| As the swine industry has adopted new technologies to raise pigs under high health conditions, there has been an increasing need to understand how pathogenic organisms are transmitted and why some diseases appear to be exacerbated in these systems.;Several experiments were carried out in order to answer some of these questions. Transmission of five common swine pathogens was studied at different levels: (1) airborne transmission of Actinobacillus pleuropneumoniae and porcine reproductive and respiratory syndrome (PRRS) virus, (2) horizontal transmission of Mycoplasma hyopneumoniae in nurseries with different pen partitions design, (3) transmission and persistence of pathogenic strains of Streptococcus suis and Haemophilus parasuis, and (4) vertical transmission of pathogenic strains of S. suis. Control of S. suis infection was further studied by experimentally exposing 5-day-old pigs to a pathogenic strain of S. suis.;Results of the studies presented in this thesis show that microbial transmission is a complex process. Airborne transmission of A. pleuropneumoniae was observed when pigs lacked antibodies against A. pleuropneumoniae -RTX-toxins. In the case of PRRS virus, transmission varied according to the strain used to inoculate the pigs.;Pen partition design (open vs. solid) was evaluated for its effect on preventing spread of M. hyopneumoniae in the nursery. Transmission was not affected by having solid partitions between pens. However, M. hyopneumoniae transmission appeared to proceed slowly, up to a time when all pigs became positive, suggesting that microbial excretion was non-linear. This raised questions on whether infected pigs can remain non-infectious during considerable periods of time.;Transmission of S. suis from sows to piglets was studied with special emphasis on colonization by specific strains responsible for causing mortality. Colonization of healthy pigs at weaning by these pathogenic strains appeared to be very low, and it was hypothesized that this was a risk factor for the pigs to develop disease later on. An experiment was designed to address this question where newborn piglets were experimentally exposed to pathogenic strains. Results from this study showed that early exposure to pathogenic strains decreased the prevalence of pigs with S. suis signs throughout the rest of the production period. |
