Chapter 11: Haemophilus, Bordetella, and Legionella

Haemophilus influenzae represents a leading cause of bacterial infection in children, despite its nomenclature suggesting a viral etiology, with clinical manifestations ranging from localized infections such as otitis media and sinusitis to life-threatening invasive disease including bacterial meningitis, acute epiglottitis, and septic arthritis. The serotype b capsule demonstrates particular virulence, historically responsible for severe pediatric disease until widespread implementation of conjugate vaccine strategies dramatically altered disease epidemiology. The chapter also addresses Haemophilus ducreyi as the etiological agent of chancroid, a sexually acquired ulcerative disease, alongside brief consideration of Gardnerella vaginalis in the pathogenesis of bacterial vaginosis, characterized by distinctive cytological findings and altered vaginal microbiota. Bordetella pertussis causes whooping cough through elaborate toxin-mediated mechanisms, particularly pertussis toxin that dysregulates cellular signaling and immune responses, with clinical disease progressing through recognizable stages including the catarrhal phase of nonspecific upper respiratory symptoms, followed by the paroxysmal stage featuring characteristic severe coughing episodes, and finally the convalescent stage of gradual recovery. Acellular pertussis vaccination through the DTaP formulation has substantially reduced disease incidence in immunized populations. Legionella pneumophila causes Legionnaires disease, a severe community-acquired pneumonia with atypical features distinct from classical bacterial pneumonia, as well as the milder Pontiac fever syndrome. Transmission occurs exclusively through environmental water aerosol inhalation rather than interpersonal spread, with the organism establishing intracellular residence within alveolar macrophages, facilitating diagnosis through specific urine antigen detection methods that enable rapid identification without requiring culture.