TRENDS IN NASOPHARYNGEAL CARRIAGE OF STREPTOCOCCUS PNEUMONIAE AMONG CHILDREN ATTENDING DAYCARE CENTERS IN SOUTHEASTERN FRANCE FROM 1999 TO 2006

Abstract

Pneumococcal nasopharyngeal carriage, serotype distribution, and penicillin-susceptibility were monitored among children attending daycare centers in France from 1999 to 2006 to assess the impact of pneumococcal conjugate vaccine and antibiotic-reducing policies. Pneumococcal carriage remained stable. Immunization rates reached 68% in 2006. Serotype distribution shifted significantly from vaccine serotypes to vaccine-related and nonvaccine serotypes. Antibiotic treatments fell by 50%.

Attendance of daycare centers (DCCs) favors nasopharyngeal bacterial colonization among children, and initial surveys conducted in this setting in the Alpes Maritimes (France) in 1997 revealed carriage rates of 55% for Streptococcus pneumoniae, 56% of which had reduced susceptibility to penicillin.^1–3 This population is highly exposed to antibiotic treatment known to select for resistant strains.⁴ To monitor trends in carriage rates, similar surveys were conducted between January and March 1999, 2002, 2004, and 2006 in the same area.

The pneumococcal conjugate vaccine (PCV7) was licensed in France in 2001, and its cost endorsed by the Social Security in December 2003. Efforts to curb unnecessary antibiotic prescriptions began in the Alpes Maritimes in January 2000, and a nationwide campaign was initiated in 2002.

The present document focuses on pneumococcal carriage, serotype distribution and antibiotic resistance trends, and PCV7 immunization from 1999 to 2006.

METHODS

Study Design.

Cross-sectional surveys were conducted on a random sample of children attending DCCs during the winter months of 1999, 2002, 2004, and 2006.

For each survey, a 2-stage cluster sample was drawn among DCCs and within those among attendees. Parents provided informed consent. The study was approved by the Nice University Hospital Ethics Committee.

Immunization status for PCV7 was recorded. Pneumococcal carriage and serotype distribution according to immunization status were studied for the last 2 surveys. Nasopharyngeal aspirates were obtained using a Vygon 522.06 canula adapted on a 1 mL syringe and transferred in a transport medium (Portagerm; Biomérieux, Lyon) to the Laboratoire d'Antibiologie of Nice University Hospital.

Microbiology.

Samples were cultured on blood agar. Pneumococcal isolates were identified by their susceptibility to optochin. Susceptibility of pneumococcal isolates to oxacillin, erythromycin, clindamycine, chloramphenicol, and tetracycline and trimethoprim-sulfamethoxazole was determined by disc diffusion. Susceptibility to trimethoprim-sulfamethoxazole was tested on separate lysed blood agar plates. For strains nonsusceptible to oxacillin, penicillin, amoxicillin, and ceftriaxone MICs were determined by E-test (BMD, France) using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints. Serotypes were determined using the quellung reaction with antisera for serotypes 6A, 6B, 9V, 14, 19A, 19F, 23F, and 24 provided by the Statens Serum Institute (Copenhagen, Denmark). In 2004, strains not reacting with any of those antisera were identified by the National Pneumococcal Reference Center in Paris. In 2006, all serotype determinations were conducted locally with pneumococcal latex pool antisera and factor antisera to identify serotypes included in the PCV7 formulation.

Statistical Analysis.

Data were analyzed using Epi-Info 6c and SPSS software.

RESULTS

A random sample of 25 DCCs was drawn for each survey. During the 4 surveys, samples were obtained from 1261 children aged 3–39 months who spent at least 3 days per week attending the DCC. Characteristics of participating children are described in Table 1.

TABLE 1: Characteristics of Study Population

Pneumococcal carriage rates remained stable. Age-adjusted carriage trends did not reveal significant differences between children aged <12 months, those aged 12–24 months, and the older group, although a drop from 58.6% in 1999 to 38.3% in 2006 was noted among younger children (P = 0.06). Ten children harbored 2 distinct strains. Carriage rates for pneumococci with diminished susceptibility to penicillin (PDSP) decreased significantly after 2002 from 37.8% to 17.0% in 2006 (P < 10⁻³), and age-adjusted carriage showed this was true for all age groups, but most significantly for children younger than 12 months of age, from 50.0% in 2002 to 8.5% in 2006 (P < 10⁻³).

Serotypes.

Vaccine serotypes accounted for 124/162 (76.5%) isolates in 1999 and 36/171 (21.0%) isolates in 2006 (odds ratio [OR]: 0.08; 95% confidence interval [95% CI]: 0.05–0.14; P < 10⁻⁵). Significant changes concerned serotypes 6B (OR: 0.20; 95% CI: 0.08–0.48; P < 10⁻³), 14 (OR: 0.35; 95% CI: 0.13–0.91; P = 0.016), and 23F (OR: 0.04; 95% CI: 0.01–0.14; P < 10⁻⁵) between 1999 and 2006, but 23F strains started declining in 2002 (OR: 0.49; 95% CI: 0.28–0.83; P = 0.005). The percentage of serotype 19F increased from 1999 (11.9%) to 2004 (18.7%) then decreased significantly in 2006 (9.4%) (2006 versus 2004; OR: 0.43; 95% CI: 0.21–0.85; P = 0.008). However, PCV7-related serotypes increased from 16/162 (9.8%) in 1999 to 33/174 (18.9%) in 2002 (OR: 2.14; 95% CI: 1.08–4.26; P = 0.02), to 65/171 (38.0%) in 2006 (OR: 5.6; 95% CI: 2.96–10.70; P < 10⁻⁵). Distribution in 2006 included 23 non-F (31.7%), 6A (28.6%), and 19A (28.6%) strains. Among non–PCV7-related types, serotype 15 contributed 9.9% in 2006, versus 3% in 1999 (P = 0.002).

