Morphologic, cytochemical, and ultrastructural features of gray eosinophils in nine cats.

Background: Gray eosinophils, resembling those in sighthound dog breeds, have not previously been reported in cats.

Objectives: We aimed to provide a morphologic, cytochemical, and ultrastructural description of gray eosinophils in cats.

Methods: Blood films examined as part of routine hematology profiles in cats from May 2015 to July 2018 were evaluated for the presence of gray eosinophils. When identified with modified Wright stain, cells were morphologically assessed and additionally stained with Diff-Quik, ALP, Luna, and Luxol fast blue stains and compared with feline controls. Two cases were prepared for transmission electron microscopy (TEM) and compared with a feline control.

Results: Gray eosinophils were identified in 9 of 2641 cats during the study period. Compared with typical feline eosinophils, these cells contained abundant round granules instead of the characteristic rod-shaped specific granules. These granules lacked the characteristic intense pink/red staining with Romanowsky stains and did not stain with ALP, Luna, or Luxol fast blue stains. On TEM, the classical electron-dense core of these granules was replaced by a core with fragmented or amorphous internal material. Typical eosinophils were not identified in any cat in which gray eosinophils were identified.

Conclusions: The distinct morphologic, cytochemical, and ultrastructural changes in gray feline eosinophils might be associated with a reduction or lack of major basic protein (MBP) in specific granule cores. Similar to canine gray eosinophils, accurate recognition of these cells is essential to prevent their misclassification as toxic neutrophils.