Resumen de Effect of porcine circovirus 2 (pcv2) sow or piglet vaccination in different pcv2 subclinical infection scenarios
Porcine circovirus 2 (PCV2) is one of the most prevalent viruses that causes great economic losses to the worldwide pig industry. This virus has a ubiquitous nature in the pig population and is the causative agent of a number of clinical and subclinical conditions named Porcine circovirus diseases (PCVDs). The most important control tool for PCVDs is the vaccination, although it should be applied together with other measures such as proper farm management practices and control of potential risk factors for disease occurrence. During the last decade, commercial PCV2 vaccines have proven to be effective in controlling PCV2 infection, reducing PCV2 systemic disease (PCV2-SD) outbreaks and improving the production parameters. Currently, the use of these vaccines remains widespread, although it is mostly applied under PCV2 subclinical infection (PCV2-SI) scenarios. In this context, the present thesis aimed to investigate some still existing gaps on the potential influence of sow vaccination on reproductive outcomes as well as on the transfer of maternally derived cellular immunity. In addition, it was also intended to expand the knowledge on piglet vaccination by comparing the infection dynamics of different vaccination timings using both serum and oral fluid (OF) samples.
The first study sought to evaluate the effect of sow vaccination against PCV2 on reproductive parameters during two consecutive reproductive cycles. The study was performed in a PCV2 subclinically infected breeding herd (evidence of PCV2 circulation but absence of major reproductive problems). Ninety-four pregnant sows were primo-immunized with a commercial PCV2 vaccine and ninety-seven were injected with phosphate-buffered saline at 6 and 3 weeks before farrowing, and then boosted at 2 weeks before the second one. Blood samples were taken throughout the study to assess PCV2 DNA load and antibodies. At farrowing, main reproductive parameters and piglet vitality index were registered. In addition, in those litters with more than three mummified or stillborn piglets, microscopic examination and PCV2 antigen detection in foetal myocardium was done. Vaccinated sows showed significantly higher antibody levels compared to the non-vaccinated counterparts. PCV2 DNA was only detected at farrowing in 2 (4.2%) non-vaccinated sows. Vaccinated sows had 1.3 more live-born piglets per litter at the second cycle than non-vaccinated counterparts. Piglets from vaccinated sows had significantly higher (+12.7%) vitality score than the ones born from non-vaccinated sows. No PCV2 compatible lesions neither PCV2 antigen were detected in the tested foetal hearts. The present study represents the first attempt to demonstrate that PCV2 sow vaccination may have a positive influence on prolificacy and vitality of the offspring in a subclinically infected breeding herd.
In the second study, the effect of PCV2 sow vaccination on humoral and cell-mediated immune responses in sows and their progeny was assessed. At 7 weeks before farrowing, fifteen PCV2 PCR negative pregnant sows with medium-low antibody values were selected and randomly distributed in two groups according to the antibody levels. Seven sows were vaccinated with a commercial PCV2 vaccine and eight were injected with phosphate-buffered saline at 6 and 3 weeks before farrowing. Blood samples were taken from sows at farrowing and their offspring at 48-72 hours of life. PCV2 DNA and antibodies were tested in sera (n=90; 6 piglets per litter), and cytokine (IFN-α, IFN-γ, IL-12p40, TNF-α, IL-1β, IL-8, IL-4, IL-6 and IL-10) levels were assessed in supernatant (n=30; 2 piglets per litter) from cultured peripheral blood mononuclear cells. All sows and piglets were negative by PCV2 PCR throughout the study. Significantly higher PCV2 antibody levels were detected in vaccinated sows after vaccination and in their offspring after colostrum ingestion compared to the non-vaccinated counterparts. Vaccinated sows did not show significant differences in cytokine secretion levels at farrowing compared to unvaccinated dams. In contrast, piglets from vaccinated sows had significantly higher levels of cytokines linked to Th1 memory cells (IFN-γ and TNF-α) in comparison to the ones from non-vaccinated dams. In conclusion, PCV2 sow vaccination, apart from triggering a humoral immune response in sows and their progeny, might be associated to an increased transfer of cell-mediated immunity from the dam to the piglet.
The purpose of the third study was to determine the PCV2 serological and virological infection dynamics in piglets vaccinated at different ages in a PCV2-SI scenario. Six hundred and forty-four 2 week-old healthy piglets were selected and distributed into four treatment groups: vaccination at 3, 6 or 10 weeks of age (3W-VAC, 6W-VAC and 10W-VAC groups, respectively) and unvaccinated pigs (NON-VAC group). Blood from a subpopulation of pigs (n=112 animals) and OF (n=40 pens) samples were taken throughout the study to assess PCV2 load, humoral immunity and viral genotyping. Generally, percentage of PCV2-DNA positive sera raised mainly by 10 weeks of age, being maximum at 14 weeks of age, and then started to decrease at 18 and 25 weeks of age. Specifically, PCV2 vaccination at 3 or 6 weeks of age yielded similar results, since they produced an earlier seroconversion and reduced, at different sampling points, the proportion of viraemic animals in comparison to the unvaccinated group. In contrast, PCV2 vaccination at 10 weeks of age only achieved such reduction at 25 weeks of age; in this case, vaccination coincided with the increase of the percentage of viraemic pigs in the population. Both antibody detection techniques used in sera and OF offered similar results with a high and statistically significant correlation. On the other hand, a higher percentage of PCV2 DNA positivity was detected in OF in comparison with sera. In conclusion, under the present study conditions, the optimal time for piglet vaccination to control PCV2 infection was at either 3 or 6 weeks of age. In addition, OF proved to be a useful matrix for the evaluation of seroconversion dynamics, however, PCV2 DNA detection in OF did not show to be an effective method for the infection control assessment during the studied vaccine programs.
