Molecular identification and characterization of a novel deltaproteobacterium as the etiologic agent of epizootic bovine abortion (foothill abortion).

摘要

Epizootic bovine abortion (EBA) has caused economic loss in beef cattle in California and other western states for more than fifty years. Due to the inability to propagate the agent in vitro, the etiology of EBA remained unknown. Suppression-hybridization polymerase chain reaction (shPCR) techniques applied to infected fetal tissues culminated in identification of a 16S rDNA fragment of a previously undescribed deltaproteobacterium closely related to members of the order Myxococcales. Development of a PCR, based upon unique sequences in the 16S rDNA, permitted the detection of this bacterium in both aborted bovine fetuses and the recognized tick vector (Ornithodoros coriaceus Koch). Taken together, this data strongly supported this unique deltaproteobacterium as being the etiologic agent of EBA. A specific non-radioactive Southern blotting procedure was subsequently developed to enhance the sensitivity of PCR-based identification of this deltaproteobacterium in the tick vector. The bacterium was identified in nymphs, male and female ticks (but not eggs and larvae). The percent of ticks identified as infected varied by geographic location. This agent of EBA was determined to reside primarily in the salivary gland of infected ticks. A blood meal, provided through artificial feeding procedures, had no significant effect in elevating the percentage of ticks that could be identified as infected. Severe combined immunodeficient mice (SCID) were developed as a laboratory animal model for studying the EBA agent. SCID mice inoculated with homogenates of infected fetal bovine thymus became moribund in 83 to 102 days and the agent identified in necropsy tissue using the EBA-specific PCR. Homogenates of these murine necropsy tissues were used to infect additional SCID mice, again resulting in clinical disease in a similar time frame to the first passage; the deltaproteobacterium was again identified in the second passage necropsy tissues. These second-passage murine tissues were used to inoculate pregnant susceptible bovine heifers. These heifers aborted close to term with their fetuses presenting with classical EBA lesions. Again, the unique deltaproteobacterium was identified by PCR.