Vaccine candidates for swine dysentery prophylaxis

THE National Pig Association considers swine dysentery as a significant concern for the pig industry. The emergence of weakly haemolytic strains of the causative bacterium Brachyspira hyodysenteriae has complicated matters because asymptomatic but positive pigs are difficult to trade. However, the appearance of these strains has also reignited an ongoing debate about using them as naturally attenuated oral vaccines against this economically devastating disease.1, 2 Yet an in vitro growth experiment shows that even if these strains were confirmed to be irreversibly attenuated, they would likely not induce immunity extending beyond the specific B hyodysenteriae strain or serotype in pigs. For this experiment, two well-described strains of B hyodysenteriae were used: the strongly haemolytic virulent strain B204, which was initially isolated in the USA in the 1970s, and the weakly haemolytic strain JR80, which was isolated from a pig with mild non-bloody diarrhoea in Germany in 2015. Strain B204 belongs to serotype 2, serogroup B. The serotype and serogroup of strain JR80 are not known. For the growth experiments, growth medium was supplemented to a final concentration of 10 per cent with different sera: horse serum; pig serum from a German high-health breeding herd (JR80 had been cultured from the conventionally kept offspring of this herd); serum from the same herd, heat-inactivated; and serum from the same herd, preadsorbed with strain JR80. The growth curves in the horse medium showed that JR80 has a lower duplication rate, meaning it reaches a lower optical density at 600 nm (OD600) after 22 hours (Fig 1). This was also reflected in the growth experiments involving different sera as supplements. As shown in Fig 2, the pig medium did not support the growth of strain JR80. In contrast, strain B204 did grow with this serum supplement, albeit to a somewhat lower OD600 than in the horse medium. Heat inactivation or preadsorption with strain JR80 resulted in the regular growth of B hyodysenteriae. Results demonstrate that the pig serum from the high-health pig herd contains immune factors, most probably antibodies, that are directed against the weakly haemolytic B hyodysenteriae strain JR80 but not against strain B204. “Long-term exposure to weakly haemolytic B hyodysen-teriae can induce adaptive immunity in pigs