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.
Virulent strains of H. parasuis, whichare able to produce Glässer’s disease, need a combination of virulence factors, most of which are still unknown, that allow them to reach and multiply in the lung for subsequent systemic spread. Bacteria then replicate on serosal surfaces, causing inflammation and damage to the host. Identification of the essential factors for attachment, evasion of the host immune system and systemic invasion will provide a better understanding of H. parasuis pathogenicity for the rational design of effective vaccines. In addition, the definition of the virulence factors of H. parasuis would greatly improve diagnosis by differentiating virulent from non-virulent strains.
More studies are needed to demonstrate the specific roles of each protein in the virulence of H. parasuis. In particular, the roles of polysaccharides should be further explored in order to determine the contribution of the capsule and biofilms to H. parasuis pathogenesis.
Costa-Hurtado M, Aragon V. Advances in the quest for virulence factors of Haemophilus parasuis. Vet J. 2013 Dec;198(3):571-6.