Efficacy of PCV2 Vaccination Under Natural Conditions: A Longitudinal Study Using PCR and Virus Isolation

Abstract

Porcine circovirus type 2 (PCV2) is the main cause of porcine circovirus-associated disease (PCVAD). Despite the widespread use of anti-PCV2 vaccines, their efficacy varies, influenced by co-infection and evaluation methods. This study assessed the efficacy of Ingelvac CircoFLEX^® PCV2 vaccine under natural conditions. One hundred serum samples were collected from vaccinated and non-vaccinated piglets aged 21 to 173 days. PCR and antibody positivity rates did not show significant differences between the two groups, but PCV2 gene load at 91 days was significantly lower (p = 0.0095) in the vaccinated group. Anti-PCV2 antibody titers were also significantly lower in the vaccinated group at 91, 145, and 173 days (p < 0.0001). PCV2 was isolated from 50% of piglets in the non-vaccinated group (50%), compared with none (0%) in the vaccinated group, suggesting that PCV2 gene load in the non-vaccinated group did not correlate with viremia. Both groups were positive for antibodies to porcine reproductive and respiratory syndrome virus (PRRSV) at 63 days, prior to the surge in PCV2 gene load, suggesting PRRSV may enhance PCV2 replication. These findings highlight that while the vaccine reduced PCVAD damage, evaluation should incorporate methods such as virus isolation instead of relying solely on PCR.

Keywords: co-infection; porcine circovirus type 2; porcine reproductive and respiratory syndrome virus; vaccine.

Figure 1

Percentages of PCV2-positive serum samples in vaccinated and non-vaccinated pigs over time, as determined with qPCR. Serum samples were obtained from five pigs in each group from day 21 to day 145 and from 10 pigs in each group on day 173. * p < 0.05 for between-group comparisons by chi-square or Fisher’s exact test. Each bar represents the mean of three independent experiments.

Figure 2

Mean PCV2 gene copy number in vaccinated and non-vaccinated pigs over time, as measured by qPCR. Error bars represent standard error. * p < 0.05 for between-group comparisons by two-way (ANOVA) followed by Tukey’s test. Each point represents the mean of three independent experiments.

Figure 3

Mean anti-PCV2 antibody titers in vaccinated and non-vaccinated pigs over time, as measured using ELISAs. The Y-axis indicates unit values based on optical density (OD), measured at 450 nm. Antibody titers (units) were calculated based on a standard curve generated from serial dilutions of a reference serum provided by the company. The post-weaning mortality rate was significantly higher in the non-vaccinated group than in the vaccinated group (18.2% vs. 0%, p = 0.036). PCV2 antibody titers at 91, 145, and 173 days of age were significantly lower in the vaccinated than in the non-vaccinated group (p < 0.0001). †: Accidental death case of piglets in the non-vaccinated group. * p < 0.0001 for between-group comparisons. Each point represents the mean of three independent experiments.

Figure 4

Confirmation of intracellular PCV2 antigen in PCR-positive serum samples from vaccinated and non-vaccinated pigs using an IFA. Red boxes and red letters show positive results, with the letters *a, ^⸭b, and ٭c indicating negative controls, vaccinated, and non-vaccinated pigs, respectively. The results shown are from three independent experiments.

Figure 5

Mean anti-PRRSV antibody titers in vaccinated and non-vaccinated pigs over time, as measured using an ELISA. Each point represents the mean of three independent experiments.