Focal outbreak of acute rheumatic fever in Italy by emm18 group A streptococci: comparison of virulence and clonal characteristics with other emm18 strains isolated in a thirty years period.

Introduction: Although cases of acute rheu- matic fever (ARF) are particularly frequent in developing countries, focal outbreaks of ARF are still present in developed countries. In March 2013, an outbreak of ARF affected seven elementary school-aged children. The same emm type (emm18) Group A streptococ- cus (GAS) strains were isolated from the throat swabs of the index case, classroom mates and contacts. GAS emm18 is considered a rheumatogenic strain; to investigate if the strains causing the ARF outbreak could have acquired particular traits, their virulence and genetic relatedness were compared to other emm18 strains respon- sible for different clinical presentations, isolat- ed in a period of about thirty years. Materials and Methods: Both emm18 GAS strains isolated from the ARF screening, in- cluding the case index and contacts (9 strains) as well as emm18 strains from a contempo- rary multicentre study on pediatric pharyngo- tonsillitis (8 strains) and emm18 strains from surveillance programs (16 strains) dating back the ‘80s were analysed for virulence traits by 69 PCR and clonal relatedness by PFGE (Pulsed Field Gel Electrophoresis), MLST (MultiLo- cus Sequence Typing) and MLVA (MultiLocus Variable number tandem repeats analysis) on selected strains. Results: The collection of emm18 GAS strains displayed ten different emm subtypes. All strains from the ARF episode were emm18.29. The emm18 strains isolated from the swab throats of children from other classrooms and community had a different subtype (emm18.32). Despite the variability of the emm locus, the emm18 strains were highly clonal: PFGE grouped all the strains but one in a single type. All emm18 strains isolated both from the out- break and community were ST42. A seven-loci MLVA typing method distinguished two groups by a single allele difference. All emm18.29 GAS strains were grouped in the same MLVA while the emm18.32 strains belonged to the other MLVA type. Four other emm18.29 strains were identified in the GAS collection, isolated from invasive in- fections in the past years. Noteworthy, one of them was isolated just four months before the ARF outbreak in a nearby geographic region. All emm18.32 GAS strains were, on the con- trary, isolated only by pediatric pharyngoton- sillitis or ARF contacts in 2013. Virulence profile (presence of superantigens speA and speC genes) were identical for all 33 emm18 strains. Discussion and Conclusion: Restricted ge- netic variation was observed among emm18 strains spanning a period of about thirty years. Indeed, two different emm18 clonal lineages (emm18.29 and emm18.32) were circulating in community at the time of the ARF occurrence. The emm18 strains responsible for the ARF outbreak were genetically related to strains re- sponsible to invasive infections dating back ten years