Studies have demonstrated that immune stimulation may trigger progression of PCV2 infection to disease and lesions characteristic of PMWS. Krakowka et al.(2001) reproduced PMWS in gnotobiotic pigs stimulated with KLH/ICFA and infected with PCV2. Allan et al. (2000) observed PMWS in 21% of PCV2-infected and vaccinated (M. hyopneumoniae at 13 and 31 days of age, and APP at 47 days of age) colostrum-fed pigs. Opriessnig et al., (2003) demonstrated enhanced PCV2 replication manifest as significantly longer PCV2 viremia, a higher copy number of PCV2 genome in serum, a wider range of tissue distribution of PCV2 antigen, and an increased severity of lymphoid depletion in conventional pigs vaccinated with commercial APP and M. hyopneumoniae vaccines compared to PCV2-inoculated, unvaccinated pigs. A study testing the effect of different adjuvants on PCV2-associated lesions found that oil-in-water adjutants increased the length of PCV2-viremia, increased the amount of PCV2 in serum and tissue, and increased the severity of lymphoid depletion compared to aqueous and aluminum hydroxide adjuvanted products (Hoogland et al., 2006). These experimental models suggest that immune stimulation in the form of adjuvanted vaccines may in some cases increase the incidence and severity of PMWS in PCV2-infected pigs. Field trials also support this hypothesis. Vaccination of piglets with a M. hyopneumoniae bacterin at 1 and 4 weeks of age resulted in a significant increase in losses associated with PMWS in the vaccinated pigs in two of five batches (Allan et al., 2001). Kyriakis et al.(2002) vaccinated a portion of the pigs in a herd experiencing a PMWS outbreak with a M. hyopneumoniae vaccine and observed PMWS in 43% of the vaccinated pigs compared with 11% of the non-vaccinated pigs. Vanderstichel et al. (2003) conducted a field trial in herds affected by PMWS. The authors used different adjuvanted M. hyopneumoniae products (two oil-in-water adjuvanted M. hyopneumoniae bacterins and one aluminum hydroxide adjuvanted bacterin). It was found that a higher number of the pigs treated with aluminum hydroxide developed PMWS compared to pigs treated with saline or with the oil-in-water adjuvant. It was concluded that the immunostimulation from the aluminum hydroxide adjuvant may have facilitated the development of clinical disease (Vanderstichel et al., 2003).
In contrast, another recent study using conventional pigs failed to demonstrate an immune-stimulation dependent difference in pigs inoculated with PCV2 and stimulated with KLH/ICFA (Ladekjær-Mikkelsen, 2002). Resendes et al. (2004) used a control group, a vaccine adjuvant group, a PCV2-infected group, and a group infected with PCV2 and injected with a vaccine adjuvant. The authors found no differences between groups (Resendes et al., 2004). Harvey(2003) reported on observational studies on two farms known to be infected by PMWS and found no significant difference between vaccinated and unvaccinated groups in terms of overall mortality rates or development of clinical PMWS. A large study using conventional pigs experimentally coinfected with PCV2 and M. hyopneumoniae demonstrated significantly increased weight gain and significantly less severe lung lesions in M. hyopneumoniae vaccinated pigs (Halbur et al., 2006).
Allan GM, McNeilly F, McNair I, O’Connor M, Meehan B, Gilpin D, Ellis J, Townsend H, Lasagna C, Boriosi G, Krakowka S: Neonatal vaccination for Mycoplasma hyopneumoniae and post-weaning multisystemic wasting syndrome: a field trial. Pig J. 48:34-41, 2001
Harvey, RE: The effect of vaccination against Mycoplasma hyopneumoniae on the mortality rate of growing pigs affected with post-weaning multisystemic wasting syndrome. Pig J. 52:201-205, 2003
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Hoogland MJ, Opriessnig T, Halbur PG: Effects of different adjuvants on porcine circovirus type 2-associated lesions. J Swine Health Prod. In Press, 2006
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Kyriakis SC, Saoulidis K, Lekkas S, Miliotis ChC, Papoutsis PA, Kennedy S: The effects of immuno-modulation on the clinical and pathological expression of postweaning multisystemic wasting syndrome. J Comp Path. 126:38-46, 2002
Ladekjær-Mikkelsen AS, Nielsen J, Stadejek T, Storgaard T, Krakowka S, Ellis J, McNeilly F, Allan G, Bøtner A: Reproduction of postweaning multisystemic wasting syndrome (PMWS) in immunostimulated and non-immunostimulated 3-week-old piglets experimentally infected with porcine circovirus type 2 (PCV2). Vet Microbiol. 89:97-114, 2002
Opriessnig T, Yu S, Gallup JM, Evans RB, Fenaux M, Pallares F, Thacker EL, Brockus CW, Ackermann MR, Thomas P, Meng XJ, Halbur PG: Effect of vaccination with selective bacterins on conventional pigs infected with type 2 porcine circovirus. Vet Pathol. 40:521-529, 2003
Resendes A, Segalés J, Balasch M, Calsamiglia M, Sibila M, Ellerbrok H, Mateu E, Plana-Durán J, Mankertz A, Domingo M: Lack of an effect of a commercial vaccine adjuvant on the development of postweaning multisystemic wasting syndrome (PMWS) in porcine circovirus type 2 (PCV2) experimentally infected conventional pigs. Vet Res. 35:83-90, 2004
Vanderstichel R, Hurnik D: The effect of a commercial vaccine and adjuvants on the prevalence of post-weaning multisystemic wasting syndrome and related circovirus lesions in a finishing barn. In: Proc Am Assoc Swine Vet. Orlando, Florida. 34:27-30, 2003