Serial passage of the etiologic agent of epizootic bovine abortion in immunodeficient mice.

Molecular studies have provided convincing evidence that a unique deltaproteobacterium is the causative agent of epizootic bovine abortion (EBA). Bovine fetuses, infected following dam exposure, are the only identified susceptible mammalian host. The inability to cultivate the bacterial agent of EBA (aoEBA) in vitro, associated with the substantial cost of bovine experimentation, drove efforts to identify an alternative laboratory animal host. Mice with severe combined immunodeficiency (SCID) were chosen as a potential host after immunocompetent mice proved resistant to infection. SCID mice inoculated with aoEBA-infected bovine fetal thymus homogenates began to show clinical signs at 2 months and became increasingly cachectic over the next 1-2 months. Following a 2nd passage (P2) through SCID mice, three susceptible pregnant heifers were inoculated with P2 murine tissue homogenates. All three fetuses presented with lesions indistinguishable from naturally occurring EBA, confirming successful passage of the bacterial pathogen in SCID mice. All murine (P1 and P2) and bovine fetal tissues contained aoEBA as determined by PCR; 16S bacterial ribosomal nucleotide sequences were identical in all murine and fetal bovine tissues examined. Bacteria in fetal bovine tissues were determined to be heavily opsonized, based upon microscopic evaluation of tissues stained with either FITC-conjugated anti-bovine IgG or biotin-conjugated anti-bovine IgG in conjunction with avidin-FITC. Unlike the near-term bovine fetus, the absence of an antibody response in infected SCID mice permits harvest of unopsonized bacteria for development of serologic assays.