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Infection with the bacterium Streptococcus suis (S. suis) may cause severe clinical signs in pigs, resulting in animal welfare problems, economic losses and abundant use of antibiotics. S. suis can also cause infections in humans. Various strains are distinguished (serotypes) of which serotype 2 is most often associated with clinical disease. The role of another serotype, i.e. serotype 9, in clinical cases has, however, increased substantially in the last two decades, and this serotype is now considered to predominate. It seems that a ‘shift’ between serotypes in the commercial pig holdings has occurred. The objectives of this thesis were to determine whether a shift in serotypes may have occurred, and to quantify the efficacy of intervention measures against serotype 9 transmission between pigs. First, a multiplex real-time quantitative PCR test was developed to enable measuring colonization of pigs with S. suis and transmission. The test was validated and it was determined whether it could differentiate between serotypes 2 and 9. In addition a hypothesis for the shift from serotype 2 to 9 was tested. This hypothesis assumed that the presence of one serotype affects the other serotype with respect to capacity of colonization and induction of clinical signs. In an experimental transmission study in pigs it was shown that simultaneous exposure of pigs to serotypes 2 and 9 increased survival of pigs after colonization in comparison to exposure to serotype 2 only. The number of bacteria of serotype 2 in the tonsils were significantly lower in the dually exposed pigs than in the pigs exposed to serotype 2 only, which might explain the observed findings. To find an alternative strategy for the abundant use of conventional antibiotics, several intervention measures were investigated: phage lysins application, vaccination of piglets with bacterins, and spatial separation between pigs. Several transmission studies were performed. In each design (about) half of the number of pigs was inoculated intranasally with serotype 9, and the other half contact-exposed. Oral and nasal application of phage lysins as well as vaccination of piglets did neither reduce spread of the pathogen, nor the capacity of serotype 9 to induce clinical disease. Spatial separation of pigs reduced the spread of serotype 9 among pigs. The reduction was, however, to such a level, that it is unlikely this intervention measure can effectively reduce the spread on commercial farms. The results of this thesis suggest that the presence of serotype 9 reduces the capacity of serotype 2 to cause mortality of pigs. This may have contributed to a shift in predominance from serotype 2 to 9. This shift may also be promoted by the extensively applied vaccination against serotype 2 in the last two decades. The clinical protective efficacy of vaccination seems lower for serotype 9 than for serotype 2. This thesis further shows that it is difficult to control the spread of serotype 9 by the intervention measures studied in this PhD research project.