Haemophilus influenzae infections

Haemophilus influenzae is an exclusively human pathogen that is a major cause of disease particularly in the context of respiratory infection. It was initially incorrectly thought to be the cause of influenza and was the first organism to have complete genome sequencing performed. Haemophilus influenzae is a gram negative bacterium that phenotypically is described as a cocco-bacillus. Its aerobic growth requires factors X (hemin) and V (adenine dinucleotide). Haemophilus influenzae may have a polysaccharide capsule and is classified into 6 major subtypes (a-f, with type b (Hib) being the most common). Strains that lack a capsule are designated as being nontypeable (NTHi). H.influenzae is spread by airborne droplets or direct contact. There is a high rate of colonization of the upper respiratory tract and most adults have NTHi strains present in the nasopharynx. This colonization is a dynamic process with constant turnover, particularly in young children. The typeable strains of H. influenzae cause systemic disease by haematogenous spread (e.g. meningitis). Nontypeable strains are mainly confined to the respiratory tract and function as mucosal pathogens. The immune response against typeable strains is predominantly dependant upon antibodies generated against the polysaccharide capsule. Hib is a major cause of systemic bacterial infection. It is an important cause of meningitis particularly in young children. Other important manifestations include epiglottidis, cellulitis and pneumonia. NontypeableHaemophilus influenzae is a leading cause of respiratory disease particularly in older children and adults. Conditions include sinusitis, tonsillitis, otitis media, pneumonia and bronchitis. It often causes chronic low-grade inflammatory disease. The most reliable way of diagnosing H. influenzae infection is by culture. Hib may be detected by assay to the capsule. Nontypeable strains may be hard to separate out from normal upper respiratory tract flora. Third generation cephalosporins (e.g. ceftriaxone) are standard therapy for the treatment of systemic Hib infections (e.g. meningitis). NTHi infection may be treated with B lactam antibiotics, macrolides or quinolones. There is a relatively high incidence of B lactam resistance. The vaccine to Hib is highly effective and increasingly is being administered to children in the first year of life. There is no clearly effective vaccine to NTHi.