Group A streptococci (Streptococcus pyogenes) are the most common bacterial cause of pharyngitis and a frequent cause of cutaneous infections including impetigo and cellulitis. More rarely, group A streptococci are implicated as the etiologic agent in patients with pneumonia, bacteremia, and a newly described toxic shock-like syndrome.' While invasive group A streptococcal infections occur in persons of all ages, the incidence of these infections is greatest in the elderly, as is the case/fatality ratio. Factors that may contribute to the increased risk of invasive group A streptococcal infections in the elderly include a higher prevalence of underlying illnesses and, possibly, decreased levels of antibodies directed against the group A streptococcal M-protein, a major determinant of virulence.
From the advent of the antibiotic era until the mid-1980s, the occurrence of severe group A streptococcal infections declined. However, since 1985, rheumatic fever outbreaks in civilian and military populations, a syndrome designated the streptococcal toxic shock-like syndrome, and an increased incidence of group A streptococcal bacteremia have been reported. These changes in the epidemiology of group A streptococcal infections have been associated with changes in the serotype distribution, with an increasing proportion of invasive infections caused by M-1 and M-3 strains, and an increase in the proportion of organisms producing pyrogenic exotoxin A, which has been linked to the toxic shock-like syndrome.