Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 15, 2002
Publication Date: N/A
Interpretive Summary: Porcine reproductive and respiratory syndrome (PRRS) is one of the most costly diseases ever faced by the United States swine industry. Although attenuated PRRS virus (PRRSV) vaccines have been shown to be highly effective under experimental conditions, they have sometimes been less effective in the field. It has been suggested that because of antigenic differences among strains of PRRSV, multi-strain vaccines might be more effective than the commonly used single-strain vaccines. In this study we compared a single-strain and a multi-strain vaccine in regard to both efficacy and safety. Under our experimental conditions both types of vaccine provided adequate protective immunity to subsequent exposure of vaccinated pigs to a highly virulent (disease producing) and unrelated strain of PRRSV. However, the potential for multi-strain vaccine to cause some degree of eventual immune dysfunction was suggested by the fact that lymph nodes became appreciably enlarged following vaccination of young pigs, presumably because such pigs had difficulty in simultaneously bringing several attenuated strains under control. Therefore it was concluded that additional safety testing would be necessary before multi-strain vaccines could be recommended for routine use.
Technical Abstract: The objective of this study was to compare the efficacy and safety of single-strain and multi-strain vaccines for the prevention of the respiratory facet of porcine reproductive and respiratory syndrome. The study comprised 6 groups of pigs (A through F, 8 pigs/group). At the beginning of the study (day 0) groups C and D were vaccinated with a single-strain vaccine, and groups E and F were vaccinated with a multi-strain vaccine. The multi-strain vaccine contained 5 attenuated strains of PRRSV including the strain used as the single-strain vaccine. On day 28 groups B (nonvaccinated/challenge control), D, and F were challenged with a highly virulent field strain of PRRSV that was unrelated to any of the strains used for vaccination. Group A was kept as a nonvaccinated/nonchallenged control. On day 42 all pigs were necropsied. Their lungs and lymph nodes were examined for virus-induced changes. Serum samples obtained at weekly intervals during the study and lung lavage fluids obtained at necropsy were tested for the presence and titer of PRRSV. Serum samples were also tested for antibody. The presence and severity of clinical signs and lesions were the primary means by which vaccine efficacy and safety were evaluated. Both vaccines provided a high level of protective immunity to challenge. However, appreciable lymph node enlargement in pigs vaccinated with multi-strain vaccine, with or without subsequent challenge, raised a question as to its safety. Collectively these results indicate that both single-strain and multi-strain attenuated PRRSV vaccines can be effective immunogens, but additional studies in regard to safety are needed before multi-strain vaccines can be recommended for routine field use.