Hematologic and clinical characteristics of dogs with circulating macrophage-like cells

Macrophage-like (ML) cells are rarely observed on blood smear examinations, and the significance of these cells has been poorly described.

The objective of this study was to retrospectively describe selected hematologic and clinical characteristics of dogs with ML cells in peripheral blood.

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.

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.

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.