An outbreak of invasive group A streptococcal disease associated with high carriage rates of the invasive clone among school-aged children

F. R. Cockerill 3rd, K. L. MacDonald, R. L. Thompson, F. Roberson, P. C. Kohner, J. Besser-Wiek, J. M. Manahan, J. M. Musser, P. M. Schlievert, J. Talbot, B. Frankfort, J. M. Steckelberg, W. R. Wilson and M. T. Osterholm
Department of Laboratory Medicine and Pathology, Division of Microbiology, Mayo Clinic and Foundation, Rochester, Minn. 55905, USA.

OBJECTIVES: To determine if a common strain of group A streptococcus was responsible for an outbreak of invasive streptococcal disease in southeastern Minnesota and to determine whether this strain was prevalent among residents of this area during the outbreak who either had streptococcal pharyngitis or were asymptomatic streptococcal carriers. DESIGN: Pharyngeal culture survey and case-contact evaluation. SETTING: Three adjacent counties in southeastern Minnesota defined as the outbreak area. Outbreak period, January 1 through March 31, 1995. PATIENTS: Seven patients with invasive streptococcal infection, 1249 patients (adults and children) with sore throat who resided in the outbreak area, children from an elementary school located in 1 community where the majority of invasive cases occurred, and 896 students from 3 schools located in Minnesota counties outside the outbreak area. MEASUREMENTS: Pulsed-field gel electrophoresis (DNA fingerprinting) of group A streptococcal 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 invasive disease had group A streptococcal isolates that were indistinguishable by pulsed-field gel electrophoresis. Additional testing showed that these isolates carried significant virulence factors including pyrogenic exotoxin A and streptococcal superantigen. Five of these 7 patients with invasive disease had underlying medical conditions; 4 developed toxic shock syndrome and died (case fatality, 57%). The outbreak-associated group A streptococcal clone was found in 69 (26.5%) of the 260 patients with sore throat from whom group A streptococcus was isolated. The frequency of the outbreak clone among pharyngeal carriers from the 3 schools outside the outbreak area was significantly less (range, 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). Four 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 the 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 community school-aged children. Cases of invasive group A streptococcal infection may therefore reflect the tip of the iceberg with regard to the burden of colonization of a specific invasive streptococcal clone in a community.

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