Streptococcus pneumoniae bacteraemia in Belgium: differential characteristics in children and the elderly population and implications for vaccine use

J. Flamaing1,*, J. Verhaegen2 and W. E. Peetermans3

1 Department of Geriatric Medicine, 2 Department of Microbiology, 3 Department of General Internal Medicine, University Hospital Leuven, Leuven, Belgium

Received 22 January 2001; returned 24 September 2001; revised 28 March 2002; accepted 17 April 2002


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The characteristics of bacteraemia with Streptococcus pneumoniae in children (0–4 years) and the elderly (>=60 years) were compared over a 7 year period (1994–2000). Of a total of 7927 isolates of invasive S. pneumoniae studied in the national reference laboratory, 74% (n = 5837) were blood isolates. Of these 5837 S. pneumoniae bacteraemias, 843 (14%) occurred in children and 3144 (54%) in the elderly. The prevalence of penicillin resistance (MIC >= 0.1 mg/L) in bacteraemic isolates rose from 8.2% to 18.9% (P = 0.03) in children and from 5.1% to 16.35% (P = 0.001) in the elderly over the study period. The prevalence of erythromycin resistance (MIC >= 1 mg/L) in bacteraemic isolates was significantly higher in children than in the elderly (44.7% versus 25.7%, P = 0.001) and rose significantly over the 7 year period in the elderly (18.6–33.65%, P = 0.001). There were more serogroups and serotypes (SGTs) among the bacteraemic isolates obtained from the elderly compared with children (36 versus 26, P = 0.03). SGTs 6, 14, 18 and 19 cause significantly more bacteraemia in children than in the elderly. The opposite is true for SGTs 3, 7, 8, 9, 11, 12, 15, 20, 22 and 35. The new 7, 9 and 11 valent conjugate vaccine formulations cover significantly more bacteraemic SGTs in children than in the elderly (82%, 89.5% and 92% versus 55.5%, 65% and 77.5%, respectively; P = 0.001). The 23 valent polysaccharide vaccine provides a theoretical coverage of 95% in the elderly population. Our data indicate consideration of a vaccination strategy in the elderly population that combines the efficacy of conjugate vaccines with the broad coverage of the 23 valent polysaccharide vaccine.


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Streptococcus pneumoniae is the leading cause of bacteraemia, meningitis, pneumonia and upper respiratory tract infection worldwide. Invasive pneumococcal disease affects mostly children, the elderly and immunocompromised individuals. The annual incidence of pneumococcal bacteraemia is estimated at 15–30 cases/100 000 population for all persons, at 45–90/100 000 in the elderly (>=65 years of age) and at >150/100 000 in children under 2 years of age.14

Resistance of S. pneumoniae to the major classes of antibiotic (penicillins, cephalosporins and macrolides) used to treat invasive disease is rising. The introduction of resistant clones as well as de novo resistance, often due to horizontal transfer of DNA between species, results in resistance.5,6 Because of the high incidence of pneumococcal disease and the problem of rising resistance, there is a need for adequate prevention of invasive disease and of transmission in risk groups. The 23 valent (23-V) polysaccharide vaccine has been shown to prevent invasive disease in the elderly and in patients with chronic underlying conditions, such as heart failure, chronic obstructive pulmonary disease, diabetes mellitus and splenectomy.7,8 Conjugate vaccines are being developed to tackle the problem of pneumococcal disease (and carriage) in infants <=2 years of age, but may be beneficial in other age groups as well.913

The 7 valent (7-V) formulation of the pneumococcal vaccine includes conjugates derived from polysaccharides or oligosaccharides from types 4, 6B, 9V, 14, 18C, 19F and 23F. The 9 valent (9-V) formulation comprises additional serotypes 1 and 5. The 11 valent (11-V) formulation also includes serotypes 3 and 7F. The 23-V polysaccharide vaccine has all serogroups and serotypes (SGTs) of the 11-V conjugate vaccine plus 2, 8, 9N, 10A, 11A, 12F, 15B, 17F, 19A, 20, 22F and 33F. Cross-reactivity of serotypes within the same serogroup was assumed in the analysis of the theoretical coverage of the vaccines.

