Experimental exposure of young pigs using a pathogenic strain of Streptococcus suis serotype 2 and evaluation of this method for disease prevention

Control of Streptococcus suis infections and associated disease have proven to be a difficult challenge under most farm conditions. The objective of t his study was to experimentally expose young pigs with a pathogenic strain of S. suis serotype 2 as a means of controlling the disease in a commercial swine farm. Prior to the start of the study, the pathogenic S. suis strain responsible for mortality in the farm was identified and used to experimen tally inoculate baby piglets. Over a 3-week period, groups of pigs were sel ected (100 pigs/wk) and divided into 2 groups: control (50 pigs/week) and e xperimentally exposed (50 pigs/week). Pigs in the experimentally exposed gr oup were inoculated at 5 d old by tonsillar swabbing with the pathogenic S. suis farm isolate. The effect of exposure with this pathogenic strain was evaluated during the nursery and finishing stages and was based on: morbidi ty (pigs with central nervous signs (CNS) and/or lameness), mortality and n umber of treatments required by pigs that had either CNS or lameness. The r elative risk (RR) of acquiring disease due to S. suis infection was also ca lculated. Results showed that morbidity in the experimentally exposed group s was lower than in the control group and these results were statistically different (P = 0.006). Experimentally exposed pigs also showed a statistica lly significant reduction in lameness problems (P = 0.012), but not in CNS (P = 0.20) or mortality (P = 0.59). Pigs in the control group had an increa sed RR of 4.76, 8.77 and 2.7 for morbidity, to have lameness or to have CNS signs, respectively. In conclusion, experimental exposure of young pigs wi th the farm's pathogenic S. suis strain at a young age, had a positive effe ct in reducing clinical signs characteristics of S. suis infection. This me thod constitutes a novel approach to the control of S. suis infections in s wine farms.