The effects of M-protein fraction of hemolytic streptococci on embryonic heart in the rat.

Wistar rats were administered M-protein fraction (MP) of group A type 12 hemolytic streptococci on day 9 of gestation. When 9 mg/kg of MP was injected, the incidence of malformations was 11.8%, and the incidence of cardiovascular malformations was 7.9%. The main malformations were ventricular septal defect, microphthalmia, and hydrocephaly. Light and electron microscopic studies revealed no myocarditis nor endocarditis in either the maternal or fetal heart. Fluorescent antibody technique demonstrated no immunological cross-reaction between MP and rat heart tissue. The results suggest that MP is slightly teratogenic in the rat.