In this article we describe the differential characteristics of pneumococcal bacteraemia in children (0–4 years of age) and the elderly (>=60 years of age) over a 7 year period (1994–2000). Penicillin and erythromycin resistance, as well as SGT distribution, are compared, and implications for vaccine formulations for both age groups are discussed.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Invasive isolates of S. pneumoniae

All invasive isolates of S. pneumoniae (from blood, CSF, pleural fluid, middle ear and various aspirates of normally sterile sites) are sent to the national reference laboratory at the University Hospital Leuven by >100 Belgian laboratories, covering >50% of the Belgian population. Isolates were mailed to the reference laboratory on blood agar together with a case report form containing information on the age and sex of the patient, isolation date, site of the original sample and outcome (cure or death). Identification of S. pneumoniae was confirmed by appearance, {alpha}-haemolysis and optochin susceptibility on blood agar.

Typing of S. pneumoniae isolates

The isolates are typed by phase-contrast microscopy using Neufeld’s reaction with 46 serotype or serogroup sera obtained from the Statens Seruminstitut (Copenhagen). According to the Danish typing system, types yielding serological cross-reactions have been classified in 20 groups, each containing two to four serotypes. The 46 test sera comprise 20 group sera and 26 single serotype sera.

Susceptibility testing

Susceptibility to penicillin and erythromycin was tested by the standardized disc diffusion test on Mueller–Hinton agar containing 5% horse blood agar according to the NCCLS recommendations.14 An inoculum density equivalent to 0.5 McFarland standard was prepared in trypticase soy broth. Plates were inoculated with a sterile cotton-tipped swab and incubated overnight at 35°C in a 5% CO2 incubator.

Oxacillin (1 µg) discs were used to screen for strains with diminished susceptibility to penicillin. For all isolates with inhibition zones <=19 mm the MICs of penicillin were determined on Mueller–Hinton blood agar plates with Etest (AB Biodisk, Solna, Sweden). The NCCLS interpretative criteria were used for the three categories of susceptibility to penicillin (<=0.06 mg/L for fully susceptible strains, 0.12–1.0 mg/L for intermediately resistant strains and >=2 mg/L for resistant strains). A MIC of >1 mg/L for penicillin with Etest correlates with a MIC of >=2 mg/L for penicillin with broth dilution.

Statistical analysis

Bacteraemic isolates of S. pneumoniae obtained from children (0–4 years of age) and the elderly population (>=60 years of age) for the period 1994–2000 were analysed. Comparisons were made between groups by {chi}2 or Fisher’s exact test where appropriate. A P value of <0.05 was considered significant.


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Bacteraemic isolates of S. pneumoniae

A total of 7927 invasive isolates of S. pneumoniae were sent to the national reference laboratory between 1994 and 2000, of which 5837 (74%) were blood isolates and 2090 (26%) came from other normally sterile sites (CSF, middle ear fluid, pleural fluid and various other normally sterile sites). Bacteraemic isolates accounted for 39% of all invasive S. pneumoniae isolates (n = 2137) in children (0–4 years of age) and for 92% of all invasive S. pneumoniae isolates (n = 3399) in the elderly (>=60 years of age) (P = 0.001) (Figure 1).



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Figure 1. Invasive S. pneumoniae isolates (Belgium, 1994–2000). Bacteraemic versus other invasive isolates: white bars, bacteraemic isolates; grey bars, other invasive isolates.

 
The number of bacteraemic S. pneumoniae isolates in children rose constantly from 85 in 1994 to 143 in 2000, whereas the number of bacteraemic S. pneumoniae isolates in the elderly rose from 296 in 1994 to 523 in 1996, declined thereafter (459 in 1998) and rose again (514 in 2000). The male/female ratio was significantly higher in children than in the elderly (1.5 versus 1.2, P = 0.04). The proportion of men with bacteraemia decreased with increasing age in the elderly age group. Sixty-two per cent of the bacteraemic S. pneumoniae isolates were recovered from October to March.

SGTs causing bacteraemia in children and in the elderly

There were more SGTs among bacteraemia isolates from the elderly than from children (36 versus 26, P = 0.03). The 10 most frequent SGTs accounted for 91.5% of S. pneumoniae bacteraemia in children and for 77.2% of S. pneumoniae bacteraemia in the elderly. No SGT was bactaeremic only in children, whereas SGTs 13, 17, 20, 29, 30, 32, 34, 35 and 37 (accounting for 2.1% of S. pneumoniae bacteraemias in the elderly) were bacteraemic only in the elderly (Figure 2). S. pneumoniae bacteraemia in children was caused more frequently by SGTs 6, 14, 18 and 19 than in the elderly population, whereas SGTs 3, 7, 8, 9, 11, 12, 15, 20, 22 and 35 were more common among isolates from the latter (Figure 2).



