The Pathogenesis of Equine Endometritis Due to Streptococcus Zooepidemicus.

The ability of uterine isolates of Streptococcus zooepidemicus to resist phagocytosis was explored by the organism\u27s ability to grow in equine fresh blood. Although isolates grew in fresh blood of many horses, any given isolate was killed by blood of certain horses. Killing required leukocytes, heat labile and heat stable components of plasma and appeared to follow phagocytosis of the organism by neutrophils. This led to the working hypothesis that killing was mediated by phagocytosis requiring complement and isolate-specific antibody to variable surface antigens. Variation in killing between horses was attributed to differences in circulating isolate-specific antibody. This is a situation analogous to requirements for phagocytosis of the human pathogenic M protein positive group A streptococci, and supports existing evidence that uterine isolates of Streptococcus zooepidemicus also carry M protein. However, in contrast to human systemic infections with the group A streptococci, killing ability in fresh blood was not closely related to resistance to equine endometritis. Horses in whose blood a given isolate grew were able to eliminate intrauterine inocula of that isolate. Similarly, horses in whose blood the isolate was killed were not necessarily resistant to intrauterine inoculation. Variations between horses and between isolates appeared to be the major determinants of disease. It was concluded that other mechanisms of resistance in addition to phagocytosis, were responsible for clearance of streptococci from the uterus. Physical clearance of carbon powder from the equine uterus was enhanced in horses which displayed mucus production at the endometrial surface. In addition, mucus appeared to block the binding of carbon to the endometrial surface. Since the equine endometrium is ciliated, it was suggested that mucociliary clearance may also contribute to removal of streptococci from the equine uterus