Group A Streptococcus carriage among close contacts of patients with invasive infections

During the past few years, the incidence of invasive group A Streptococcus (GAS) infection has been increasing, However, there are presently no clear recommendations regarding antibiotic prophylaxis for close contacts of inde x patients. The aims of this study were 1) to determine the prevalence of c arriage of the same GAS strain as the patient's among contacts of patients with invasive infections and 2) to assess the importance of exposure durati on. From March 1995 to March 1998, the authors prospectively included in th e study all patients with invasive GAS infection, as defined by the Working Group on Severe Streptococcal Infections, who came to Hopital Maisonneuve- Rosemont in Montreal, Quebec, Canada. An epidemiologic investigation was sy stematically carried out for each index case. Contacts were divided into tw o groups: those who had spent 24 hours or more with the index patient durin g the week preceding the beginning of his or her illness and those who had spent 12-24 hours with the index patient during that week. Strains of GAS w ere examined by serotyping (proteins M and T and the presence or absence of the serum opacity factor) and by characterization of streptococcal pyrogen ic exotoxins (exotoxins A, B, and C). One hundred and two contacts of 17 in dex cases with invasive GAS infection were systematically screened. Contact s were considered positive if they carried the same strain of the bacterium and the same streptococcal pyrogenic exotoxin as the index case. Among the contacts who had spent at least 24 hours per week with their respective in dex cases, 13 out of 48 (27%) were found to be harboring the same serotype of GAS as the index patient (95% confidence interval 14.5-39.5). By compari son, only one of the 54 contacts in the 12- to 24-hour group (1.8%) was fou nd to be carrying the same strain of the bacterium (95% confidence interval 0-5.3). This difference between the two groups was statistically significa nt (p < 0.001). The median age of the positive carriers (10 years) was sign ificantly lower than the median age of the noncarriers (39 years) (p less t han or equal to 0.0005). This study showed that close contacts who had spen t 12-24 hours with the index patient were rarely colonized with GAS. If ant ibiotic prophylaxis against GAS is recommended, it should probably target c ontacts who spent at least 24 hours with an infected patient during the wee k preceding illness onset.