Pulmonary haemophilus influenzae infection

Last revised by Liz Silverstone on 13 Jan 2024

Pulmonary haemophilus influenzae (Hi) infection refers to infection of lung with Haemophilus influenzae, a small gram-negative cocco-bacillus.

Haemophilus (from the Greek, ‘blood-loving’) requires erythrocyte factors for growth. Hi was first isolated in the 1890s and was so named because it was erroneously thought to be the cause of the 1918 - 1920 influenza pandemic. (Viruses were unknown before the 1930s.)

Non-invasive disease is due to contiguous spread and includes pneumonia without bacteremia as well as otitis media and sinusitis.

Invasive disease refers to infection of normally sterile sites and includes pneumonia with bacteremia. Bacteria can then cause sepsis and seed other sites.

These common infections can exacerbate chronic obstructive pulmonary disease and are a common cause of community- and hospital-acquired pneumonia (CAP and HAP) and ventilator-associated pneumonia (VAP). Risk factors for pneumonia include age over 65, smoking, chronic lung disease, alcohol abuse, diabetes and heart disease.

H. influenzae type b (Hib) is the most virulent strain and was the cause of around 95% of invasive disease such as meningitis, empyema, pericarditis, and septic arthritis. The Hib polysaccharide capsule is the main virulence factor. Conjugate vaccine has reduced the incidence of Hib, almost eliminating Hib meningitis. Now, non-encapsulated (NTHi) strains are the most common pathogenic strains in vaccinated populations and they commonly colonize the nasopharynx. Various infectious manifestations occur either by contiguous non-invasive spread (e.g. otitis media, sinusitis and pneumonia without bacteremia) or invasive disease involving sterile sites. Invasive disease usually begins with bacteremia which can lead to sepsis, meningitis, empyema, and so on.

Respiratory droplet transmission from carriers or patients can infect others.

The organism frequently colonizes the human upper respiratory tract, especially the nasopharynx, and is considered to form part of the normal respiratory flora. Most H. influenzae pneumonias are the result of direct extension from the nasopharynx to the lower respiratory tract.

Non-encapsulated (NTHi) types now predominate and they form biofilms, a structural barrier to antibiotics. This can lead to chronic pulmonary colonization which can exacerbate COPD.

Bronchitis/bronchopneumonia and/or lobar pneumonia patterns are seen:

Pleural effusions may be more frequent in lobar pneumonia. Lymph node enlargement is uncommon.

IV third-generation cephalosporin is the treatment of choice until antibiotic sensitivities are known.

Non-invasive CAP has a good prognosis however H. influenzae pneumonia can progress to invasive disease. Bacteremia can lead to sepsis, empyema, pericarditis, endocarditis, peritonitis, osteomyelitis and septic arthritis.

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