Streptococcus pneumoniae Invasive Disease, 2001

In 2001, 340 cases of invasive Streptococcus pneumoniae infection were reported among residents of the seven-county Twin Cities metropolitan area, representing a 22% decrease from the 438 cases reported in 2000. Rates of invasive disease varied considerably by age, and rates of illness among children less than 2 years of age decreased most dramatically during 2001. Figure 7 compares mean annual rates by age group from the years 1996-1999 with those from 2000 and 2001. This decrease in rates among infants and young children likely is attributable to use of the new pediatric polysaccharide-protein conjugate vaccine (PCV-7), Prevnar®, licensed in 2001. This vaccine covers seven of the serotypes that most frequently cause invasive pneumococcal infections in children.

Pneumonia accounted for 173 (51%) of the invasive pneumococcal infections reported in 2001, including only those accompanied by bacteremia or isolation of pneumococci from another sterile site, such as pleural fluid. In 2001, 17 (5%) cases of pneumococcal meningitis occurred among Twin Cities residents. Bacteremia without another known focus of infection accounted for 125 (37%) cases. Although the total number of cases fell in 2001, the proportions of common infections among total cases was similar to prior years. Thirty (9%) case-patients died, representing a slightly but not significantly higher proportion of deaths among cases than in 2000.

Of 72 case isolates from children less than 5 years of age, 39 (54%) were serotypes included in the PCV-7 vaccine. Of 102 isolates from adults 65 years of age or older, 82 (80%) were serotypes included in the 23-valent polysaccharide pneumococcal vaccine.

Among 304 invasive S. pneumoniae case isolates with drug susceptibility results, 69 (23%) were non-susceptible to penicillin, which is slightly but not significantly less than the comparable percentage in 2000 (27%; 109/410). The percentage of isolates from young children that were non-susceptible to penicillin also decreased from 2000 to 2001, falling from 38% (39/103) to 30% (12/40) among children less than 2 years of age and from 35% (46/133) to 28% (20/71) among children less than 5 years of age, respectively. However, neither change was statistically significant. This reduction or leveling-off of penicillin non-susceptibility contrasts with prior years. In 1996, 14% (60/425) of all case isolates were penicillin-non-susceptible, and this percentage rose each year through 2000.

Similarly, the percentage of case isolates resistant to erythromycin fell from 25% (104/410) in 2000 to 19% (58/304) in 2001. Prior to 2001, resistance to this macrolide antibiotic had increased quickly; the percentage of erythromycin-resistant isolates increased each year, from 9% (38/425) in 1996 through 2000. The decrease in erythromycin-resistant isolates in 2001 occurred principally among young children. The percentage of erythromycin-resistant isolates among children less than 2 years of age decreased from 42% (43/103) in 2000 to 18% (7/40) in 2001 (p<0.01); among children less than 5 years of age, the comparable percentages decreased from 37% (49/133) to 21% (15/71) during the same period (p= 0.02). Among isolates from persons 5 years of age or older, the reduction in erythromycin resistance was not statistically significant (20% [55/277] in 2000 vs. 18% [43/233] in 2001).

Prior to 2001, multidrug-resistant invasive pneumococcal isolates (i.e., those that are resistant to two to six drug classes) have been increasingly common. In 1996, 12% (51/425) of isolates were multidrug-resistant; this proportion increased each year through 2000 to 24% (97/410) but decreased to 19% (59/304) in 2001. The percentage of multidrug-resistant isolates among children less than 2 years of age decreased from 36% (37/103) in 2000 to 23% (9/40) in 2001; among children less than 5 years of age, the comparable percentage decreased from 33% (44/133) to 24% (17/71) during the same period. Since 1996, multidrug-resistant strains have been isolated more frequently from young children with invasive infections than from older individuals (Figure 8). The predominance of resistance among isolates from young children also is seen for most antibiotics tracked in our surveillance system and has been reported elsewhere. Also, the actual incidence of drug-resistant invasive infections among children less than 2 years of age decreased drastically in 2001.

The seven pneumococcal serotypes in the PCV-7 vaccine account for a large proportion of pediatric strains and represent the majority of antibiotic-resistant infections. Therefore, the decreased occurrence of these resistant strains among invasive case isolates may be attributed to increasing use of this new pediatric vaccine.

Additional information on antimicrobial susceptibility results for pneumococcal isolates submitted to MDH from 1996 to 2001 is available on the MDH web site.

• For up to date information see>> Streptococcus pneumoniae (Pneumococcal Disease)
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2001