Nontypeable Haemophilus influenzae Invasive Disease in the Netherlands: A Retrospective Surveillance Study 2001-2008

Background. Nontypeable (unencapsulated) strains of Haemophilus influenzae (ntHi) are usually involved in respiratory tract infections and otitis media but may also cause invasive disease. The epidemiology, the course of disease, and the outcome of ntHi invasive disease are not well established. For prevention, risk groups that might benefit from vaccination have to be defined. Methods. All patients with ntHi invasive disease confirmed by culture of samples collected by the Netherlands Reference Laboratory for Bacterial Meningitis from 41 sentinel hospitals and representative of similar to 45% of all Dutch hospitalized ntHi case patients over the period from 2001 through 2008 were included in the study. Data on clinical presentation, course of disease, and outcome as well as patient characteristics and comorbidity were retrospectively retrieved from hospital records. Results. Clinical presentations of 396 cases included mainly invasive pneumonia (190 cases [48%]) and bacteremia without a clinical focus (75 cases [19%]). Comorbidities were present in 327 [83%] and immunodeficiency in 173 [44%] of all cases. The overall case fatality rate within the first month after diagnosis was 12% and the lowest (2%) was among patients aged 5-54 years. The highest extrapolated age-specific incidence rates occurred within the first 6 weeks of life (19.0 cases per 100,000 persons), concerning mostly prematurely born infants with bacteremia within 24 h after birth, and in the first year of life (5.6 cases per 100,000 persons). The highest rate in adults was among elderly patients aged >65 years (2.2 cases per 100,000 persons). Conclusions. This study provides a detailed overview of invasive ntHi disease cases in the Netherlands. Risk groups are prematurely born infants, elderly patients aged >65 years, and immunocompromised patients.