Surveillance for hospital outbreaks of invasive group A streptococcal infections in Ontario, Canada, 1992 to 2000

Background: Streptococcus pyogenes can cause severe disease in the individual patient and dramatic hospital outbreaks. Objective: To describe the epidemiology of hospital outbreaks of invasive group A streptococcal infection in order to understand the potential benefit of proposed outbreak investigation and management strategies. Design: Prospective, population-based surveillance. Setting: Short-term care hospitals in Ontario, Canada. Patients: Persons with a positive culture for group A streptococcus from a normally sterile site between 1 January 1992 and 31 December 2000. Measurements: Laboratory-based surveillance identified patients with nosocomial invasive group A streptococcal infection. Epidemiologic and microbiological investigations were used to detect transmission. Results: Of 2351 cases of invasive group A streptococcal disease, 291 (12%) were hospital acquired. Twenty-nine (10%) nosocomial cases occurred as part of 20 outbreaks. Seventy percent (14 of 20) of outbreaks involved nonsurgical, nonobstetric patients. Community-acquired cases initiated 25% of outbreaks; most were cases of necrotizing fasciitis in patients admitted to the intensive care unit. Outbreaks were small (median, 2 cases [range, 2 to 10 cases]) and short (median duration, 6 days [range, 0 to 30 days]). The median time between the first 2 cases was 4.5 days. The most common mode of propagation was patient-to-patient transmission. A staff carrier was the primary mode of transmission in 2 (10%) outbreaks, but 1 or more health care workers were colonized with the outbreak strain in 6 of 18 (33%) other outbreaks. Limitations: Some outbreaks with 1 case of invasive disease may have been missed; advice provided to participating hospitals may have reduced the number and size of outbreaks. Conclusions: Practices to prevent hospital transmission of group A streptococci should include isolation of patients admitted to the intensive care unit with necrotizing fasciitis, investigation after a single nosocomial case, and emphasis on identifying and treating health care worker carriers on surgical and obstetric services and patient reservoirs on other wards.