Advances in the quest for virulence factors of Haemophilus parasuis

Haemophilus parasuis colonises healthy pigs and is the aetiological agent of Glässer's disease. The pathogenicity of H. parasuis is poorly characterised, while prevention and control of Glässer's disease continues to be challenging. Understanding the pathogenicity of H. parasuis is essential for determining how this bacterium produces disease and to better distinguish between virulent and non-virulent strains. Infection by H. parasuis requires adhesion to and invasion of host cells, resistance to phagocytosis by macrophages, resistance to serum complement and induction of inflammation. Identification of virulence factors involved in these mechanisms has been limited by difficulties in producing mutants in H. parasuis. Recent advances in understanding the pathogenesis of H. parasuis are due in part to the production of deletion mutants, although most of the potential virulence factors described so far require further characterisation. Data supporting the role of lipooligosaccharide, capsule formation, porin proteins, cytolethal distending toxin and trimeric autotransporters (VtaA), among other molecules, in the virulence of H. parasuis have been described. This review provides an overview of the current knowledge of virulence factors of H. parasuis.