Histophilus somni: Pathogenicity in cattle. An update

Histophilus somni (H. somni) is a Gram-negative bacterium currently classified as a member of the Haemophilus-Actinobacillus-Pasteurella group. Clinical syndromes associated with H. somni infection involve thromboembolic meningoencephalitis, pneumonia and disease of the reproductive tract in cattle. Animals can be carriers of non-pathogenic variants of the organism, mainly in the genital mucosa. The causes of these differences in virulence between strains are not defined. Several determinants of virulence of the pathogen are proposed. However, many of these factors cannot be clearly related to clinical disease. H. somni avoids killing by phagocytic cells. Thus, it is able to evade the immune response by intracellular survival in the infected host. The bovine adaptive immune response against H. somni is not completely characterized. IgG2 antibodies are thought to be protective. However, the major antigen determinants of the bacterium are still unknown. Studies with H. somni bacterins have inconsistent results, especially because the factors involved in pathogenesis and immune response remain unclear.