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Figure 2. Cumulative percentage of serogroups and serotypes (SGTs) causing bacteraemia in children and in the elderly in Belgium (1994–2000). Ovals, bacteraemic SGTs in children; diamonds, bacteraemic SGTs in the elderly. Darkened ovals, SGTs included in the 7-V conjugate vaccine formulation; double-lined ovals, additional SGTs in the 9-V conjugate vaccine formulation; black ovals, additional SGTs in the 11-V conjugate vaccine formulation; darkened diamonds, SGTs included in the 23-V polysaccharide vaccine formulation. White stars, SGT significantly more bacteraemic in children; black stars, SGT significantly more bacteraemic in the elderly.

 
Bacteraemia with vaccine SGTs

When more serotypes were included in the vaccine formulations, a higher percentage of bacteraemic SGTs was covered. The theoretical coverage of the 7-V, 9-V, 11-V and 23-V vaccine formulations is 82%, 89.5%, 92% and 97% in children and 55.5%, 65%, 77.5% and 95% in the elderly, respectively. The difference of vaccine coverage for bacteraemic S. pneumoniae between vaccine formulations is significant (P < 0.04). For all vaccine formulations the coverage of S. pneumoniae bacteraemia is significantly higher in children than in the elderly (P < 0.003). After the introduction of the 23-V polysaccharide vaccine in the elderly in 1996, we observed a significant decrease in the prevalence of bacteraemia caused by the following SGTs that are included in the vaccine: SGT 1 (8.8% in 1996 and 3.7% in 2000, P = 0.01) and SGT 5 (7.6% in 1996 and 0.4% in 2000, P = 0.001). A significant increase occurred in the prevalence of bacteraemia caused by SGT 19, also included in the vaccine (7.1% in 1996 and 11.7% in 2000, P = 0.03). There were no significant changes in the prevalence of bacteraemic SGTs not included in the vaccine.

Penicillin and erythromycin resistance in S. pneumoniae bacteraemia

Overall resistance to penicillin (both intermediate and full resistance) rose from 7.6% in 1994 to 17.65% in 2000. For S. pneumoniae bacteraemia in children and in the elderly, the penicillin resistance rose from 8.2% to 18.9% (P = 0.03) and from 5.1% to 16.7% (P = 0.001), respectively. There were no significant differences in penicillin resistance between the two age groups. Before 1996, penicillin MICs were between 0.12 and 1.0 mg/L. In children the proportion of highly penicillin-resistant isolates (MIC > 1.0 mg/L by Etest) among penicillin-resistant bacteraemic isolates rose from 0% in 1996 to 48% in 1999 (P = 0.003) and decreased to 14% in 2000 (P = 0.01). In the elderly, the proportion of highly penicillin-resistant isolates among bacteraemic isolates rose from 4.4% in 1996 to 43% in 2000 (P = 0.001) (Figure 3). In children and in the elderly penicillin resistance was noted in seven and 13 SGTs, respectively.



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Figure 3. Penicillin and erythromycin resistance in S. pneumoniae bacteraemia in children and the elderly. White squares, penicillin resistance in children; black squares, penicillin resistance in the elderly; white circles, erythromycin resistance in children; black circles, erythromycin resistance in the elderly. Penicillin intermediate resistance, MIC 0.12–1 mg/L (dashed lines); penicillin high resistance, MIC > 1 mg/L by Etest (full lines). Erythromycin resistance, MIC >= 1 mg/L. Triangles, combined penicillin and erythromycin resistance in children; diamonds, combined penicillin and erythromycin resistance in the elderly.

 
The overall resistance to erythromycin rose from 22.8% in 1994 to 36.4% in 2000. For S. pneumoniae bacteraemia in children and in the elderly, the erythromycin resistance increased from 40% to 47.6% (not significant) and from 18.6% to 33.65% (P = 0.001), respectively. There was a significantly higher percentage of erythromycin resistance in children with S. pneumoniae bacteraemia than in the elderly (1994–2000: 44.7% versus 25.7%, P = 0.001) (Figure 3). In children and in the elderly erythromycin resistance was found in 14 and 22 SGTs, respectively.

