Identification of novel potential virulence-associated factors in Haemophilus parasuis.

Haemophilus (H.) parasuis is best known as the cause of Glässer's disease, a potentially fatal polyserositis in pigs. The mechanisms underlying virulence differences on the molecular level are largely unknown to date. We have compared the serotype 5 (causes polyserositis) and 11 (described as avirulent) reference strains by modified representational difference analysis, and identified five potentially virulence associated factors present in the invasive serotype 5 strain, but not in the avirulent serotype 11 strain. Among these, a putative hemolysin operon, hhdBA, was identified, which is also present in half of the serotype reference strains described as virulent, but not in those reference strains that were reported to cause no disease in animal infection experiments. The presence of all identified genes was investigated in serotype reference strains as well as in 26 field isolates from clinically ill pigs. Determining the presence of these genes may be useful in H. parasuis diagnostics to judge a strain's potential to cause disease.