Antibiotic Susceptibility.

PDSP accounted for 63% of all pneumococcal isolates in 1999 and for 34% in 2006. In 2006, 70.2% of PDSP isolates displayed penicillin MICs in the intermediate range, with no significant change compared with 1999. PSDP serotype distribution shifted from 23F, 14, and 6B strains accounting for 48.5%, 16.8%, and 15.8% of isolates, respectively, in 1999, to a majority of 19A, 19F, 15, and 6A isolates concerning 20.0%, 18.3%, 13.3%, and 11.7%, respectively, in 2006.

No strain was resistant to amoxicillin or ceftriaxone during the study period, but a significant increase in amoxicillin nonsusceptibility was observed in 2006 compared with 1999, from 14% to 37% of PDSP isolates (P < 10⁻³), with a shift from a majority of serotypes 23F and 6B in 1999 to 19A, 19F, 6A, and 14 in 2006. A similar change in serotype distribution was observed for ceftriaxone nonsusceptibility, with 49.1% of PSDP isolates not fully susceptible in 2006.

Susceptibility of SP Isolates to Other Antibiotics.

Susceptibility improved overall for all tested antibiotics. Among PCV7 serotypes, resistance dropped significantly, as did multidrug resistance to 3 or more antibiotics. No significant change was observed among the remaining serotypes, except for improved susceptibility to trimethoprim-sulfamethoxazole among non-PCV7 or PCV7-related serotypes.

Carriage According to Immunization Status.

Carriage of SP was 49.3% among vaccinees versus 58.5% among nonvaccinees (OR: 0.69; 95% CI: 0.38–1.26; P = 0.19). Likewise, carriage rates for PDSP were comparable in both groups: 16.6% among vaccinees versus 18.5% among nonvaccinees (OR: 0.88; 95% CI: 0.41–1.93; P = 0.72).

Among pneumococcal carriers, 20/113 (17.7%) of those immunized carried vaccine types, versus 10/38 (26.3%) of those known not to be immunized (OR: 0.60; 95% CI: 0.23–1.58; P = 0.25). Similarly, carriage of vaccine-related serotypes was comparable between vaccinees and nonvaccinees (32.7% versus 39.5%, respectively; OR: 1.34; 95% CI: 0.58–3.06; P = 0.45).

There was no difference in pneumococcal carriage rate according to the number of vaccine doses: 52.9%, 50.0%, and 48.6% after 1, 2, and 3 doses, respectively. Likewise, no difference was observed concerning the proportion of PDSP isolates: 11.8%, 16.7%, and 16.6% after 1, 2, and 3 doses, respectively.

DISCUSSION

These consecutive surveys revealed changing patterns of pneumococcal carriage among children attending group daycare.

Although overall carriage rate remained stable, the proportion of PDSP decreased by 47% between 2002 and 2006. Unchanging carriage rates were reported among children below 7 years attending pediatric or family practice physician offices in Massachusetts between 2001 and 2004.⁵ However, Cohen et al⁶ found a significant drop in pneumococcal carriage in France between 2001 and 2005, with increasing penicillin susceptibility, but their study focused on children with acute otitis media, suggesting that in that particular context other pathogens are now more frequently involved.

Serotype distribution shifted in favor of nonvaccine strains, namely serotype 15, but also through increasing colonization with vaccine-related serogroup 23, and serotypes 6A and 19A, as described in other settings since PCV7 implementation.^7,8 Significant reduction in the percentage of 23F strains, and increasing proportions of vaccine-related serotypes, occurred as early as 2002 (ie, at a time when fewer than 5% of children had received at least 1 vaccine dose) although no change in PDSP carriage rates was noted before 2004, so that other determinants may also be involved in serotype distribution patterns.

Carriage did not differ between vaccinees and nonvaccinees, contrary to observations concerning otitis media.⁷ This may be related to the daycare setting where high immunization rates limit the spread of vaccine serotypes to nonvaccinees.⁹

The fact that PDSP carriage subsequently decreased despite stable pneumococcal carriage rates may result from clearing of resistant strains by the vaccine. The role of lower antibiotic prescription rates achieved in France and in particular in our study area, where a local intervention promoting prudent antibiotic use was established in 2000 and followed in 2002 by a nationwide campaign, is difficult to elicit. Such interventions have proved effective in France for an older age-group.¹⁰ In the present study, fewer children had received antibiotics in 2002 than in 1999, and the proportion of treated children continued to fall subsequently.

Overall antibiotic susceptibility increased for all tested antibiotics. We found no change in penicillin-susceptibility among nonvaccine types nor among PCV7-related types. One may expect replacement serotypes to in turn display reduced susceptibility if submitted to sustained selective pressure. If so, the beneficial effect of the vaccine could be lost, particularly for invasive disease.

The universal distribution of S. pneumoniae and its numerous serotypes, its potential for mutation, serotype switching, and development of antibiotic resistance mechanisms provide this organism with ample resources for survival and sustained pathogenicity. It is unlikely that selective pressure through immunization, for all its immediate benefits, would limit invasive pneumococcal disease in the long run. Maintaining cautious therapeutic strategies therefore remains essential to avoid confronting antibiotic-resistance. This argues strongly in favor of continuing public health campaigns for prudent antibiotic use.

ACKNOWLEDGMENTS

The authors thank all the daycare staff, parents, and their children for taking part in this study. We are grateful to Dr Emmanuelle Varon of the French National Reference center for pneumococci for her contribution in the identification of pneumococcal serotypes.