Background
Although the type of infectious diseases caused by Haemophilus influenzae has changed considerably in recent years because of the widespread and routine immunization of children against type b organisms, H influenzae remains a significant pathogen. The recent vaccine shortage in the United States from December 2007 to September 2009 is a reminder that despite the decline in the incidence of invasive disease, Hib immunization remains critical in the control of the disease.
First isolated in 1892 by Robert Pfeiffer from the sputum of patients with pandemic influenza infection, H influenzae was thought to be the infectious agent responsible for flu. In 1920, the organism was named Haemophilus influenzae (from the Greek haemophilus, meaning “blood-loving”) to reflect the fastidious growth requirement of the organism, as well as its apparent association with influenza. In 1933, the discovery of the viral etiology of influenza eventually refuted this erroneous association. Nevertheless, subsequent findings revealed that H influenzae was responsible for a wide spectrum of clinical diseases.
In the 1930s, Margaret Pittman defined 2 major categories of H influenzae: the unencapsulated strains and the encapsulated strains. The unencapsulated strains were chiefly responsible for infections at mucosal surfaces, including otitis media, conjunctivitis, bronchitis, and sinusitis. In contrast, one of the 6 antigenically distinct encapsulated strains, strain type b, was associated with invasive diseases (eg, septicemia, meningitis, cellulitis, septic arthritis, epiglottitis, pneumonia).
Prior to the availability of an effective vaccine, H influenzae type b (Hib) was the most common cause of pediatric bacterial meningitis in the United States.