Epidemiologic and immunologic characteristics of Streptococcus equi infection in foals.

A 2-phase study was performed to characterize the effects of Streptococcus equi infection in unexposed and previously exposed foals. In phase I, 22 weanling foals involved in a naturally occurring S equi epizootic were studied, along with a comparison group of 11 unexposed foals, matched for age, sex, and breed. Six months later (phase II), an epizootic was experimentally induced in previously exposed and unexposed foals from phase I. The prevalence and duration of clinical signs, the relative risk of developing disease, bacteriologic culture results, hematologic responses, and mucosal and serum immunologic responses were determined. Disease protection in phase-I and -II foals was associated with high values for serum S equi M protein-specific IgG at the onset of the epizootic (P < 0.02 for phase 1 and P < 0.01 for phase II), and with a rapid (within 2 weeks of exposure) mucosal S equi M protein-specific IgG response (P < 0.05 for phase I and P = 0.01 for phase II).