from october to november 2002 , a cluster of skin and soft tissue infections due to mrsa involving pediatric and maternity patients occurred at a new york city hospital . the hospital has a labor and delivery unit and 2 units that house both healthy newborns and maternity patients . after the outbreak was recognized , the following interventions were implemented : 1 ) nursing and medical personnel from the involved areas were informed of the outbreak and potential modes of transmission of staphylococci , 2 ) contact precautions were emphasized for all patients with suspected or proven skin infections , 3 ) alcohol - based hand sanitizers were placed in involved areas , 4 ) healthcare workers from involved units were screened for nasal mrsa colonization , and 5 ) environmental surfaces ( including cord clamps , antitheft transponders , and temperature sensors of baby warmers ) were tested for mrsa contamination . healthcare workers colonized with mrsa were treated with intranasal mupirocin and furloughed until repeat cultures were negative . to identify any other potential case - patients , letters concerning the outbreak were sent to pediatricians who cared for newborns discharged from the affected units during the outbreak period . cases were defined as mrsa infections in patients who stayed on the labor and delivery , nursery , or maternity units at any time from october 2002 to december 2002 . the medical records of the patients were reviewed for information regarding prior healthcare exposures , receipt of antimicrobial agents , underlying medical conditions , treatment , and clinical outcome . cultures related to the outbreak were grown on tryptic soy agar plates supplemented with 3% sheep blood ; colonies consistent with s. aureus were identified according to standard techniques . all isolates underwent susceptibility testing with the etest method ( ab biodisk , solna , sweden ) . ribotyping was performed with the riboprinter microbial characterization system ( qualicon , wilmington , de , usa ) , as previously noted ( 13 ) . in addition , isolates of mrsa collected during the outbreak were fingerprinted by pulsed - field gel electrophoresis ( pfge ) , as previously described ( 13 ) . sccmec typing was performed by using multiplex polymerase chain reaction ( pcr ) , under conditions described by oliveira et al . primers to detect the meca gene were included as an internal positive control ( 15 ) . multilocus sequence typing ( mlst ) was performed on selected isolates as described by enright et al . bidirectional dna sequencing of 7 amplified housekeeping genes was performed with an automated fluorescent dye - terminator sequencing system ( applied biosystems , foster city , ca , usa ) . the presence of genetic sequences encoding several staphylococcal toxins was also investigated for the outbreak isolates . based on the previously reported distribution of enterotoxins in ca - mrsa from the united states ( 7 ) , the following toxins were selected for investigation : staphylococcal enterotoxin a ( sea ) , b ( seb ) , c ( sec ) , h ( seh ) , and k ( sek ) . in addition , strains were screened for pvl and toxic shock syndrome toxin-1 ( tsst-1 ) . previously published primers and conditions were used to detect sequences encoding for sea , seb , sec , seh , pvl , and tsst-1 ( 1719 ) . genes encoding for sek were detected with the following primers : sek forward : 5-tggatcaatggaaatcacaaaa-3  and reverse : 5-tttggtagcccatcatctcc-3  ( predicted product size 287 bp ) . the identification of mw2 in the outbreak of the neonatal - maternity unit prompted a retrospective investigation to determine the regional prevalence of mrsa resembling this strain . in 1999 , 2001 , and 2003 , surveillance studies were performed in brooklyn , new york . each surveillance study involved collecting all single - patient isolates of s. aureus from clinical microbiology laboratories during a 3-month interval . susceptibility testing was performed in the central research laboratory by using the agar dilution method according to nccls methodology ( 20 ) . all mrsa isolates were then screened for a phenotype of susceptibility to clindamycin and ciprofloxacin ( typical for mw2 ) . the study was approved by the institutional review board at the state university of new york ( suny ) health science center and maimonides medical center . from october to november 2002 , a cluster of skin and soft tissue infections due to mrsa involving pediatric and maternity patients occurred at a new york city hospital . the hospital has a labor and delivery unit and 2 units that house both healthy newborns and maternity patients . after the outbreak was recognized , the following interventions were implemented : 1 ) nursing and medical personnel from the involved areas were informed of the outbreak and potential modes of transmission of staphylococci , 2 ) contact precautions were emphasized for all patients with suspected or proven skin infections , 3 ) alcohol - based hand sanitizers were placed in involved areas , 4 ) healthcare workers from involved units were screened for nasal mrsa colonization , and 5 ) environmental surfaces ( including cord clamps , antitheft transponders , and temperature sensors of baby warmers ) were tested for mrsa contamination . healthcare workers colonized with mrsa were treated with intranasal mupirocin and furloughed until repeat cultures were negative . to identify any other potential case - patients , letters concerning the outbreak were sent to pediatricians who cared for newborns discharged from the affected units during the outbreak period . cases were defined as mrsa infections in patients who stayed on the labor and delivery , nursery , or maternity units at any time from october 2002 to december 2002 . the medical records of the patients were reviewed for information regarding prior healthcare exposures , receipt of antimicrobial agents , underlying medical conditions , treatment , and clinical outcome . cultures related to the outbreak were grown on tryptic soy agar plates supplemented with 3% sheep blood ; colonies consistent with s. aureus were identified according to standard techniques . all isolates underwent susceptibility testing with the etest method ( ab biodisk , solna , sweden ) . ribotyping was performed with the riboprinter microbial characterization system ( qualicon , wilmington , de , usa ) , as previously noted ( 13 ) . in addition , isolates of mrsa collected during the outbreak were fingerprinted by pulsed - field gel electrophoresis ( pfge ) , as previously described ( 13 ) . sccmec typing was performed by using multiplex polymerase chain reaction ( pcr ) , under conditions described by oliveira et al . primers to detect the meca gene were included as an internal positive control ( 15 ) . multilocus sequence typing ( mlst ) was performed on selected isolates as described by enright et al . bidirectional dna sequencing of 7 amplified housekeeping genes was performed with an automated fluorescent dye - terminator sequencing system ( applied biosystems , foster city , ca , usa ) . the presence of genetic sequences encoding several staphylococcal toxins was also investigated for the outbreak isolates . based on the previously reported distribution of enterotoxins in ca - mrsa from the united states ( 7 ) , the following toxins were selected for investigation : staphylococcal enterotoxin a ( sea ) , b ( seb ) , c ( sec ) , h ( seh ) , and k ( sek ) . in addition , strains were screened for pvl and toxic shock syndrome toxin-1 ( tsst-1 ) . previously published primers and conditions were used to detect sequences encoding for sea , seb , sec , seh , pvl , and tsst-1 ( 1719 ) . genes encoding for sek were detected with the following primers : sek forward : 5-tggatcaatggaaatcacaaaa-3  and reverse : 5-tttggtagcccatcatctcc-3  ( predicted product size 287 bp ) . the identification of mw2 in the outbreak of the neonatal - maternity unit prompted a retrospective investigation to determine the regional prevalence of mrsa resembling this strain . in 1999 , 2001 , and 2003 , surveillance studies were performed in brooklyn , new york . each surveillance study involved collecting all single - patient isolates of s. aureus from clinical microbiology laboratories during a 3-month interval . susceptibility testing was performed in the central research laboratory by using the agar dilution method according to nccls methodology ( 20 ) . all mrsa isolates were then screened for a phenotype of susceptibility to clindamycin and ciprofloxacin ( typical for mw2 ) . the study was approved by the institutional review board at the state university of new york ( suny ) health science center and maimonides medical center . from october 18 to november 28 , 2002 , a total of 8 patients with skin and soft tissue infections due to mrsa were identified . during this period , 3.5 cases of mrsa infection occurred each month in the nursery and maternity units . in contrast , no mrsa infections had been reported from the involved units in the 10 months before the outbreak . two patients were mothers , and 6 were neonates ; in no instance were both the mother and her child infected . all had been hospitalized on an involved unit at some point from october 16 to november 6 , 2002 . review of medical records showed that none of the patients had prior hospital exposure , underlying chronic medical conditions , or recent antibiotic therapy . patients stayed on the unit for an average of 5 days ( range 212 days ) . clinical infection developed in 4 of the newborns and 1 mother while in the hospital . symptoms developed in 2 newborns and 1 mother 2 , 10 , and 24 days , respectively , after discharge . definitive therapy generally consisted of topical or systemic antimicrobial agents active against mrsa ; 1 patient required surgical drainage . time course of hospitalizations and onset of methicillin - resistant stpahylococcus aureus illness during the outbreak at hospital a. solid bars represent period of hospitalization ; arrows represent onset of clinical infection . the first was in an infant , born in november 2002 , who was seen as an outpatient for pustulosis ; however , the site was not cultured . the second case involved another infant , also born in november 2002 , who was readmitted to the hospital 4 days later for treatment of