Haemophilus influenzae is a very common commensal in the upper respiratory tract, mostly as the non-capsulate variety. From different surveys, mainly in urban communities, rates of 60 to 80 per cent have been obtained for carriage of non-capsulate strains in the throat or nasopharynx of young children and rates of 3 to 4 per cent for the carriage of capsulate strains, notably type b; rather lower carrier rates (30 to 80 per cent) of non-capsulate strains were obtained in older children and adults ; capsulate strains were rarely found in these older age-groups. Much higher carrier rates of capsulate haemophili may occur in semi-isolated communities of young children without overt disease.
Capsulate Haemophilus Influenzae
Capsulate Haemophilus influenzae, particularly type b, may act as a primary pathogen in producing nasopharyngitis (sore throat with fever), laryngo-epiglottitis (croup), acute bronchitis, pneumonia, otitis media, septicaemia, meningitis, septic arthritis and pericarditis.
Age seems to be an important factor in the incidence of clinical infection with the capsulate haemophili ; acute purulent meningitis occurs mostly in the age range 2 months to 3 years and laryngo-epiglottitis (a form of croup with greatly enlarged and inflamed epiglottis) mostly in the age-range 2 to 7 years. Both types of infection have an associated bacteriaemia and blood culture should be done when either is suspected.
The route of infection of the meninges is probably from the nasopharynx via the blood. The restriction of these infections to infants and young children is probably related to the finding by Fothergill and Wright that bactericidal antibodies against a capsulate strain of Haemophilus influenzae from a case of meningitis could not be demonstrated in blood samples from children between the ages of 2 months and 3 years but were commoner in children between 3 and 10 years and constantly present in the blood of older children and adults.
When type-b Haemophilus influenzae first infects a baby after it has lost protective maternal antibody at 2 to 3 months of age, it commonly causes naso-pharyngitis with fever and in some cases may spread to cause infection elsewhere in the respiratory tract or invade the blood and cause meningitis or other metastatic infection. Subsequently these children are protected by the active production of specific type-b antibodies.
Non-capsulate haemophilus Influenzae
Non-capsulate haemophili have in the past two decades been frequently found in the sputum of cases of chronic bronchitis, particularly during acute exacerbations, and in cases of bronchiectasis.
Difficulty in the demonstration of haemophili in sputum because of their irregular distribution has been overcome by homogenization of the specimen by pretreatment with a mucolytic enzyme such as pancreatin or by shaking the specimen with sterile water and glass beads for 15 to 30 minutes.
Although Haemophilus influenzae is not regarded as a primary etiological factor in the causation of chronic bronchitis, its importance as a secondary infecting agent has been demonstrated by the studies of May and others. Three points may be emphasized:
(1) The more frequent association of Haemophilus influenzae with purulent than with mucoid sputum in cases of chronic bronchitis.
(2) The clinical improvement with antimicrobial therapy that suppresses, temporarily, Haemophilus influenzae, and the association of clinical relapse with its return.
(3) the presence of high levels of species-specific antibody to Haemophilus influenzae in the blood of patients with chronic bronchitis, particularly those with purulent sputum, and the low level of such antibody in patients with asthma and in healthy subjects.
Both Haemophilus influenzae and pneumococci are associated with acute exacerbations of chronic bronchitis, just as both are important agents in the causation of secondary broncho-pneumonia. In young children, particularly, non-capsulate haemophili are often found in pure culture in cases of paranasal sinusitis, and less commonly, in otitis media; in such cases, the haemophili are presumably secondary pathogens after a primary virus infection.