Atypical Presentation of Bordetella pertussis in a Six-Month-Old with Cleaved Lymphocytes and Mild Respiratory Distress.

Bordetella pertussis, the causative agent of whooping cough, typically presents in infants with paroxysmal coughing, inspiratory whoop, and post-tussive vomiting; however, atypical presentations are increasingly recognized, particularly in partially or recently immunized individuals. We present the case of a six-month-old previously healthy, fully immunized female infant who was evaluated for a three-day history of low-grade fever, feeding difficulties, increased irritability, and mild respiratory symptoms, in the absence of the characteristic pertussis cough or respiratory distress. Physical examination was largely unremarkable, with no signs of cyanosis, retractions, or abnormal lung sounds. Laboratory testing revealed leukocytosis with marked lymphocytosis, while a peripheral blood smear showed the unexpected presence of cleaved lymphocytes, a finding typically associated with viral infections or lymphoproliferative disorders rather than with pertussis. Despite the non-classical clinical and hematologic features, B. pertussis was confirmed by polymerase chain reaction (PCR) testing of a nasopharyngeal swab. The patient was treated with azithromycin and supportive care, resulting in clinical improvement and resolution of hematologic abnormalities. This case highlights the diagnostic challenges of pertussis in infants presenting without hallmark symptoms and introduces cleaved lymphocytes as a potential, though uncommon, hematologic feature of the infection.