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Haemophilus somnus" has been identified in the etiology of bovine abortion on the basis of the isolation of the organism from aborted fetal and placental tissues. To investigate the role of hematogenous dissemination of "H. somnus" in the pathogenesis of abortion and to monitor the humoral immune response to infection, 19 pregnant cows (gestation ages, 1.4 to 7 months) were challenged intravenously (11 cows) or intrabronchially (8 cows). Five cows challenged intravenously aborted, and one cow challenged intrabronchially resorbed her fetus. "H. somnus" was isolated in large numbers from aborted tissues, and placental lesions were similar to those reported in a field case of "H. somnus" abortion. Antibody titers in serum were measured by the microagglutination test (MAT) and by enzyme-linked immunosorbent assay (ELISA). A response to challenge was measured by MAT; it was also measured by ELISA within the immunoglobulin Gl (IgGl), IgG2, and IgM isotypes. On comparison of pre-and postchallenge antibody titers, the greatest and most persistent response was detected within the IgG2 isotype. Prechallenge antibody titers (measured by MAT and by IgG2 ELISA) were lower in animals that aborted than in normal calving animals, indicating that IgG2 antibody may have a role in limiting hematogenous dissemination of "H. somnus."

The ability of the aerobic bacterial flora from the normal bovine respiratory and reproductive tracts to enhance or inhibit the growth of Pasteurella haemolytica, P. multocida, and Haemophilus somnus was tested in vitro. Six strains of each of these pathogens were cross streaked with each isolate of bovine normal flora. Flora which enhanced the growth of these pathogenic bacteria outnumbered inhibitors four to one. An intermediate number of isolates produced no effect on pathogen growth. Most enhancers were gram positive (Micrococcus, Staphylococcus, Corynebacterium, or Rhodococcus isolates), although several isolates of Moraxella and Actinobacter were also good enhancers. For H. somnus, there were proportionally more organisms which produced marked enhancement among the preputial flora than among the nasal flora, which may account for the greater number of genital carriers than nasal carriers. Bacillus isolates were the most significant inhibitors among the nasal flora, whereas no genus or species from the reproductive tract was noted to produce appreciable inhibition. It is proposed that changes in ratios of inhibitors to enhancers may determine, in part, whether a carrier state or disease occurs. Also, suggestions are made for in vitro use of this phenomenon for diagnostic tests.

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