The goals of the present thesis project were: (1) to improve the diagnosis of H. parasuis using a species-specific polymerase chain reaction (PCR); (2) to evaluate the epidemiology of H. parasuis in the field using phenotype and genotype-based techniques; and (3) to use epidemiological data to design control strategies to reduce nursery mortality. In Section B, Chapter 2, a PCR test was developed to improve the diagnosis of H. parasuis in clinical samples. The developed PCR was more sensitive than H. parasuis isolation, indicating that this test can be used to define of the prevalence of H. parasuis in affected herds. In Chapter 3, the within and between-herd epidemiology of H. parasuis were studied based on the characterization of isolates using serotyping and genotyping. Genotyping was more discriminative than serotyping for strain characterization and comparison. It was demonstrated that only 1 to 3 prevalent strains of H. parasuis were involved in nursery mortality in the studied herds and that herds with similar sources of animals shared similar prevalent strains. In Chapter 4, whole cell protein profiles of H. parasuis field isolates were studied using computer-based analysis. The existence of a unique group of major proteins in H. parasuis isolates recovered from affected animals was confirmed. In Chapter 5, an alternative model to study H. parasuis infections was developed and tested. The naturally-farrowed, colostrum-deprived (NFCD) model is a viable alternative to specific-pathogen free (SPF), cesarean-derived, colostrums-deprived (CDCD) and conventional pigs for vaccine testing and study of H. parasuis systemic infection. In Chapter 6, an uncapsulated H. parasuis vaccine was tested against homologous and heterologous challenges. There was no cross-protection between different H. parasuis strains sharing similar protein profiles. Chapters 7 and 8 evaluated the use of controlled exposure of young pigs to live, virulent H. parasuis strains as an alternative measure to control nursery mortality. This method successfully reduced mortality in the nursery caused by H. parasuis, being more effective than traditional methods such as commercial and autogenous vaccines.
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