Haemophilus are small gram-negative bacilli with complex growth requirements. H. Influenza is a major cause of bacterial meningitis in children. It is also associated with otitis media and exacerbations of chronic bronchitis, and pneumonia. H. Ducreyi causes chancroid and H. Aegyptius may cause endemic or epidemic purulent conjunctivitis.
Small gram-negative bacilli
Non-motile and non-spore forming
Requires X and V factors present in blood for growth
Grows slow on horse blood agar but well on chocolate agar
Polysaccharide capsule has 6 antigenic types: a to f
Type b is the most important pathogen for humans
Virulence is due to capsule formation
The disease caused by haemophilus typically begins as a nasopharyngitis, probably precipitated by a viral infection of the upper respiratory tract. It may be followed by sinusitis or otitis media and may lead to pneumonia, the latter is often complicated by empyema. Bacteraemia occurs early in severe cases and frequently result in the involvement of one or more joints or in the development of acute bacterial meningitis. Indeed in children, H. Influenzae is the commonest cause of acute bacterial meningitis. H. Influenzae type b is associated with epiglottis or obstructive laryngitis which may be fatal.
In all cases of suspected bacterial meningitis, a sample of blood as well as the CSF should be cultured. The CSF could be examined directly by gram-strain. If exposure of the organism in the CSF to specific antiserum result in a quellung reaction, the result is established. The specimen should be streaked onto chocolate agar and incubated in a candle jar.
Ampicillin was the drug of choice. However 20% of strains are resistant to ampicillin. Therefore initial therapy now consists of a cephalosporin and ampicillin/chloramphenicol. The mortality rate of treated H. Influenzae is less than 10% and most children acquire effective immunity by age 10. However a significant proportion of children (up to 30%) develop neurological sequelae and 5% end up being institutionalised. A vaccine is now available against haemophillus influenzae type b and is used in children.