On a commercial farrow-to-finish operation, a total of 1,792 weaned pigs at 18 days of age were included in four replicates of the trial. Study pigs were housed in nursery barns containing 64 pens (28 in each), with shared feeders and drinkers, for five to six weeks. Using a randomized complete block design, pigs were allocated to blocks and pens according to sex and weight. Pens were randomly assigned to either treatment with potassium penicillin G (Treated) in-water, or no treatment (Control). Treatment occurred at two time-points: upon moving to the nursery barn, and 21 days after moving into the nursery barn. Tonsillar swabs were taken for bacterial identification of S suis from two randomly selected pigs from each pen at three time-points: prior to treatment and one day after the completion of each treatment period. All pigs were observed for any clinical abnormalities at least twice each day beginning at assignment to the study. A final dose of 297,000 IU of potassium penicillin G per liter of water was administered via the drinking water for five days over two periods of treatment.
In total, 56.2% of the tonsillar swabs had at least one colony of S suis serotypes 2 or 1/2 per plate. At least one colony per plate was isolated from 50.5% and 62% of the swabs taken from Treated pigs and Control pigs, respectively. The results of the logistic regression analysis indicated that the odds of a positive swab for S suis in the Control group was 1.6 times higher than in the Treated pigs (P < 0.05). Swabs taken from the Control group had a 2.3 times greater probability of having ≥ 1000 S suis colonies per plate compared to having zero per plate, than swabs that were taken from the Treated pigs (P < 0.05). Overall, a total of 53 pigs in the Control group and 29 pigs in the Treated group died as a result of S suis infection. Mortalities from all causes included 59 (6.6%) pigs from the Control group and 32 (3.6%) in the Treated group. The risk of mortality due to S suis in the Control group was 1.9 times greater than in the Treated pigs (P < 0.05)). Similarly, risk of total mortality was significantly greater in the Controls as compared with the Treated group (Risk ratio = 1.9, P < 0.03).
Previous studies have found that S suis is frequently isolated from the tonsillar area of piglets before and after weaning. Attempts to remove the tonsillar carrier state of S suis in early-weaned pigs comparing several antimicrobials have not been successful. The implementation of control measures as described in this investigation may be a more appropriate action for the control of S suis infection. In the present study, risk of mortality from all causes, as well as bacterial counts of S suis from tonsillar swabs, were reduced with mass in-water treatment with potassium penicillin G. Results may have been moderated by the fact that the Treated and Control groups were housed in the same room, therefore sharing airspace and allowing for the possibility of a heavier overall challenge than if only the Treated pigs were housed together.
The results of this study of weaned pigs raised in typical on-farm conditions indicate that potassium penicillin G administered in drinking water is effective in reducing mortality associated with S. suis infection and reducing tonsillar carriage of S suis that could lead to disease and that may be a source of pig-to-pig transmission of infection.
C. Byra, P. Gadbois, W.R. Cox, M. Gottschalk, V. Farzan, S.A. Bauer, J.B. Wilson. Evaluation of potassium penicillin G administered in-water for the control of Streptococcus suis in weaned pigs. AASV, 2010.