Haemophilus Influenzae Infections

Characteristics and Classification

H. influenzae is a small, pleomorphic, facultatively anaerobic gram-negative bacillus with complex nutritional requirements, characterized by its need for both hemin (X factor) and NAD (V factor) for growth, according to the American Academy of Otolaryngology-Head and Neck Surgery 1
The bacterium can be either encapsulated (typeable, serotypes a-f) or unencapsulated (nontypeable), as stated by the American Academy of Otolaryngology-Head and Neck Surgery 1
Invasive disease caused by H. influenzae can produce several clinical syndromes, including meningitis, bacteremia, epiglottitis, or pneumonia, as reported by the Centers for Disease Control and Prevention 2

Laboratory confirmation requires isolation of H. influenzae from a normally sterile site (e.g., blood, cerebrospinal fluid, joint fluid, pleural fluid, or pericardial fluid), as recommended by the Centers for Disease Control and Prevention 2
A probable case of invasive H. influenzae can be diagnosed with detection of H. influenzae type b antigen in cerebrospinal fluid, according to the Centers for Disease Control and Prevention 2

Beta-lactamase production is the primary mechanism of resistance to ampicillin and amoxicillin, with prevalence ranging from 30-40% in the United States, as reported by the American Academy of Otolaryngology-Head and Neck Surgery 3
H. influenzae has intrinsically poor susceptibility to macrolides and azalides due to efflux pumps mediated by acrAB genes, according to the American Academy of Otolaryngology-Head and Neck Surgery 1

For susceptible (beta-lactamase-negative) H. influenzae infections, high-dose amoxicillin (80-90 mg/kg/day in 2 divided doses) is the recommended first-line treatment, as suggested by the American Academy of Pediatrics 4
Essentially all H. influenzae isolates, including beta-lactamase-producing strains, are susceptible to high-dose amoxicillin-clavulanate, as reported by the American Academy of Otolaryngology-Head and Neck Surgery 3

The Haemophilus influenzae type b (Hib) vaccine has dramatically reduced invasive H. influenzae type b disease, including meningitis and pneumonia, as stated by the European Society of Clinical Microbiology and Infectious Diseases 5

For respiratory infections where H. influenzae is suspected but not confirmed, empiric therapy should cover both H. influenzae and S. pneumoniae, as recommended by the American Thoracic Society 6 and the American Thoracic Society 7