a clinical case is defined as illness in a person who has clinical features of scarlet fever ( fever and fine , sandpaper rash of characteristic distribution that blanches on pressure , with or without strawberry tongue , desquamation , or sore throat ) . a confirmed case is defined as a clinical case with positive throat or wound culture for s. pyogenes or antistreptolysin titer > 200 . epidemiologic , clinical , and laboratory data were collected by standard questionnaire for every reported case . a cluster was defined as > 2 cases in persons sharing the same residential or school address within the incubation period . we compared epidemiologic , clinical , and microbiological features of the scarlet fever cases from january july 2011 ( outbreak period ) with features of those reported during 20082010 ( baseline period ) . we used spss version 14.0 ( spss inc . , chicago , il , usa ) for analyses ; p<0.05 was considered significant . for comparison we extracted records of patients hospitalized during january 2008july 2011 who had diagnoses that are known complications of scarlet fever , including toxic shock syndrome , acute rheumatic fever , and acute glomerulonephritis . bacterial culture of s. pyogenes was performed on diagnostic specimens in hospital laboratories and the public health laboratory centre of the department of health ; the latter serves as the diagnostic and public health reference laboratory in hong kong . antimicrobial drug susceptibility testing , emm typing , and detection of various virulence genes were performed at the public health laboratory centre on s. pyogenes isolates received during 2011 and archived during 20082010 ( 1 ) . pulsed - field gel electrophoresis ( pfge ) was performed on the basis of the gram - positive protocol , and pfge profiles were analyzed by using bionumerics 5.0 software ( applied maths , sint - martens - latem , belgium ) . in june 2011 , the department of microbiology of the university of hong kong announced the discovery of a unique 48-kb insertion sequence in the genome of s. pyogenes isolated from a blood specimen from a 7-year - old girl who died of scarlet fever ( 2 ) . we tested for this insert in a sample of strains collected during 20082011 using the method provided by the university of hong kong . during january 1july 31 , 2011 , a total of 996 cases of scarlet fever were reported , greatly exceeding the annual number of cases reported during 2008 ( 235 ) , 2009 ( 187 ) , and 2010 ( 128 ) . outbreak activity in 2011 peaked at week 26 ( week ending june 25 ) ( figure 1 ) . during the outbreak period ( january  july 2011 ) , the annualized incidence rate was 24.0/100,000 population , 9 higher than the average annualized incidence rate of 2.62/100,000 population during the baseline period of 20082010 . during the previous 2 decades , baseline annual incidence rates ranged from 0.0351 to 3.37 cases/100,000 population . weekly number of scarlet fever cases , by onset date , hong kong , january white bars indicate clinically diagnosed but not laboratory - confirmed cases ; solid bars indicate laboratory - confirmed cases . solid triangle indicates may 30 dissemination of press release about first fatal case ( in a 7-year - old girl ) ; open triangle indicates june 21 dissemination of press release about second fatal case ( in a 5-year - old boy ) ; circle indicates june 23 launch of health education campaign . table 1 compares the epidemiologic features , clinical features , and laboratory results for scarlet fever cases reported during 2011 and 20082010 . highest incidence ( 547 cases/100,000 population ) was reported for children 47 years of age ( table 1 ) . clinical features , complications , and case - fatality rate for cases reported in 2011 were largely comparable to those reported during the baseline period . the proportion of case - patients requiring hospitalization during 2011 was lower , and mean duration of hospital stay was 0.5 days shorter than for the baseline period . * complications include toxic shock syndrome , septicaemia , parapharyngeal abscess , rheumatic fever , quinsy and hepatitis . tss , toxic shock syndrome ; na , not available ; + , positive ;  , negative . among the 996 scarlet fever cases reported during january july 2011 , s. pyogenes isolates from samples from 90 patients ( mostly throat swab specimens ) were characterized . strains found belonged to the following emm types ( number and percentage