Epidemiology and carriage of streptococcus pneumoniae and staphylococcus aureus among young children in Hong Kong

Abstract

Background Streptococcus pneumoniae and Staphylococcus aureus are important pathogens causing bacterial infections in children worldwide. At least 93 different serotypes have been identified and recognized in Streptococcus pneumoniae. In Hong Kong, the 7-valent-pneumococcal vaccine (PCV7) has been introduced into the childhood immunisation programme since September 2009, but widespread use of this vaccination may bring about epidemiological changes in the bacteria commonly carried by children.

Objectives (i) To examine the changes in carriage, serotype distribution and antibiotic resistance of nasopharyngeal pneumococcal isolates in children before and after the introduction of PCV7 in Hong Kong.

(ii) To analyse the association between S. pneumoniae and S. aureus colonisation among young children in Hong Kong following the use of pneumococcal conjugate vaccine.

Methods Nasopharyngeal swabs were collected from 1978 and 2211 children aged 2 to 6 years attending day care centres (DCCs) and kindergartens (KGs) in all 18 school districts in Hong Kong in period 1 (1999-2000) and period 2 (2009-2010), respectively. Nasal samples were also collected from the participants in period 2 for detection of S. aureus. E-test and disc diffusion method were used to determine the antibiotic susceptibility. Sequential multiplex PCR and/or the quellung reactions were used for serotyping. A standardized questionnaire was also used to obtain information from children’s parents so as to study the variables associated with the carriage of these two bacteria.

Results In the first part of our study, the PCV7 vaccine penetration (at least one dose) in period 2 was 28.1% (622/2211). The nasopharyngeal carriage rate of PCV7 isolates decreased from 12.8% in period 1 to 8.6% in period 2 (P < 0.001). Vaccine serotypes 14 and 18C had decreased significantly while non-vaccine serotypes 19A, 6A, 6C 23A and 15B had increased significantly from period 1 to period 2. Dual penicillin/erythromycin non-susceptibility rates for 19F, 14, 6A and 23A has also been found to increase over the two time periods. In the second part of our study, 27.6% of the children were found to carry S. aureus and the nasal MRSA carriage rate was 1.3%. Molecular typing including staphylococcal cassette chromosome mec (SCCmec), sequence type (ST) and clonal cluster (CC) showed that all the 28 MRSA isolates belonged to SCCmec IV (n = 13) or V (n = 15). Twelve of these isolates had community-associated MRSA genotypes (ST59/SCCmec IV, ST30/SCCmec IV, ST88/SCCmec V), ten isolates had healthcare-associated MRSA genotypes (ST45/SCCmec IV/V, CC5/SCCmec IV and ST630/SCCmec V) and six isolates had novel genotypes (ST10/SCCmec V, CC1/SCCmec IV). spa typing results also indicated an intra- and inter-school transmission of certain MRSA and methicillin-sensitive S. aureus strains. Significant association between S. pneumoniae and S. aureus carriage was not found in this study.

Conclusion Our study found that carriage, serotype distribution and antibiotic resistance of nasopharyngeal pneumococcal isolates in children have changed after the use of PCV7 in Hong Kong while association between S. pneumoniae and S. aureus colonisation among young children in Hong Kong was not found. Further studies investigating the five increasing non-PCV7 serotypes are warranted to guide future vaccine policy.