Nasopharyngeal carriage of Streptococcus pneumoniae and pneumococcal urine antigen test in healthy elderly subjects

Abstract

Background: Children frequently carry Streptococcus pneumoniae (pneumococcus) in their nasopharynx, even when healthy. Lower carriage rates have been reported in adults and only sparse data are available for the elderly. We sampled healthy elderly subjects for nasopharyngeal carriage to assess the prevalence of pneumococcal carriage using various assays. Methods: A deep nasopharyngeal swab sample was taken from 590 healthy elderly subjects aged ≥ 65 y. The samples were stored in STGG (skim milk–tryptone–glucose–glycerol) medium and cultured directly and after incubation in enrichment broth using routine identification methods. Real-time polymerase chain reaction (PCR) assays specific for pneumolysin and pneumococcal surface antigen A genes was performed on the same samples. Urine was also collected and assayed using the commercial Binax Streptococcus pneumoniae NOW urine antigen test. Results: The prevalence of pneumococcal carriage in healthy elderly persons was 1.5% for encapsulated pneumococci and 5.3% for all presumptive pneumococci. The use of the enrichment broth did not increase the yield of positives. PCR assays gave higher numbers of positives, but pneumolysin PCR in particular gave probable false-positive results. Only 1 urine antigen test was positive, and this was in a person not carrying pneumococcus. Conclusions: Nasopharyngeal carriage of pneumococci in the elderly was rare. Identification of presumptive pneumococci in culture requires further confirmation, e.g. by serotyping. The urine antigen test was not affected by concurrent carriage. Low carriage prevalence suggests that encapsulated pneumococci detected in a respiratory tract sample during sickness may be the true cause of disease, since contamination from asymptomatic nasopharyngeal carriage seems unlikely.

Keywords::

• Streptococcus pneumoniae
• carriage
• colonization
• elderly

Acknowledgements

Outi Hall, MD is acknowledged for obtaining the samples, Mervi Mannila and Päivi Siren for the urine sample assays, and Eeva-Liisa Korhonen for the bacteriological assays.

Declaration of interest: This study was conducted as part of a clinical vaccine trial supported by GlaxoSmithKline Biologicals. Arto A. Palmu has had travel paid for and has received honoraria from GlaxoSmithKline to attend expert group meetings. He is the head of the Clinical Research Unit at the National Institute for Health and Welfare, which has received research funding from GlaxoSmithKline and Solvay Pharmaceuticals. Tarja Kaijalainen has no conflicts of interest. Annika Saukkoriipi has no conflicts of interest. Maija Leinonen has no conflicts of interest. Terhi M. Kilpi has led research projects for which her employer has received funding from GlaxoSmithKline and has had travel paid for by Sanofi Pasteur MSD to attend expert group meetings.