Capsular immunity is necessary for protection against some but not all strains of Glaesserella parasuis

Glaesserella parasuis is the causative agent of Glässer's disease in pigs and results in significant losses to the swine industry annually. Due to the serovar and strain specific response associated with many bacterin vaccines, there has been difficulty generating broad heterologous protection. Here, an unencapsulated G. parasuis mutant (HS069∆cap) was assessed as a bacterin vaccine and compared to a bacterin made from the encapsulated parent strain, against challenge with the homologous, parent strain (serovar 5) as well as four heterologous challenge strains (serovar 1, 4, 5, and 14). Both the HS069 and HS069∆cap bacterins generated high titers to the homologous and heterologous strains. The HS069∆cap bacterin was able to provide protection against the parent strain as well as 12939 (serovar 1), 2170B (serovar 4), and MN-H (serovar 13), but was unable to protect animals from challenge with Nagasaki (serovar 5). In contrast, the HS069 bacterin was able to provide protection against all challenge strains, showing the importance of serovar specific immunity against the challenge strain Nagasaki. This appears to be due to the production of a more abundant and well-organized capsule in Nagasaki as compared to HS069. This study indicates HS069∆cap is a good candidate strain for bacterin development; however, it may be less able to provide protection against highly encapsulated strains of G. parasuis.