Pulmonary persistence of Haemophilus somnus in the presence of specific antibody.

Chronic experimental Haemophilus somnus pneumonia was produced in five 8- to 12-week-old calves to investigate host-parasite relationships in the respiratory tract. Calves were depressed and pyrexic and coughed intermittently for 3 days and then recovered except for sporadic coughing. Bacteria persisted in the lung for 6 to 10 weeks or more. Immunoglobulin G1 (IgG1), IgG2, and IgM but no IgA antibodies specific for H. somnus were detected in serum. Bronchoalveolar lavage samples contained detectable IgG1, IgG2, IgM, and IgA antibodies specific for H. somnus throughout most of the experiment. The kinetics of the isotypic antibody response against H. somnus in serum and bronchoalveolar lavage fluids differed, suggesting that both local and systemic antibody responses had occurred. Persistence of pulmonary infection for 10 weeks or more in the presence of antibody may be due to an inappropriate distribution of isotypes, toxicity of H. somnus for bovine macrophages, and perhaps other factors. Three of the calves were challenged with a 10-fold-higher dose of H. somnus at 10 weeks after the original inoculation. Immunity against H. somnus was indicated by the rapid clearance of bacteria from the lungs and the presence of minimal pneumonia at necropsy 3 days after bacterial challenge.