Resumen de New insights on PCV2 vaccination: thinking out of the box
Hua Feng
Porcine circovirus type 2 (PCV2) is the etiological agent of PCV2 systemic disease (PCV2-SD), which causes severe clinical signs in nursery and fattening pigs and, in consequence, leads to considerable economic losses. Besides PCV2-SD, this virus is also associated to other diseases, named collectively as porcine circovirus diseases (PCVDs). Nowadays, the control of PCVDs depends on vaccines and, to a lesser degree, on improving management strategies as well as controlling risk factors. Until now, a number of commercial vaccines are available in the swine production industry worldwide. Most of those products are based on PCV2a genotypes, and have been shown to be highly effective in controlling PCV2 infection, PCVDs and improve productive parameters. This thesis aimed to assess two poorly or non-explored aspects related with PCV2 vaccination. On one hand, the putative interference of different maternally derived antibody (MDA) levels at the time of vaccination on the average daily weight gain (ADWG) evolution was studied. On the other hand, the possibility of PCV2 eradication by means of a mass vaccination strategy was tested for the first time. In both studies, the same subunit PCV2 vaccine was used. The first study aimed to compare the efficacy of a PCV2 commercial vaccine in terms of seroconversion, infection dynamics and ADWG in pigs with different MDA levels. A total of 337 animals from a PCV2 subclinically infected farm were distributed into two groups based on weight and PCV2 antibody levels (high [H] or low [L]) at 2 weeks of age. One week later, these animals were subdivided in four groups according to the treatment received. Vaccinated (V) pigs (H-V and L-V) received 1 mL of a commercial vaccine and non-vaccinated (NV) ones (H-NV and L-NV) received 1 mL of PBS. All piglets were subsequently bled at 7, 12, 18, 22 and 25 weeks of age and weighted at 12 and 25 weeks of age. V animals showed significantly lower PCV2 infection rate and viral load, and higher ELISA S/P ratios and ADWG than NV ones. Compared with H-V piglets, L-V pigs showed a numerically lower PCV2 infection rates, lower area under the curve of viral load, an earlier seroconversion and a numerically higher (but no statistically significant) ADWG. The worst growth rates were observed in the L-NV group. In this study, MDA did not seem to interfere significantly with the effect of PCV2 vaccination on ADWG. However, only when a small subpopulation of pigs with the highest ELISA S/P ratios was considered, an apparent interference of vaccine efficacy on ADWG was noticed. Therefore, the impact of this possible interference under field conditions is probably negligible for most farms. In the second study, the feasibility to eradicate PCV2 in a conventional PCV2 infected farm by vaccinating both sows and piglets using a commercially subunit vaccine was assessed. Vaccination strategy implied that all sows, boars and gilts of the farm were vaccinated every four months, and all piglets were vaccinated and revaccinated with the same vaccine at 4 and 7 weeks of age, respectively. This vaccination strategy was applied during 12 consecutive months. Blood samples from 15 piglets of 4, 8, 12, 16, 20 and 24 weeks of age and 15 sows were taken monthly PRE, DURING and POST mass vaccination strategy. From all the collected sera (n = 1796), a representative proportion of them (n = 1235, 69%) were analysed (n = 1121 from piglets and n = 114 from sows). All these samples were tested by PCV2 ELISA and PCV2 PCR (and quantitative-PCR when PCR positive). All tested sows were negative by PCR but seropositive. ELISA mean OD values of sows decreased throughout the study. Percentages of PCV2 PCR positive samples in piglets were 8% (12/150), 0.9% (6/659) and 3.5% (11/312) PRE, DURING and POST application of the mass vaccination program, respectively. ELISA mean OD values of PCV2 seropositive animals progressively decreased until the end of the mass vaccination period, but a clear seroconversion was observed after stopping such strategy. In conclusion, one year period of mass PCV2 vaccination (without implementing further farm management practices or biosafety measures) was not able to clear out PCV2 infection. In deed the virus became detectable again when vaccination was stopped.
