Haemophilus Influenzae  (Invasive Disease, All Serotypes)

Surveillance Case Definition

CDC’s case definitions for infectious conditions under public health surveillance include a clinical description of the disease as well as laboratory criteria for diagnosis. According to the clinical definition, invasive disease due to Haemophilus influenzae (H. flu) may produce any of several clinical syndromes, including meningitis, bacteremia, epiglottitis, and pneumonia. Laboratory criteria require isolation of H. flu from a normally sterile site or detection of H. flu type b antigen in cerebrospinal fluid (CSF) for a probable case. Only H. flu meningitis was reportable in Maryland through 1987. In 1988, invasive disease due to H. flu type b (Hib) became reportable in Maryland for all health care providers. All invasive H. flu became nationally notifiable from 1991. Noninvasive infections caused by H. flu, such as bronchitis, sinusitis, and otitis, are not nationally notifiable.

Active surveillance for all invasive H. flu in Maryland commenced in November 1991 with the Bacterial Invasive Diseases Surveillance (BIDS) Project, a CDC-funded joint project of The Johns Hopkins University, School of Hygiene and Public Health and EDCP. That project now continues under the Emerging Infections Program (EIP) with the University of Maryland as an additional partner in the collaborative effort.

EIP/BIDS conducts active surveillance for data on H. flu isolates from sterile sites from all medical laboratories in the state and performs annual laboratory audits. Prior to this, EDCP obtained data on H. flu isolates on a regular basis from only the DHMH laboratory. Thus, although invasive disease due to H. flu of types other than b, non-typable H. flu, and cases in which typing was not done, are not reportable in Maryland in spite of being nationally notifiable, these cases are detected and reported through the EIP/BIDS Project. EIP/BIDS project staff believe that only a few cases with Hib antigen detected in CSF but without H. flu isolated from a normally sterile site (and thus not covered by EIP/BIDS) are being missed by their surveillance system.

Immunization

A polysaccharide vaccine with no efficacy in children under 18 months of age and uncertain efficacy in older children was used in the United States from 1985 to 1988. Two conjugate vaccines with greater efficacy in infants and children were licensed in late 1990. In January 1991, the CDC recommended the introduction of routine immunization in infancy for Hib. Three or four doses are currently recommended by age 15 months, depending on the vaccine.Maryland law currently requires age-appropriate Hib immunization for all children less than 60 months of age for entry into child care centers and preschool programs. In the 1998-99 school year 98.5% of Maryland preschool enrollees had completed the required Hib immunizations.

Photo Courtesy of the
Children's Immunization Project

Epidemiology, 1988 - 1999 (see graphs below)

In the past, H. flu was the most common cause of bacterial meningitis in the United States, accounting for an estimated 12,000 cases annually, primarily among children under five years of age. The case fatality rate is 5%, and neurologic sequelae are observed in 25% to 35% of survivors. Virtually all cases of H. flu meningitis among children are caused by strains of type b (Hib), the only serotype for which immunization is available. The incidence of H. flu in children has fallen dramatically since the late 1980’s, coincident with the licensure of the conjugate vaccines. The CDC reports that the incidence of H. flu disease among children under five years of age in the United States decreased 97% between 1987 and 1997, while the incidence of Hib disease decreased by 99% in the same group. Hib disease among children under five years of age is now targeted for elimination in the United States, as a result of widespread immunization of infants and children.

As with the national rates, there was a decline in the incidence of reported invasive H. flu disease in Maryland from 1988 to 1991; however, there was a sharp increase in 1992. That increase may have been associated with the start of active case detection in November 1991. Thus, the striking increase in rates observed for the over 20-year-old age group from 1992 onwards can also be attributed in part to the improved surveillance, while continued increase in rates that would have been observed for children under 5 years of age was quelled as a result of the introduction of Hib vaccination into the routine immunization schedule in 1991. In general, rates have remained steady from 1992 to 1999 with an average of 69 cases per year. In 1999, there were 71 cases of H. Flu in Maryland (Figures 1 & 2).

Serotype-specific data (from the EIP/BIDS Project) is available for 82% of cases with first positive cultures in 1999. Among the 58 cases of all ages with information on serotype, 17% were serotype b (Hib - Figure 3). In 1999, there were only two cases of Hib in children under five years of age. Most H. flu and Hib disease cases in 1999 were in persons aged ten years and over (median age was 55 years).

Figure 2. H Flu. Disease, Cases by Age and Year of Report. Culture Positive Cases, 1988-1999.

Figure 3. H. Flu Type b Disease by Age and Year. Culture Positive Cases, 1991-1999.  

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Maryland Department of Health & Mental Hygiene — Epidemiology & Disease Control Program