Peripheral smear clues for Bordetella pertussis.

A 2-month-old girl, who was current on her immunizations and had no prior illness, presented to the emergency department with a 2-week history of progressive cough and shortness of breath. A chest radiograph revealed bilateral peribronchial thickening. Complete blood count results revealed a white blood cell count of 38.6 3 10/L (71% lymphocytes, 19% segmented neutrophils, 6% monocytes, 3% bands, and 1% metamyelocytes); hemoglobin level was 10.7 g/dL, and platelet level was 425 3 10/L. Morphologic examination of the peripheral blood smear revealed numerous mature lymphocytes with scant cytoplasm, condensed chromatin, and clefted nuclei—characteristic of Bordetella pertussis lymphocytosis. Real-time polymerase chain reaction was positive for B pertussis and negative for respiratory syncytial virus and influenza A and B. The patient was treated with azithromycin and prednisolone.