AN OUTBREAK OF INVASIVE GROUP-A STREPTOCOCCAL DISEASE-ASSOCIATED WITHHIGH CARRIAGE RATES OF THE INVASIVE CLONE AMONG SCHOOL-AGED CHILDREN

Objectives.-To determine if a common strain of group A streptococcus w as responsible for an outbreak of invasive streptococcal disease in so utheastern Minnesota and to determine whether this strain was prevalen t among residents of this area during the outbreak who either had stre ptococcal pharyngitis or were asymptomatic streptococcal carriers. Des ign.-Pharyngeal culture survey and case-contact evaluation. Setting.-T hree adjacent counties in southeastern Minnesota defined as the outbre ak area. Outbreak period, January 1 through March 31, 1995. Patients.- Seven patients with invasive streptococcal infection, 1249 patients (a dults and children) with sore throat who resided in the outbreak area, children from an elementary school located in 1 community where the m ajority of invasive cases occurred, and 896 students from 3 schools lo cated in Minnesota counties outside the outbreak area. Measurements.-P ulsed-field gel electrophores is (DNA fingerprinting) of group A strep tococcal isolates obtained from patients with invasive disease, throat swabs of patients with sore throat, and throat swabs of asymptomatic school-aged children. Results.-All patients with outbreak-associated i nvasive disease had group A streptococcal isolates that were indisting uishable by pulsed-field gel electrophoresis. Additional testing showe d that these isolates carried significant virulence factors including pyrogenic exotoxin A and streptococcal superantigen. Five of these 7 p atients with invasive disease had underlying medical conditions; 4 dev eloped toxic shock syndrome and died (case fatality, 57%). The outbrea k-associated group A streptococcal clone was found in 69 (26.5%) of th e 260 patients with sore throat from whom group A streptococcus was is olated. The frequency of the outbreak done among pharyngeal carriers f rom the 3 schools outside the outbreak area was significantly less (ra nge, 0%-10%) than in children from the school in the outbreak area (78 %; relative risk, 29; 95% confidence interval, 11.1-78. 1; P<.001). Fo ur of the 7 patients with outbreak-associated disease had contact with children who attended the school in the outbreak area, Conclusions.-A single clone of group A streptococcus was responsible for 7 cases of invasive streptococcal disease during an outbreak in Minnesota and for a significant number of pharyngitis cases that also occurred during t he outbreak invasive disease occurred most frequently in persons with underlying medical conditions. This outbreak was also associated with increased carriage rates of the invasive streptococcal clone among com munity school-aged children, Cases of invasive group A streptococcal i nfection may therefore reflect the tip of the iceberg with regard to t he burden of colonization of a specific invasive streptococcal clone i n a community.