High pharyngeal carriage rates of Streptococcus pyogenes in Dunedin school children with a low incidence of rheumatic fever

Abstract

Aim: To document the incidence and type distribution of Streptococcus pyogenes in a group of Dunedin children throughout the 1997 school year.

Methods: The 780 children recruited from ten primary schools had their throats swabbed on each reporting of pharyngitis. Additional pharyngeal swabbings were obtained monthly from a representative subset of these children, referred to as group 1. All swab samples were plated on CNA-P, a blood agar medium that facilitates detection of haemolytic streptococci. S. pyogenes isolates were classified according to the RFLP patterns of PCR products of their emm genes (ERP typing). Representative isolates of each ERP pattern were also emm-typed, a sequence typing method that correlates with serological M-typing.

Results: 28% of the group 1 children were found to carry S. pyogenes for more than two months. This carriage rate is similar to that previously detected in Dunedin and Waikato schoolchildren, but is higher than that generally reported in other countries. Although the predominant S. pyogenes types detected in Dunedin are similar to those in North Island populations, some of the types frequently associated with North Island cases of rheumatic fever and glomerulonephritis were absent or isolated infrequently from the Dunedin children.

Conclusion: The high pharyngeal carriage rates of S. pyogenes in Dunedin schoolchildren, without the concomitant increased occurrence of post-streptococcal sequelae observed in North Island populations may, in part, be due to a relatively lower occurrence of the M-types most commonly implicated in these diseases.