Streptococcus pneumoniae Invasive Disease, 2003

Statewide active surveillance for invasive Streptococcus pneumoniae (pneumococcal) disease began in 2002, expanded from the Twin Cities metropolitan area, where active surveillance has been ongoing since 1995. In 2003, 607 cases of invasive pneumococcal disease were reported, including 314 cases among Twin Cities metropolitan area residents, and 293 cases among residents of Greater Minnesota. Incidence rates overall, and by age group were similar between these two geographic regions. For example, there were 11.6 cases of invasive pneumococcal disease per 100,000 Twin Cities metropolitan area residents, and 12.6 cases per 100,000 residents of Greater Minnesota. By age group, annual incidence rates per 100,000 Twin Cities area residents and Greater Minnesota residents were, respectively, 26.3 and 18.9 cases among children aged 0-4 years; 2.4 and 4.0 cases among children and adults aged 4-34 years, 11.2 and 10.0 cases among adults 35-64 years, and 43.6 and 41.4 cases among adults aged 65 years and older.

In 2003, pneumonia accounted for 357 (59%) cases of invasive pneumococcal disease among all cases, (i.e., those infections accompanied by bacteremia or isolation of pneumococci from another sterile site such as pleural fluid). The 194 pneumonia cases among Twin Cities area residents accounted for a higher proportion of all invasive disease in that group (62%), than the 163 cases among residents of Greater Minnesota (56%). Bacteremia without another focus of infection accounted for 178 (29%) cases statewide, including 75 (24%) cases in Twin Cities area residents and 103 (35%) cases in Greater Minnesota residents. Pneumococcal meningitis accounted for 37 (6%) cases statewide, including 24 (8%) of cases in Twin Cities area residents and 13 (4%) cases in Greater Minnesota residents. Seventy-two patients with invasive pneumococcal disease died (12%), 41 (13%) were Twin Cities area residents and 31 (11%) were Greater Minnesota residents.

Compared with 1999, the year before the pediatric conjugate vaccine (PCV- 7, Prevnar, Wyeth-Lederle) was licensed, a major downward trend in incidence of invasive pneumococcal disease was evident in 2000-2002 among children under age 5 years from the Twin Cities metropolitan area (Figure 3). The incidence rate in this age group increased slightly in 2003 (26.3 cases per 100,000, up from 22.5 cases per 100,000 cases the year before). Based on the distribution of serotypes among isolates from these cases, this increase only occurred in disease due to non-vaccine serotypes (i.e. serotypes other than the seven included in PCV-7, and other closely related serotypes). This trend bears further monitoring because increases due to non-vaccine serotypes, also called “replacement disease” are hypothetically possible, and a small degree of replacement disease among children has been observed elsewhere in the United States.

In 2003, among those in other age groups (especially aged 65 years and older), rates of invasive pneumococcal disease were substantially lower than those in 1999. This has also been observed elsewhere in the United States, and is likely attributable to herd immunity from use of the conjugate vaccine in children. PCV-7 reduces pneumococcal colonization by the vaccine serotypes, so it is possible that the reservoir of circulating pneumococcal strains in the community might now include fewer of the PCV-7 serotypes, leading to these disease reductions. For example, of invasive isolates from cases in Twin Cities area residents aged 65 years and older in 2003, 29% (32/110) were serotypes included in PCV-7, compared to 51% (72/142) in 1999. The 23-valent pneumococcal polysaccharide vaccine (PPV-23, Pneumovax, Merck, and Pnu-Immune 23, Wyeth-Ayerst Laboratories) is recommended for adults over age 64, and for other individuals with certain chronic conditions, but increasing use of this vaccine is unlikely to have caused this change in disease rates. There are 16 serotypes included in PPV-23 that are not included in PCV-7. In 2003, 43% (47/110) of isolates from those aged 65 years and older were of one of these 16 serotypes, compared with 30% (42/142) in 1999. Therefore, only the PPV-23 serotypes also included in the PCV-7 have declined in this age group.

Out of 567 isolates submitted for 2003 cases, 47 (8%) were highly resistant to penicillin and 32 (6%) exhibited intermediate-level resistance; 70 isolates (12%) exhibited multi-drug resistance (i.e. high-level resistance to two or more drug classes). The proportion of isolates submitted from Greater Minnesota residents with higher intermediate-level resistance to penicillin (40/264, 15%) was somewhat higher than the proportion from Twin Cities area residents (39/303, 13%); however, this was not a statistically significant difference. From Twin Cities area cases in 2003, the proportion of multi-drug resistant isolates (11%) represented a continuing decline, a trend that began in 2001 (Figure 4). From Greater Minnesota cases, over two years, the proportion of invasive multi-drug resistant isolates also declined, from 16% in 2002 to 14% in 2003. The decreased incidence of invasive disease due to drug resistant strains is also very likely due to use of PCV-7. This is because the majority of drug resistant strains are isolates with serotypes included in the PCV-7. In 2003, of 79 isolates with intermediate or high-level resistance to penicillin, 67 (85%) were either included in the PCV- 7 or were closely related to PCV-7 serotypes (i.e. were immunologically similar to PCV-7 serotypes). Streptococcus pneumoniae is one of several pathogens included in the MDH Antibiogram, which gives detailed antimicrobial susceptibility results of isolates tested at the Public Health Laboratory from 2003 cases

• For up to date information see>> Streptococcus pneumoniae (Pneumococcal Disease)
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2003