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Hib was a leading cause of infections among children world-wide before Hib vaccines became available. After the avail-ability of the Hib vaccine, the frequency of invasive Hib disease has reduced remarkably even in the developing coun-tries. The frequency of Hib infection has come down to only 2–3 cases/100,000 children below 5 years. So, majority of Hib infection now occurs in (a) nonimmune children due to incom-plete vaccination or poor antibody response to the vaccine and (b) in elderly people.
In the developed countries with successful implementation of Hib vaccination program, disease caused by other serotypes of encapsulated bacteria and by nontypable strains of H. influ-enzae is being increasingly recognized.
H. influenzae is a strict human pathogen. Humans are the onlynatural hosts. Nonencapsulated strains are regular commen-sals of the nasopharyngeal mucosa and of the oropharynx. They colonize at the site in virtually all children within the first few months of life. In contrast, H. influenzae serotype b is rarely found in the upper respiratory tract or, if found, is present in few children (1–5% of cases).
Human cases are important reservoir of infection. The infected nasopharyngeal mucosal secretion is the most common source of infection. Transmission is by inhalation of respiratory tract droplets or by direct contact. Transmission to neonates occurs through the maternal genital tract. Nontypable H. influenzae biotype 4 can colonize the genital tract and is a major cause of invasive disease in neonates.
H. influenzae has been subdivided into eight biotypes on thebasis of three biochemical reactions: (a) indole production, (b) urease activity, and (c) ornithine decarboxylase activity. Most of the clinical isolates belong to the biotypes I–III. Type b H. influenzaebelongs to biotype I.
H. influenzae is of four phage types. These are HP1, HP3, S2,and N3. Furthermore, Hib produces bacteriocin, which is known as “hemocin.” This bacteriocin is active against other serotypes of capsulated strains, other species of Haemophilus, and againstEscherichia coli.
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