Society for Veterinary Clinical Pathology.