Histopathologic changes in bovine fetuses after repeated reintroduction of a spirochete-like agent into pregnant heifers: association with epizootic bovine abortion.

A spirochete-like organism was found in the plasma of bovine fetuses affected with epizootic bovine abortion (EBA). The spirochete-like organism was frequently found in abattoir-collected fetuses as an inapparent infection, and EBA was found in cattle on foothill rangeland where the vector tick Ornithodorus coriaceus could repeatedly reintroduce the infectious agent into pregnant cattle (superinfection). Epizootic bovine abortion resembled a naturally acquired superinfection in circumstances where the agent was frequently present in the environment under conditions favoring transmission. Therefore, to determine whether fetal lesions could be experimentally induced in utero, spirochete-like organisms collected from clinically normal fetuses at an abattoir were inoculated IV and subcutaneously into 2 pregnant heifers 5 times over a 4-month period to mimic repeated tick transmission in the field. Macroscopic and microscopic examinations of tissues from 2 cesarean-collected fetuses and from 3 calves born at term with the naturally acquired spirochete infection indicated that the calves had evidence of an infection that caused morphologic changes compatible with immunologic stimulation and mild reticuloendothelial hyperplasia. Compared with findings in the calves, lesions in the superinfected fetuses were more severe, and the lesion distribution in various organs was more extensive. The spirochete-like organism appeared to be a mild pathogen because of its persistence in the host. Clinical disease from the infection may only develop with repeated superinfections. Therefore, a relationship between this microorganism and EBA is probable.