omphalitis . no additional cases were reported from december10 , 2002 , to december 31 , 2003 . susceptibility testing showed that all 8 isolates were susceptible to clindamycin , ciprofloxacin , trimethoprim - sulfamethoxazole , rifampin , doxycycline , linezolid , and vancomycin . of the 8 clinical isolates , 7 ( isolates p1-p7 ) belonged to 1 ribotype that was identical to the prototypical mw2 strain ( figure 2 ) . pfge confirmed that the 7 isolates were identical and closely related to mw2 ( figure 2 ) . since the 7 isolates appeared identical , mlst was performed on one of the isolates and showed sequence type 1 . the pfge and mlst pattern are the same as ca - mrsa clone usa 400 , which also includes mw2 ( 21 ) . among these 7 isolates , all contained sek and pvl , 6 contained sec and seh , and 5 contained sea . the eighth clinical isolate , from a catheter - site infection , was distinct from the outbreak strain by ribotyping and pfge ( figure 2 ) . for this isolate , sccmec was nontypable , and mlst typing confirmed a distinct allelic profile . a ) ribotype and b ) pulsed - field gel electrophoresis patterns of 8 clinical isolates of methicillin - resistant staphylococcus aureus . clinical isolate p8 and the 3 isolates from healthcare workers ( s1s3 ) are unrelated to the outbreak strain . a total of 189 healthcare workers worked on the involved units during the outbreak period . screening cultures of the anterior nares were performed in 176 of the workers in november 2002 . three of the cultures were positive for mrsa , including 2 from the nursing staff and 1 from a pediatrician . the 3 mrsa strains possessed a susceptibility pattern typical for the multidrug - resistant hospital strains , with resistance to clindamycin and ciprofloxacin . they belonged to ribotypes distinct from the outbreak clone , and pfge confirmed these isolates were unrelated to mw2 ( figure 2 ) . none of the 27 environmental samples collected in november 2002 yielded positive cultures for mrsa . a total of 4,345 isolates of s. aureus were collected in the 3 surveillance studies conducted in 1999 , 2001 , and 2003 ; susceptibility data for these isolates are given in table 2 . the 1,913 mrsa isolates , 118 ( 6% ) possessed the screened phenotype ( susceptible to both clindamycin and fluoroquinolones ) . among the 118 isolates , a total of 11 isolates possessed the same ribotype pattern as the outbreak clone , mw2 . sources of the cultures included skin and soft - tissue in 7 patients , blood / sterile body fluid in 3 patients , and the genital tract in 1 patient . the number of isolates resembling mw2 remained relatively constant during the 3 surveillances ( 4 in 1999 , 3 in 2001 , and 4 in 2003 ) . from october 18 to november 28 , 2002 , a total of 8 patients with skin and soft tissue infections due to mrsa were identified . during this period , 3.5 cases of mrsa infection occurred each month in the nursery and maternity units . in contrast , no mrsa infections had been reported from the involved units in the 10 months before the outbreak . two patients were mothers , and 6 were neonates ; in no instance were both the mother and her child infected . all had been hospitalized on an involved unit at some point from october 16 to november 6 , 2002 . review of medical records showed that none of the patients had prior hospital exposure , underlying chronic medical conditions , or recent antibiotic therapy . patients stayed on the unit for an average of 5 days ( range 212 days ) . clinical infection developed in 4 of the newborns and 1 mother while in the hospital . symptoms developed in 2 newborns and 1 mother 2 , 10 , and 24 days , respectively , after discharge . definitive therapy generally consisted of topical or systemic antimicrobial agents active against mrsa ; 1 patient required surgical drainage . time course of hospitalizations and onset of methicillin - resistant stpahylococcus aureus illness during the outbreak at hospital a. solid bars represent period of hospitalization ; arrows represent onset of clinical infection . the first was in an infant , born in november 2002 , who was seen as an outpatient for pustulosis ; however , the site was not cultured . the second case involved another infant , also born in november 2002 , who was readmitted to the hospital 4 days later for treatment of omphalitis . no additional cases were reported from december10 , 2002 , to december 31 , 2003 . susceptibility testing showed that all 8 isolates were susceptible to clindamycin , ciprofloxacin , trimethoprim - sulfamethoxazole , rifampin , doxycycline , linezolid , and vancomycin . of the 8 clinical isolates , 7 ( isolates p1-p7 ) belonged to 1 ribotype that was identical to the prototypical mw2 strain ( figure 2 ) . pfge confirmed that the 7 isolates were identical and closely related to mw2 ( figure 2 ) . since the 7 isolates appeared identical , mlst was performed on one of the isolates and showed sequence type 1 . the pfge and mlst pattern are the same as ca - mrsa clone usa 400 , which also includes mw2 ( 21 ) . among these 7 isolates , all contained sek and pvl , 6 contained sec and seh , and 5 contained sea . the eighth clinical