Experimental transmission of epizootic bovine abortion (foothill abortion).

Advances in defining the biology of epizootic bovine abortion (EBA), including identification of the etiologic agent, have been hampered by the inability to reproduce the disease with confidence. Experimental reproduction of EBA, by feeding the tick vector Ornithodoros coriaceus on susceptible pregnant heifers, is not reliable. The primary objectives of this study were to identify specific tissue(s) obtained from EBA-infected fetuses that could transmit the disease, and then utilize such an infectious challenge system to better define the pathogen, host immunity and geographic distribution of the agent. Described here is the ability to routinely reproduce EBA following inoculation of cryopreserved suspensions of homogenized thymus into susceptible pregnant heifers. This challenge system permitted experiments demonstrating the agent was non-filterable, inactivated upon sonication and susceptible to antibiotics. These findings suggest a prokaryotic microbe and represent a major advance in EBA research. Additional experiments demonstrated that inoculation of the cryopreserved EBA-infectious tissue into heifers, prior to breeding, conferred immunity. Furthermore, such immunized heifers were resistant to challenge with heterologous sources of infectious tissue, suggesting monovalent vaccine development might be feasible. Lastly, challenge studies employing animals from Central Nevada, an area considered free of EBA, demonstrated partial immunity, suggesting the pathogen, and possibly the disease, enjoy a broader distribution than previously thought.

Diagnosis of epizootic bovine abortion in Nevada and identification of the vector.

In the 43 years since the first description in California, epizootic bovine abortion (EBA) has been considered but not definitively diagnosed as a cause of late-term abortions on Nevada ranches. Examination of aborted full-term bovine fetuses obtained from Nevada ranches revealed gross abnormalities consistent with EBA (enlarged lymph nodes, petechial hemorrhages of the oral mucosa and conjunctiva, ascites, and splenohepatomegaly), and EBA was confirmed by histologic examination of fetal tissues. The histologic thymic changes were characteristic of EBA and included severe histocytic thymusitis with depletion of thymocytes, interlobular hemorrhage, and fibrinocellular exudation. The gross enlargement of lymph nodes was the result of cortical follicular hyperplasia and histiocytic lymphadenitis. In addition, widespread, predominately nonsuppurative histologic lesions typical of EBA were observed in most organs, including the brain, lung, heart, liver, and spleen. Furthermore, the presence of Ornithodorus coriaceus, the argasid tick vector of EBA, was established by tick collection using CO2 traps. The tick was identified on ranches and in geographic areas (northern and northwestern counties of Nevada) coincident with diagnosis of multiple cases of EBA. This study establishes the presence of EBA as a cause of late-term abortion in Nevada. Additionally, identification of the EBA tick vector, O. coriaceus, in the same areas as the abortions provides strong evidence that the disease is endemic.