28 June 2022: Disseration of Carlos Neila (IRTA): Epidemiological and pathological studies for Streptococcus Suis.

Streptococcus suis is a major swine pathogen with a worldwide distribution. Despite being a natural inhabitant of the pig respiratory tract, it can sometimes cause disease in weaning piglets, characterized by arthritis, meningitis, and/or sudden death. It is also a zoonotic pathogen, particularly important in Southeastern Asian countries. The impact of the disease in porcine production seems to be significant, but there are no accurate estimations. In this thesis the prevalence and cost of the S. suis-associated disease were estimated for three of the main pig-producing countries in Europe: Germany, the Netherlands, and Spain (Study I). Our results showed that the disease is highly prevalent in pig farms in these countries, with substantial differences in prevalence and mortality between phases, and higher prevalence in the post-weaning period. Costs were higher in Germany, followed by the Netherlands and Spain, and they were essentially explained by the measures applied to prevent or control S. suis (autovaccines and antimicrobials), although the value of the animals that die because of the pathogen was also important in the estimation, especially in the post-weaning period. Pigs in commercial farms get colonized by S. suis early in life, but only a fraction of farms and pigs develop the disease later. Because of the endemic aspect of S. suis and the estimated prevalence, different factors that have traditionally been associated with the appearance of
S. suis disease were studied in two Catalan farms. Results showed that animal stress and higher humidity before weaning, presence of porcine respiratory and reproductive syndrome virus, lower temperature after weaning, and lower parity of the dams increased the risk of developing clinical signs associated with the disease. Host susceptibility and the virulence of the strains present in the farms also play an important role in the development of the disease. The pig response against this early colonizer was studied in cesarean-derived colostrum-deprived piglets intranasally inoculated either with a virulent or a non-virulent S. suis strain. This infection allowed the observation for the first time of the innate immune response against the colonization in nasal mucosa, blood, lung, trachea, liver, and spleen by transcriptional analysis. On the one hand, the host response to the nonvirulent S. suis strain was characterized by rapid control at the site of inoculation. On the other hand, the piglet local inflammatory response was maintained longer in nasal mucosa for the virulent strain, demonstrating that the host response depends on the virulence of the strain. In addition, the virulent strain was detected deep in the cribriform plate of the ethmoid bone, suggesting that this may be an alternative route of invasion for S. suis. As indicated above, antimicrobials are one of the tools used for control of S. suis disease, with the consequent problems in the emergence and spread of resistances. Recently, the microbiota was identified as a factor involved in health. Thus, in order to face the current challenge of reducing the use of antimicrobials in livestock, the protective efficacy of a mixture of nasal probiotics was tested against a lethal challenge with a virulent S. suis strain. Commensals were selected based on different criteria, such as relative abundant in healthy piglets, in vitro S. suis inhibition, or adherence ability. Despite the lack of significant differences between groups, piglets inoculated with a combination of three commensals showed the best survival rate and less severe clinical signs and lesions after challenge. This study lays the ground for future studies with porcine probiotics, an alternative to prevent the diseases caused by pathogens that are early colonizers of the upper respiratory tract.