The epidemiology of acute rheumatic fever is essentially the epidemiology of group A streptococcal pharyngitis. Rheumatic fever is most frequently observed in the age group most susceptible to group A streptococcal infections, children from 5{endash}–15 yr of age. However, susceptibility to rheumatic fever is also evident in older age groups, as is noted by the outbreaks of acute rheumatic fever that have occurred in specific closed populations such as military recruits. Increased numbers of cases also occur in socially and economically disadvantaged groups.

This has been attributed to crowding, which is more frequent in this segment of the population. Furthermore, the increased incidence of group A streptococcal pharyngitis in fall, winter, and early spring is associated with an increased number of cases of acute rheumatic fever during these same periods of the year.

Group A streptococcal impetigo does not result in acute rheumatic fever, but infection of the upper respiratory tract or the skin may lead to another nonsuppurative complication of streptococcal infection, acute poststreptococcal glomerulo-nephritis.

The reasons for this are not fully understood. Hypotheses relating to differences in rheumatogenic potential of "skin strains" and "throat strains" as well as observed differences in the immunologic response to group A streptococcal impetigo compared to streptococcal upper respiratory tract infection have been proposed to explain the contrast.

The major epidemiologic risk factor for development of acute rheumatic fever is group A streptococcal pharyngitis. The major reservoir for group A streptococci is the upper respiratory tract of humans.

The attack rate of acute rheumatic fever following group A upper respiratory tract infection is approximately 3% of individuals with untreated or inadequately treated infection. This figure has been remarkably constant, and the occasionally reported lower rates probably reflect inclusion of group A streptococcal carriers. Many children who harbor the group A Streptococcus are carriers of group A streptococci in the upper respiratory tract. The group A streptococcal carrier is at much reduced risk for development of acute rheumatic fever and for spread of the organism to close family or school contacts.

Of particular epidemiologic interest is the resurgence of acute rheumatic fever that occurred in the United States in the middle and late 1980s. Although the annual incidence of acute rheumatic fever in many communities in the United States was less than 1 in 100,000 in the years through the early 1980s, beginning in the mid-1980s, outbreaks of acute rheumatic fever occurred in numerous areas across the United States. The initial and largest outbreak was reported from Utah, but subsequent reports from eastern states, including Ohio and Pennsylvania, indicated that this resurgence was multifocal. One survey of pediatric cardiologists in large referral medical centers in the United States suggested that an increase in numbers of cases of acute rheumatic fever between 1985 and 1989 occurred in approximately 25 states. There were also at least two outbreaks of acute rheumatic fever in military recruit populations in the United States between 1985 and 1988.

The reasons for this resurgence of acute rheumatic fever in the United States remain unknown. Although rheumatic fever has been associated with socially and economically disadvantaged populations, the 1980s' resurgence has been associated with middle-class, often suburban and rural families. In addition, serotypes of group A streptococci that have been isolated only rarely during the previous 2 or 3 decades have emerged and spread. These serotypes began to be isolated in greater numbers when rheumatic fever cases were being reported. An increased number of isolates from rheumatic fever patients or simultaneously from their household contacts and siblings were shown to be M types 1, 3, 5, 6, and 18. These types have historically been associated with rheumatic fever. Very mucoid strains, especially strains of M type 18 group A streptococci, have appeared in a number of communities prior to the appearance of rheumatic fever. Mucoid strains have historically been associated with virulence.