Genetic Diversity of Haemophilus parasuis14 May 2014
New research by the University of Guelph in Canada and presented at the 33rd Centralia Swine Research Update in January 2014 sheds further light the pathogen most associated with Glässer’s disease.
Glässer’s disease, caused by the bacterium Haemophilus parasuis, is a major source of loss in the swine industry, reported Patrick Boerlin. The majority of pigs carry H. parasuis in their nose, most frequently without any sign of disease.
Our lack of understanding of the factors contributing to the development of Glässer’s disease makes its control difficult. It is assumed that environmental, host and pathogen factors interact to trigger the disease, but these factors are still unknown.
The objectives of this study were to develop and apply a new efficient typing method for H. parasuis, and to assess the presence of associations between newly described virulence factors and the propensity of H. parasuis strains to cause disease.
Materials and Methods
A H. parasuis collection consisting of 54 clinical isolates, 25 isolates from the nose of healthy pigs, and the reference strains of the 15 known serotypes was set up.
A new multilocus variable number of tandem repeat analysis (MLVA) scheme was developed and used to type the 94 isolates from the collection.
The isolates were also tested by PCR for the presence of nine recently described putative virulence genes.
The presence of association between the source of the isolates (i.e. sick versus healthy pigs) and specific MLVA types, clusters of types or putative virulence factors was assessed.
Fifty-four MLVA types were identified among the 94 isolates tested (discrimination index of 97.4 per cent).
These types clustered into two major genetic groups. Serotype 5, 13 and 14 all belong to one of the groups and serotype 2 isolates to the other, while isolates of serotype 4 where evenly distributed between the two groups. A strong association was observed between seven out of the nine putative virulence genes investigated and one of the two genetic groups.
However, no significant association was observed between the source of the isolates and the two major genetic groups or the presence of specific virulence genes. One single MLVA type lacking the majority of the putative virulence genes investigated contained 20 per cent of all the clinical isolates examined.
The results of this study also suggest that exchange of genetic material occurs between strains within each one of the two major genetic groups but not frequently between the two groups.
Overall, the results of this study demonstrate the diversity of the H. parasuis population present in Ontario and the ability of MLVA to type large numbers of H. parasuis isolates efficiently at an affordable cost.
The results also show the lack of correlation between H. parasuis genetic groups or the presence of recently described putative virulence genes and the ability to cause disease in pigs.
However, this study has identified a specific H. parasuis strains frequently involved in disease, which may be worth investigating in more details to identify what makes it different from the others.
An efficient and affordable typing method was developed in the course of this study, which will allow the tracing of H. parasuis strains and provide a better understanding of the transmission and epidemiology of this pathogen at the animal, farm, and local level.
Furthermore, this study has identified candidate strains for comparative genomics studies needed for the identification of new virulence genes potentially important for control strategies for Glässer’s disease.
Acknowledgments This research project was funded by NSERC. The authors thank G. Chalmers, V. Nicholson, and G. Soltes for their technical assistance in this project.
Boerlin P., J. MacInnes, Z. Poljak and J. Gallant. 2014. Genetic diversity of Haemophilus parasuis. Proceedings of 33rd Centralia Swine Research Update.