Treatment and vaccination protocols for control and prevention of Streptococcus suis serotype 2 infection in nursery pigs
Streptococcus suis (S. suis) is a ubiquitous gram positive streptococcus found in swine throughout the world and commonly causes septicemia, arthritis, meningitis, and has been reported to be an important cause of pneumonia in young pigs. Coinfection with porcine reproductive and respiratory syndrome virus (PRRSV) is common and especially problematic. Considerable research has been conducted to create a successful prevention method to prevent mortality associated with PRRSV/S. suis coinfection. The studies presented in this thesis evaluate the effectiveness of treatment and prevention protocols for acute infection of S. suis and PRRSV in nursery age pigs in a high morbidity and high mortality PRRSV/S. suis coinfection model. Two antimicrobials (ampicillin and ceftiofur), a modified-live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine, a low dose exposure to Streptococcus suis (S. suis), three experimental S. suis vaccine candidates, an autogenous S. suis bacterin, and a commercial S. suis vaccine were tested in this model to determine their effectiveness in reducing clinical disease associated with PRRSV/S. suis coinfection. The three experimental vaccine candidates tested included a ceftiofur-killed bacterin, a capsular polysaccharide conjugate vaccine, and a streptomycin-dependent S. suis mutant. The ceftiofur killed bacterin showed the greatest reduction in mortality of all experimental vaccines tested and approached a level of significance compared to the positive controls (p=0.06). To date, significant (p<0.05) reductions in mortality have been observed when pigs were treated for three consecutive days post S. suis challenge and every third day post S. suis challenge with ceftiofur hydrochloride and when pigs were exposed to a reduced dose of the challenge isolate 19 days before challenge. A vaccine based on reduced dose exposure to virulent S. suis would not be practical due to S. suis'zoonotic potential and residual virulence that was observed. These results are consistent with practitioner reports and field experience and suggest that the efficacy of vaccinating nursery pigs for S. suis is limited. More research on a better vaccine is clearly needed.