Haemophilus Influenzae, Invasive Disease
2015 Case Definition

Background

Haemophilus influenzae is a bacterium that has encapsulated (typable) or unencapsulated (nontypable) strains. Encapsulated strains express 1 of 6 antigenically distinct capsular polysaccharides (type a, b, c, d, e or f). Haemophilus influenzae non-type b strains can cause invasive disease clinically similar to type b (Hib) disease (pneumonia, bacteremia, meningitis, epiglottitis, septic arthritis, cellulitis, or purulent pericarditis). Nontypable strains can also cause invasive disease but more commonly cause mucosal infections such as otitis media, conjunctivitis, and sinusitis.

Hib was once the most common cause of bacterial meningitis in children aged ≤5 years in the United States. Since the introduction of Hib conjugate vaccines in the United States, the incidence of invasive Hib disease in children aged <5 years decreased by 99%, to less than 1 case per 100,000. During 2000-2012, the average annual incidence rate of invasive Hib disease in children younger than 5 years of age in the United States remained below the Healthy People 2020 goal of 0.27/100,000.

The epidemiology of invasive Haemophilus influenzae disease in the United States has shifted in the post-Hib vaccination era. Nontypable Haemophilus influenzae now causes the majority of invasive disease in all age groups, with the greatest burden of disease among the youngest and oldest age groups. From 1999 through 2008, the annual incidence of invasive nontypable Haemophilus influenzae disease was 1.73/100,000 in children younger than 5 years of age and 4.08/100,000 in adults ≥65 years of age. Rates of invasive Haemophilus influenzae and Hib remain higher among Native American and Alaska Native (AI/AN) children than non-Native children. During 1998–2009, the average annual incidence of Hib disease in children aged <5 years in the United States was 8–10 times higher (1.3/100,000) among AI/AN children than it was among white (0.16/100,000) and black (0.12/100,000) children, respectively. Invasive Haemophilus influenzae is associated with severe outcomes, especially in older adults; among ≥65 year-olds the overall case fatality ratio (CFR) is estimated to be 19.5% and increases with age, ranging from 10.2% to 27.5%.

Clinical Criteria

Invasive disease may manifest as pneumonia, bacteremia, meningitis, epiglottitis, septic arthritis, cellulitis, or purulent pericarditis; less common infections include endocarditis and osteomyelitis.

Laboratory Criteria For Diagnosis

• Detection of Haemophilus influenzae type b antigen in cerebrospinal fluid [CSF]
• Detection of Haemophilus influenzae-specific nucleic acid in a specimen obtained from a normally sterile body site (e.g., blood or CSF), using a validated polymerase chain reaction (PCR) assay; or
• Isolation of Haemophilus influenzae from a normally sterile body site (e.g., cerebrospinal fluid [CSF], blood, joint fluid, pleural fluid, pericardial fluid)

Epidemiologic Linkage

Not applicable for case classification.

Probable

• Meningitis WITH detection of Haemophilus influenzae type b antigen in cerebrospinal fluid [CSF]

Confirmed

• Isolation of Haemophilus influenzae from a normally sterile body site (e.g., cerebrospinal fluid [CSF], blood, joint fluid, pleural fluid, pericardial fluid) OR
• Detection of Haemophilus influenzae-specific nucleic acid in a specimen obtained from a normally sterile body site (e.g., cerebrospinal fluid [CSF], blood, joint fluid, pleural fluid, pericardial fluid), using a validated polymerase chain reaction (PCR) assay

Case Classification Comments

Positive antigen test results from urine or serum samples are unreliable for diagnosis of Haemophilus influenzae disease and should not be used as a basis for case classification.

Isolates of Haemophilus influenzae are important for antimicrobial susceptibility testing.

• Haemophilus Influenzae, Invasive Disease | 1997 Case Definition