Pathology of congenital syphilis in rabbits.

We have developed a model for congenital syphilis in the rabbit. This report provides additional information on newborn tissue pathology in animals that were infected in utero. A total of 35 pregnancies were evaluated, each containing 6 to 12 newborns. In the infected group, the mortality was approximately 50%; of the live newborns, half appeared normal and half were hyperreflexic, weak, and runty. Gross pathology in the sickly newborns was quite prevalent and involved enlarged spleens with isolated spots of necrosis; enlarged livers that were overtly congested and hemorrhagic and had numerous granular, white spots; and brains with hemorrhage in the occipital area. Histopathology was apparent in different tissues. Lymphocytes, plasma cells, and vacuolated macrophages were prominent in livers, spleens, brains, and bones. A few actively motile treponemes were visualized by dark-field microscopy within extracts of spleen and within cerebrospinal fluid. Low numbers of treponemes were also demonstrated in sections of brain and liver by using the Warthin-Starry silver stain technique. Blood hematocrits were decreased, and extramedullary hematopoiesis was prominent within spleens and livers; this is consistent with anemia. This rabbit model exhibits many of the same pathologic features commonly found in human congenital syphilis.