Streptococcal disease and the host–parasite relationship

Streptococcal evolution has been shaped by human history, social change and microbial selection. Streptococci with pathogenicity for people likely diversified within the last 5–10,000 years, jumping from animal hosts to infect the larger aggregations of people seen in towns and cities following the rise of agriculture. Streptococci are subject to continuing selection through changes in population immunity, hygiene, living conditions, medical interventions, and family size. Such selective effects can account, in part at least, for the virtual disappearance of scarlet fever, for changes in the prevalence and virulence of strains, and for the greatly reduced rates of rheumatic fever in affluent populations. In host populations of finite size which develop immunity, the distribution and survival of particular streptococcal strains is greatly influenced by competition from related strains, and by stochastic loss and turnover of multiple strains in small communities, as seen in child-care centres and Aboriginal settlements. Although multiple strains of GAS compete, they also help each other by boosting the immune response to shared T-cell epitopes, which can promote tissue inflammation and bacterial growth. This concept of multiple strain (multiple hit) sensitisation is applied also to help explain the age-specific incidence of RF and the explosive epidemics of RF seen in military camps in World War II. Ecological principles, such as those developed in this paper, could be more applied more deliberatively in the control of streptococcal diseases.