Vaccine efficacy against PCV2-related reproductive pathology in gilts

PCV2 is involved in reproductive failure in swine that includes clinical and subclinical forms. Subclinical PCV2 in utero infection is diagnosed by the detection of PCV2 DNA or specific antibodies in foetal tissues, presuckling serum or foetal thoracic fluid without the presence of microscopic lesions or indication of reproductive failure. We have evaluated the efficacy of two vaccines against subclinical PCV2-related reproductive pathology and detected modifications in the classic target (lymphoid tissues) of the infected gilts. Specific Ab titres in serum, viraemia, foetus and foetal membranes/fluids samples positivity to PCV2 were compared in four groups of conventional gilts. Six gilts (group VAI) received 2mL, IM of a commercial inactivated PCV2 vaccine licensed for sows and piglets and 6 gilts (group VBI) received 1 mL, IM of a commercial vaccine based on an ORF2 capsid protein expressed in a baculovirus system, and licensed for use in piglets. Both vaccines were administered in gilts at 120 and 150 days of life. Nine additional gilts (group NVI, n=6 and group CTR, n=3) were kept unvaccinated. All animals received Regumate® for 18 days followed by an oestrus synchronization and superovulation protocol. Gilts in VAI, VBI and NVI groups were inseminated with a double (24 h apart) dose of PCV2-negative semen spiked with a PCV2b (0.2 ml of suspension containing 103.9 TCID50/25 μl of virus) strain isolated in a PMWS outbreak in Italy. CTR gilts were fecundated with a double dose of PCV2-free semen. Necropsies were performed 30 days or 54 days following insemination depending on the gestational status of the gilts: samples of lymph nodes (Lfns: superficial inguinal, mesometrial, tracheobronchial and mesenteric), tonsils and spleen in the dam as well as placenta, amniotic fluid and tissues (heart, liver, spleen) from foetuses were collected for histology, immunohistochemistry to PCV2 and its quantitation by real time PCR. Lymphoid tissue was graded according to a previous system. Pearson Chisquare test was used for statistic. No statistically significant differences in antibody titres nor viraemia were revealed among groups during the trial. Mainly mesometrial (3/12 VAI; 4/12 VBI; 5/12 NVI; 0/6 CTR) Lfns, and in a lesser extent superficial inguinal (1/12 NVI) Lfns, showed grade 3 (depletion and presence of giant cells) while the other lymphoid tissues were normal or at least with grade 1 depletion. Four out of 6 gilts were pregnant in the VAI group, 3 out of 6 in VBI, 3 out of 6 in NVI and 2 out of 3 in CTR, from which were collected 23, 19, 33 and 15 foetuses, respectively and the corresponding placenta membranes and amniotic fluid. CTR group showed the lowest positivity proportion of all sample types compared to the other groups. In the experimentally infected animals, the percentage of positive fetuses was significantly lower in the 2 vaccinated groups compared to NVI, while the percentage of PCV2 positive placentas and amniotic fluids was significantly lower in VAI compared to VBI. Immunohistochemical stain to PCV2 was found in one foetus of the VBI group in placenta, heart and liver and in the superficial inguinal lymph node of a gilt of the NVI group. From the results of the trial it was successfully produced a subclinical form of PCV2 reproductive failure, in which it was possible to demonstrate the presence of PCV2 specific lesion mainly in loco-regional Lfns (those mesometrial). In the present experimental conditions with a very severe challenge, vaccination with a commercial inactivated PCV2 vaccine licensed for sows and piglets (vaccine A) did not eliminate but significantly reduced the risk of foetus infection. Considering the most common way of foetus infection through the foetal membranes/fluids, the protective role of the two vaccines against the subclinical form of PCV2-associated reproductive failure does not seem to be similar. The lowest proportion of placentas and amniotic fluid infected in VAI compared to VBI groups may explain the lowest foetal positivity to PCV2 in this group of gilts compared to NVI animals.