Combined resistance to penicillin and erythromycin in bacteraemic S. pneumoniae isolates rose from 0.95% in 1994 to 5.8% in 2000 in children (P = 0.005) and from 0.9% to 9.6% in the elderly (P = 0.001) (Figure 3). Table 1 shows SGTs exhibiting penicillin and erythromycin resistance. SGTs 6 and 14 showed significantly more penicillin resistance in the elderly than in children. In children the erythromycin resistance was significantly higher in S. pneumoniae bacteraemia caused by SGTs 4, 6, 9, 23 and 24. In the elderly the erythromycin resistance was significantly higher in S. pneumoniae bacteraemia caused by SGT 19. The combined penicillin and erythromycin resistance per SGT was not different between the two groups.


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Table 1. SGTs exhibiting penicillin and erythromycin resistance
 
Bacteraemia caused by penicillin- and erythromycin-resistant vaccine SGTs

The 7-V, 9-V and 11-V conjugate vaccine formulations cover 97.3% of bacteraemic SGTs exhibiting penicillin resistance in children, whereas the 7-V, 9-V and 11-V conjugate and the 23-V polysaccharide vaccine formulations cover 76.3%, 81.7%, 81.7% and 97.1% of bacteraemic SGTs exhibiting penicillin resistance in the elderly. The 7-V, 9-V and 11-V conjugate vaccine formulations cover 87.3%, 92.2% and 95.5% of bacteraemic SGTs exhibiting erythromycin resistance in children, whereas the 7-V, 9-V and 11-V conjugate and the 23-V polysaccharide vaccine formulations cover 59.3%, 69.4%, 82.7% and 97.1% of bacteraemic SGTs exhibiting erythromycin resistance in the elderly.


    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The yearly incidence of pneumococcal bacteraemia is highest in the age group >=65 years in European countries.4 The mortality of pneumococcal bacteraemia in this age group is higher than in children (>=65 years of age: 20%, and >=85 years of age: 40% versus 2% in children).3 In our data set, 54% of bacteraemic isolates came from the elderly (>=60 years of age) and 14% from children (<=4 years of age). A similar distribution was reported in other European countries.1522 In children there were significantly more non-bacteraemic isolates than in the elderly, mainly due to otitis media isolates and the fact that blood cultures are less frequently taken in children.22 The seasonal variation, with a high incidence of S. pneumoniae bacteraemia in the winter months has also been noticed by others.23 Viral infections and crowding favour transmission and subsequent infection with S. pneumoniae in winter months.

Penicillin resistance in blood and other invasive isolates rose from 7.6% in 1994 to 17.6% in 2000. Introduction of resistant clones as well as de novo resistance, often due to horizontal transfer of DNA between species, results in penicillin resistance.5,6 Penicillin resistance is mainly of intermediate level (only 11% of resistant strains have a MIC > 1 mg/L by Etest).24 A significant rise in the proportion of highly penicillin-resistant bacteraemic isolates has occurred in children and the elderly since 1996. Between 1994 and 2000, 42% of bacteraemias in the age groups investigated was caused by SGTs with >10% of penicillin resistance. The highest rate of infections with penicillin-resistant S. pneumoniae occurred in children. Risk factors for infections with penicillin-resistant S. pneumoniae are recent, prolonged and prophylactic antibiotic use, young age, recent hospitalization, day care centre attendance and non-invasive disease.2527 In 1998, 28.2% of middle ear isolates were penicillin resistant, compared with only 9.9% of blood isolates.24 A pronounced rise in penicillin resistance in bacteraemic isolates was also observed in the elderly population. This trend has also been noticed by others.28 Old age, antibiotic use, hospitalization and nursing home residency are risk factors for infections with penicillin-resistant S. pneumoniae in the elderly.26,29 SGTs 3, 6, 9, 19 and 23 are associated with a higher mortality and occur for at least 54% in the elderly population.24

Erythromycin resistance in blood and other invasive isolates rose from 22.9% in 1994 to 36% in 2000. In 1998, 54.3% of middle ear isolates showed erythromycin resistance but only 25.3% of blood isolates.24 Erythromycin resistance in bacteraemic S. pneumoniae is twice as high in children as in the elderly. SGTs 4, 6, 9, 23 and 24 (causing 39% of bacteraemias in children) show significantly more erythromycin resistance in children. In Belgium, monotherapy with macrolides for pneumococcal infections must be discouraged because of the high prevalence of erythromycin resistance. More than 90% of erythromycin-resistant pneumococci in Belgium carry the ermAM gene, leading to target modification and cross-resistance for all macrolides.30 Combined penicillin and erythromycin resistance is also rising significantly in both age groups in SGTs that frequently cause bacteraemia (SGT 6, 9, 14 and 23).

