Haemophilus influenzae is a Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. It is generally aerobic but can grow as a facultative anaerobe. H. influenzae was mistakenly considered to be the cause of influenza until 1933 when the flu virology became apparent.
It was the first free-living organism to have its entire genome sequenced. The project was completed and published in 1995. Two major categories were defined: the unencapsulated strains and the encapsulated strains. There are six generally recognized types of encapsulated H. influezae: a, b, c, d, e, and f. Unencapsulated strains have a greater genetic diversity than the encapsulated group.
The pathogenesis of H. influenzae infections is not completely understood. Epiglottitis is known to be a major factor in virulence. Unencapsulated strains are almost always less invasive although they can produce an inflammatory response in humans. Hib conjugate vaccine is effective in preventing Hib infections, although, there are several other vaccines are available. Most strains are opportunistic pathogens meaning they live in a host but only cause disease when other factors create an opportunity.
Naturally-acquired disease caused by H. influenzae seems to occur in humans only. In infants and young children it can cause bacteremia, pneumonia, and acute bacterial meningitis. Occasionally it causes cellulites, osteomyelitis, epiglottitis, and infection arthritis.
Through Hib conjugate vaccine the incidence of invasive Hib disease has decreased to 1.3/100,000 in children. It does remain as a major cause of lower respiratory tract infections in infants. Unencapsulated H. influenze causes ear infections, eye infections, and sinusitis in children.
Diagnosis is typically performed by bacterial culture or latex particle agglutinations. Diagnosis is confirmed when the organism is isolated from a sterile body site. Bacterial culture is performed on agar plates preferably chocolate agar. Colonies appear as convex, smooth, pale, grey or transparent colonies. Use of antibiotics prior to sample collection greatly reduces the isolation rate by killing the bacteria before identification is possible.
Latex particle agglutination test is a more sensitive method to detect H. influenzae. Effective vaccines have been available since the early 1990s. Unfortunately Hib vaccines cost about seven times the total cost of vaccines against measles, polio, tuberculosis, diphtheria, tetanus, and pertussis.