The Incidence and Pathogenesis of Myocarditis in Rabbits after Group a Streptococcal Pharyngeal Infections

Rabbits subjected to single pharyngeal infections with group A streptococci developed cardiac lesions characterized by myofiber necrosis and a non-granulocytic cellular reaction with histiocytes, lymphocytes, and Anitschkow myocytes. The histopathologic changes were demonstrable in some animals within 24 hours of inoculation, apparently were maximal 72 hours after induction of infection (at which time they were seen in the hearts of all nine rabbits studied), and thereafter healed in the course of the following 2 weeks. The extent of involvement was variable, and with healing the necrotic areas were replaced by fibrous tissue. When intradermal infections with the same organisms were produced in rabbits, cardiac lesions, indistinguishable from those observed in the pharyngeally infected group, appeared in a much smaller number of animals. The hearts of five of six rabbits sacrificed a month or more following the last of a series of streptococcal pharyngeal infections exhibited lesions characterized chiefly by fibrosis, although mononuclear cellular infiltrations were also noted. In these repetitively infected animals the presence of occasional multinucleated giant cells and a few small foci of calcification were features not encountered in the single infection group. In a second series of rabbits sacrificed 3 days after the last of three pharyngeal infections with different strains of streptococci, acute as well as more chronic changes were observed. In none of the lesions in rabbits subjected to single or multiple streptococcal infections were bacteria demonstrable, either in histologic sections or in cultures of myocardial tissue. A large number of control animals was studied concomitantly, and in only one instance was a lesion, considered comparable to those described in the streptococcal series, encountered. The implications of these findings, particularly in terms of the non-suppurative sequelae of streptococcal infections in man, are discussed.