3.14 Haemophilus Influenzae

Microbiology review for the USMLE Step 1 exam

• Haemophilus influenzae is a gram negative coccobacilli that is often encapsulated, encapsulated strains are broken up into subgroups a through f, non-encapsulated strains are referred to as non-typable
• The most common pathogenic strain is type b, which causes disease in children and immunocompromised individuals, it is the strain that we vaccinate against
• The group B H. influenzae vaccine is a conjugated polysaccharide vaccine recommended as a routine childhood vaccination in the United States, provides protection against the group B strain of the bug
• H. influenzae mainly affects the extremes of age, very young children (younger than 5) and elderly adults (>65)
• In young children, it most commonly causes epiglottitis, otitis media, and meningitis, in elderly adults it is a cause of lower respiratory tract infections, especially in patients with COPD
• H flu infections in young children can be a medical emergency and requires treatment with antibiotics and intubation in some cases
• Otitis media is the most common H flu infection in children, caused by non-group B H flu, it is still the number one cause of otitis media in children