Haemophilus influenzae

Haemophilus influenzae is a Gram negative coccobacillus that can cause serious invasive disease in children and adults. The organism can be differentiated into six capsulated serotypes (Hia-Hif) and non-encapsulated (ncHi) strains. Prior to routine immunisation, H. influenzae serotype b (Hib) accounted for > 80% of invasive H. influenzae disease and was a leading cause of bacterial meningitis, mainly in children aged  H. influenzae infections and occurs mainly in adults.

Non-encapsulated (ncHi; also known as nontypeable H. influenzae, NTHi) usually causes non-invasive respiratory tract infections, such as otitis media and sinusitis. Occasionally, however, ncHi can cause invasive disease, mainly in individuals with underlying medical conditions such as malignancy, immunosuppression and chronic diseases. In addition, the incidence of invasive ncHi in the first month of life (neonatal disease) is currently ten times higher than Hib following the success of vaccination. Understanding the epidemiology of ncHi is important because of a recently licensed 10-valent pneumococcal vaccine conjugated to the immunogenic outer membrane Protein D of H. influenzae which may potentially prevent ncHi disease in children and high-risk adults. Other H. influenzae serotypes (Hia, Hic, Hid, Hie and Hif) rarely cause invasive disease and, like ncHi, usually affect those with co-morbidities.