Haemophilus influenzae is an exclusively human pathogen whose ecologic niche is the human respiratory tract. The species H. influenzae includes strains with six antigenically distinct polysaccharide capsules designated a through f. Serotype b strains cause serious invasive disease in infants, including meningitis and bacteremia. Polysaccharide–protein conjugate vaccines have virtually eliminated disease caused by type b strains in countries where the vaccine is widely used. However, invasive disease caused by H. influenzae type b is still a significant problem worldwide in countries where the vaccine is not used.
Strains of H. influenzae that lack a polysaccharide capsule are called nontypeable because they are nonreactive with the typing sera directed at each of the six capsular polysaccharides. Nontypeable strains of H. influenzae demonstrate enormous genetic diversity and are an important cause of human respiratory tract disease.
Because type b and nontypeable strains of H. influenzae differ from one another in epidemiology, clinicalmanifestations, and treatment, they are considered separately in each section of this chapter.
Epidemiology and respiratory tract colonization
H. INFLUENZAE TYPE B
Prior to the widespread use of the H. influenzae conjugate vaccines, approximately 3% to 5% of infants were colonized in the nasopharynx by type b strains, with higher rates observed in day-care centers. The conjugate vaccines have resulted in a marked decrease in the colonization rate, contributing to the dramatic decrease in invasive type b infections in the United States.
NONTYPEABLE H. INFLUENZAE
Nontypeable strains of H. influenzae frequently colonize the nasopharynx of healthy children, with higher rates in day-care centers. Nasopharyngeal colonization begins in infancy and essentially every child is colonized at some time. Frequent transmission of strains occurs among children in day-care centers and colonization with nontypeable H. influenzae in the first several months of life is associated with recurrent otitis media. Different antibiotics have different effects on the dynamics of colonization.