Haemophilus influenzae type b
- Haemophilus influenzae is a cause of severe bacterial infection, particularly in infants.
- Vaccination is only recommended for children under 5 – older children and adults usually do not need a Hib vaccine.
National Immunization Survey Data:
Percent Children 19-35 Months Who Have Received > 3 HiB
(2020 Goal: 90.0%)
For Your Patients
From the CDC’s Pink Book.
Haemophilus influenzae is a gram-negative coccobacillus. It is generally aerobic but can grow as a facultative anaerobe. In vitro growth requires accessory growth factors, including “X” factor (hemin) and “V” factor (nicotinamide adenine dinucleotide [NAD]). Chocolate agar media are used for isolation. H. influenzae will generally not grow on blood agar, which lacks NAD.
H. influenzae has encapsulated (typeable) and unencapsulated nontypeable strains. The outermost structure of encapsulated H. influenzae is composed of polyribosyl-ribitol-phosphate (PRP), a polysaccharide that is responsible for virulence and immunity. Six antigenically and biochemically distinct capsular polysaccharide serotypes have been described; these are designated types a through f. There are currently no vaccines to prevent disease caused by non-b encapsulated or nontypeable strains. In the prevaccine era, type b organisms accounted for 95% of all strains that caused invasive disease.
Incidence is strikingly age-dependent. In the prevaccine era, up to 60% of invasive disease occurred before age 12 months, with a peak occurrence among children 6-11 months of age. Passive protection of some infants is provided by transplacentally acquired maternal IgG antibodies and breastfeeding during the first 6 months of life. Children 60 months of age and older account for less than 10% of invasive disease. The presumed reason for this age distribution is the acquisition of immunity to Hib with increasing age.
Invasive disease caused by H. influenzae type b can affect many organ systems. The most common types of invasive disease are meningitis, epiglottitis, pneumonia, arthritis, and cellulitis. Meningitis is infection of the membranes covering the brain and spinal cord and is the most common clinical manifestation of invasive Hib disease, accounting for 50%-65% of cases in the prevaccine era. Hallmarks of Hib meningitis are fever, decreased mental status, and stiff neck (these symptoms also occur with meningitis caused by other bacteria). Hearing impairment or other neurologic sequelae occur in 15%-30% of survivors. The case-fatality rate is 3%-6%, despite appropriate antimicrobial therapy.