Haemophilus parasuis – virulence genes and serovars

Glässer’s disease, caused by Haemophilus parasuis, is a significant disease of pigs worldwide, causing polyserositis, polyarthritis and meningitis. There are 15 known serovars of H. parasuis (Olvera et al. 2006). The correlation between pathogenic and non-pathogenic isolates based on serovar, originally established by Kielstein and Rapp-Gabrielson (1992) through inoculation of pigs with the serovar reference strains, has been challenged (Olvera et al. 2006). This has prompted research into virulence genes and other genotyping methods to be able to predict virulence. The hypothesis of the current study was that there is a correlation between six potential virulence genes and the known virulence of the 15 serovar reference strains.

The reference strain for each of the 15 recognised serovars was examined via polymerase chain reaction (PCR) for the presence of the following potential virulence genes: vtaA (virulence-associated trimeric autotransporter), hhdAB (putative hemolysin operon), lsgB (lipopolysaccharide sialytransferase gene), fhuA (ferric hydroxamate receptor) and capD (polysaccharide biosynthesis protein). The virulence of each of these strains has already been determined by Kielstein and Rapp-Gabrielson (1992).

The results of the presence or absence of these virulence genes for all 15 reference strains are shown in Table 1. No single gene was present in all 10 pathogenic strains and was absent in all five non-pathogenic strains. The best correlation with any single gene was for vtaA, which was present in nine out of 10 pathogenic strains and absent in three out of five non-pathogenic strains. The next best correlation occurred with the hhdAB gene, which was present in six out of 10 pathogenic strains and absent in all 5 non-pathogenic strains. Overall, none of the tested genes by themselves or in combination were adequate to distinguish between pathogenic and non-pathogenic strains. Current studies were focussed on alternative typing technologies such as enterobacterial repetitive intergenic consensus sequence-based PCR and multi-locus sequence typing to see if these technologies are more useful in predicting pathogenicity in combination with the yes/no approach of the current work. Further, identification of the genes responsible for the high minimal inhibitory concentration (MIC) levels to some antimicrobials recently found in some Australian H. parasuis isolates (Dayao et al. 2014) is being sought.

Table 1.  Results of PCR assays for virulence genes for the serovar reference strains. The virulence is given as highly virulent ++, moderately virulent +, not virulent (according to Kielstein and Rapp-Gabrielson 1992)