of strains ) : emm12 ( 70 , 77.8% ) , emm1 ( 14 , 15.6% ) , emm4 ( 2 , 2.2% ) , emm22 ( 2 , 2.2% ) , emm2 ( 1 , 1.1% ) , and emm3 ( 1 , 1.1% ) . all strains were susceptible to penicillin , but 77 ( 85.6% ) strains were resistant to erythromycin . of 59 strains tested for the 48-kb insert , 78.0% ( 46 strains ) tested positive ; 39 were emm12 strains and 7 emm1 strains . antimicrobial drug susceptibility results were available for 39 strains positive for the 48-kb insert ; 3 ( 7.70% ) were susceptible to erythromycin . conversely , among all erythromycin - resistant emm12 strains in 2011 tested for the 48-kb insert , 6/42 ( 14.3% ) yielded a negative result . forty - eight emm12 isolates during january  june 2011 that were subjected to virulence gene profiling showed 5 virulence gene profiles . no particular virulence gene profile was dominant among the 9 scarlet fever cases associated with medical complications ( table 2 ) . among 26 emm12 strains subjected to pfge , 7 patterns were detected ; the emm12 strain from 1 of the 2 fatal cases exhibited a unique pfge pattern ( figure 2 ) . for the other fatal case , pulsed - field gel electrophoresis patterns of 26 emm type 12.0 streptococcus pyogenes strains , hong kong , 2011 . toxin profile results are shown as corresponding to the genes spea speb , spec , spef , speh , and ssa . scale bar indicates percent similarity . of the archived s. pyogenes strains collected during 20082010 , few strains were from patients diagnosed with scarlet fever ; therefore , we analyzed s. pyogenes strains isolated from throat and superficial wound specimens from outpatients < 15 years of age . among 28 such strains , emm28 was detected in 9 strains ; emm4 in 4 strains ; emm1 in 3 strains ; emm12 , 22 , and 89 in 2 strains each ; and 6 other emm types in 1 strain each . all strains were susceptible to penicillin ; the erythromycin resistance rate was 10.7% ( 3/28 ) . the 48-kb insert was found in 10 ( 35.7% ) strains : 3 strains of emm28 , and 1 strain each of emm types 1 , 2 , 4 , 22 , 44 , 89 , and stg485 . the 2011 s. pyogenes outbreak in hong kong attracted heightened media coverage , which might have increased reporting of cases ; however , the higher proportion of laboratory - confirmed cases in 2011 than those during 20082010 suggests the upsurge was genuine . we found insufficient evidence that a particular emm type of virulence gene profile or presence of the 48-kb insert was associated with increased incidence or severity . the 48-kb insert identified in 2011 was found in s. pyogenes strains isolated in 20082010 , albeit at lower rate ( 35.7% in 20082010 vs.78% in 2011 ) . a shift in prevailing emm type that occurred in 2011 might have contributed to fluctuations in the number of cases ( 3 ) . a higher rate of erythromycin resistance in s. pyogenes ( > 80% ) was found in 2011 than in the reported previous years ( 20%30% ) ( 4 ) . because all erythromycin - resistant strains were also resistant to clindamycin ( data not shown ) , we deduced the resistance mechanism to be resistance to macrolides , lincosamides , and streptogramins b system , as encoded by the erm genes ( 5 ) . the 48-kb insert provided a mechanism for macrolide resistance among s. pyogenes in hong kong , but our laboratory investigation found macrolide - resistant s. pyogenes strains and the macrolide - susceptible strains that bore them negative for this insert . mutation of the pcr primer binding site might explain the former strains ; further investigation is needed to explore this possibility . the upsurge in scarlet fever cases in hong kong during 2011 likely reflects a regional phenomenon ; a marked increase in cases was also observed in mainland china ( 6 ) and macao ( 7 ) during this period . high resistance rates against macrolides were also observed for the outbreak in mainland china ( 8) . we recommend close monitoring and surveillance of disease activity , genetic testing , antimicrobial susceptibility profiling , and maintaining scarlet fever s statutory notifiable status .
more than 900 cases of scarlet fever were recorded in hong kong during january  july , 2011 . six cases were complicated by toxic shock syndrome , of which 2 were fatal . 
 pulsed - field gel electrophoresis patterns suggested a multiclonal epidemic ; emm12 was the predominant circulating type . 
 we recommend genetic testing of and antimicrobial resistance monitoring for this reportable disease .