Question
Bordetella pertussis.
Answer
Bordetella pertussis is a small, Gram-negative coccobacillus, the causative agent of whooping cough (pertussis), a highly contagious respiratory disease transmitted by droplet infection, particularly severe in unimmunized infants.
Virulence factors/pathogenesis: the organism attaches to ciliated respiratory epithelium via filamentous haemagglutinin (FHA) and pertactin, without invading deeper tissue. It produces pertussis toxin (an AB exotoxin that ADP-ribosylates a G-protein, raising intracellular cAMP, and also causes lymphocytosis-promoting and histamine-sensitizing effects) and tracheal cytotoxin (damages and destroys ciliated epithelial cells, impairing mucociliary clearance and provoking the characteristic paroxysmal cough as the airway attempts to expel accumulated mucus).
Clinical stages: catarrhal stage (mild, coryza-like, most infectious period), paroxysmal stage (bursts of severe coughing ending in an inspiratory “whoop,” often with post-tussive vomiting — highest risk of complications, particularly apnoea in young infants), and convalescent stage (gradual resolution).
Laboratory diagnosis: culture of nasopharyngeal swab/aspirate on selective Bordet-Gengou medium or charcoal-cephalexin agar (best yield in catarrhal stage, before antibiotics); PCR — now the preferred method, more sensitive and faster than culture; direct fluorescent antibody testing; serology (rise in antibody titre) in later stages when culture yield falls.
Prevention: active immunization with acellular or whole-cell pertussis vaccine as part of the DPT schedule (primary series plus boosters); a booster (Tdap) in pregnancy protects the neonate via passive transfer of maternal antibody; macrolide (erythromycin/azithromycin) chemoprophylaxis for close contacts.