More SGTs cause bacteraemia in the elderly compared with children (36 versus 26). The 10 most frequent SGTs cause 91.5% of bacteraemia in children and 77% of bacteraemia in the elderly and this finding is comparable to those in other European countries.31,32 SGTs 6, 14, 18 and 19 are more frequent in children. In the elderly SGTs 3, 7, 8, 9, 11, 12, 15, 20, 22 and 35 cause bacteraemia more often than in children.

Retrospective studies (case–control and indirect cohort studies) showed an effectiveness of the 23-V polysaccharide vaccine of 50–80% in preventing bacteraemic disease with vaccine-related serotypes. Epidemiological surveys indicate that the 23-V vaccine formulation provides a theoretical coverage of at least 90% for isolates causing invasive disease in adults. Since 95% of the bacteraemic serotypes in the elderly in Belgium are included in the 23-V polysaccharide vaccine, the vaccine confers a similar protection in our country.8,3335 The 23-V polysaccharide vaccine was introduced in Belgium at the end of 1995, when it was recommended for use in risk groups and all persons >=60 years of age by the Belgian High Council of Public Health and a consensus conference of scientific societies.8 This resulted in a sharp increase in vaccine uptake, with up to 30% of the target population being vaccinated.36,37 The reversal of the yearly increase of the number of blood isolates obtained from the elderly and the decrease in the bacteraemic prevalence of SGT 1 and 5 (both included in the vaccine) in the elderly that we have observed since 1997 may be reflecting the vaccine response.

Pneumococcal conjugate vaccines have been developed for use in children and are very efficacious (90% efficacy) in preventing invasive pneumococcal disease.1013,38,39 The SGTs in the 7-V, 9-V, 11-V protein–polysaccharide conjugate vaccine formulations are responsible for 82%, 89% and 92%, respectively, of all bacteraemias in children in Belgium. The expected clinical efficacy in preventing bacteraemia with vaccine-related SGTs in Belgium can be expected to be at least comparable to the efficacy reported by the study of the Kaiser Permanente Vaccine Study Center Group.13 There is also a high coverage (>87%) of bacteraemic SGTs exhibiting penicillin and erythromycin resistance for all vaccine formulations in children. Unlike the 23-V polysaccharide vaccine, the 7-V and 9-V conjugate vaccines reduce the acquisition and carriage of vaccine serotypes and include pneumococci exhibiting penicillin resistance.40,41 Replacement with exogenously acquired or unmasked non-vaccine serotypes occurs,41 although the impact of reduced vaccine serotype and enhanced non-vaccine serotype carriage on invasive pneumococcal disease is currently unknown. The reduction in vaccine serotype carriage, however, may reduce transmission of vaccine serotypes and the burden of the antimicrobial resistance of pneumococcal disease.

The high efficacy of the conjugate vaccine in young children makes its use in adult risk groups and in the elderly population appealing. In the elderly, the clinical efficacy of the 23-V polysaccharide vaccine seems to decrease with advancing age and increasing interval between vaccinations.34 The antibody response is not equal for all SGTs, resulting in less protection for some crucial SGTs (6B, 18C, 19F, 23F).35,4244 Although revaccination is safe, higher pre-existing antibody levels and shorter intervals can create more side effects.45,46 Inducing immunological memory by a conjugate vaccine regimen (including the SGTs that induce inadequate responses in a 23-V polysaccharide formulation) and boosting with a 23-V polysaccharide vaccine, thereby broadening the protection by the 23-V vaccine to include less frequent SGTs, seems a logical approach. The best regimen for adult risk groups and the elderly remains to be established. Because of the high incidence and the resistance problem of invasive pneumococcal infections in the elderly, these data are urgently needed.


    Acknowledgements
 
This work was supported in part by the R. van Furth Chair in Infectious Diseases and the G. Rolinson Chair in Respiratory Inflammation at the Katholieke Universiteit Leuven, obtained by W.E.P.


    Footnotes
 
* Corresponding author. Tel: +32-16-33-79-40; Fax: +32-16-33-79-41; E-mail: johan.flamaing{at}uz.kuleuven.ac.be Back


    References
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 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
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