isolate , from a catheter - site infection , was distinct from the outbreak strain by ribotyping and pfge ( figure 2 ) . for this isolate , sccmec was nontypable , and mlst typing confirmed a distinct allelic profile . a ) ribotype and b ) pulsed - field gel electrophoresis patterns of 8 clinical isolates of methicillin - resistant staphylococcus aureus . clinical isolate p8 and the 3 isolates from healthcare workers ( s1s3 ) are unrelated to the outbreak strain . a total of 189 healthcare workers worked on the involved units during the outbreak period . screening cultures of the anterior nares were performed in 176 of the workers in november 2002 . three of the cultures were positive for mrsa , including 2 from the nursing staff and 1 from a pediatrician . the 3 mrsa strains possessed a susceptibility pattern typical for the multidrug - resistant hospital strains , with resistance to clindamycin and ciprofloxacin . they belonged to ribotypes distinct from the outbreak clone , and pfge confirmed these isolates were unrelated to mw2 ( figure 2 ) . none of the 27 environmental samples collected in november 2002 yielded positive cultures for mrsa . a total of 4,345 isolates of s. aureus were collected in the 3 surveillance studies conducted in 1999 , 2001 , and 2003 ; susceptibility data for these isolates are given in table 2 . the 1,913 mrsa isolates , 118 ( 6% ) possessed the screened phenotype ( susceptible to both clindamycin and fluoroquinolones ) . among the 118 isolates , a total of 11 isolates possessed the same ribotype pattern as the outbreak clone , mw2 . sources of the cultures included skin and soft - tissue in 7 patients , blood / sterile body fluid in 3 patients , and the genital tract in 1 patient . the number of isolates resembling mw2 remained relatively constant during the 3 surveillances ( 4 in 1999 , 3 in 2001 , and 4 in 2003 ) . this report characterizes the nosocomial transmission of the ca - mrsa strain mw2 among healthy newborns and , possibly , a postpartum woman . symptoms developed in 3 patients 224 days after hospitalization ; 2 may have acquired the bacteria in the hospital or the community . an eighth patient , a mother with catheter - site infection , had an unrelated strain with a pattern suggestive of a hospital - associated strain . the source of the outbreak and mechanism of transmission were not evident , as no cultures of staff members or the environment yielded this particular strain of mrsa . transmission may have occurred after mw2 was introduced into the hospital by transient colonization of healthcare workers or by contamination of shared medical equipment . the infection control measures enacted in response to the initial cases may have had a role in controlling the outbreak . widespread screening of healthcare workers for mrsa did not detect the outbreak strain in this and another report ( 12 ) . while a potential role for this practice can not be excluded , current evidence does not support routinely implementing widespread screening for ca - mrsa . in the pediatric population , risk factors associated with mrsa infections include premature birth or low birth weight , chronic underlying diseases , prolonged hospitalization , invasive or surgical procedures , indwelling catheters , and prolonged use of antimicrobial agents ( 2225 ) . prior outbreaks involving the pandemic strain phage type 80/81 were characterized by high colonization rates among infants discharged from nurseries and subsequent transmission to family members ( 26 ) . in this report , infection developed in the outpatient setting for 2 patients ( following an admission on the involved unit ) , which suggests carriage of mw2 from the hospital back into the community . unrecognized ca - mrsa colonization during hospitalization could become an additional method of its dissemination in the community . increased prevalence of ca - mrsa has been reported in chicago , los angeles , texas , and minnesota ( 2,3,27,28 ) . in new york city , ca - mrsa appears less common ; 1 investigation reported mrsa carriage in 0.26% of children and their guardians ( 29 ) . in our present report , a retrospective analysis of isolates collected from citywide surveillance studies conducted from 1999 to 2003 suggests that 1% of all mrsa isolates in brooklyn are genotypically related to the prototypical north american ca - mrsa , mw2 . since only mrsa isolates that were susceptible to both clindamycin and ciprofloxacin were analyzed , this analysis probably underestimates the true prevalence . other strains of ca - mrsa ( e.g. , usa 300 ) and usa 400 strains that acquired resistance to these antimicrobial agents would have been missed by our screening methods . the introduction of ca - mrsa strains into neonatal units represents an especially serious challenge . many of the infections caused by these strains , including some in our report , can be unusually severe and life - threatening ( 11 ) . careful vigilance involving surveillance , identification of these dangerous strains , and implementation of infection control measures , should be helpful in preventing further transmission both within and outside of the hospital .