Experimental abortion and the systemic immune response to "Haemophilus somnus" in cattle.

"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 G1 (IgG1), 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."