Evaluation of Porcine Circovirus type 2 vaccine

Abstract

Porcine circovirus type 2 (PCV2) is one of the most important swine infectious pathogen which cause a variety of swine diseases called porcine circovirus associated diseases (PCVAD) such as postweaning multisystemic wasting syndrome (PMWS), porcine respiratory disease complex (PRDC), porcine dermatitis, nephropathy syndrome (PDNS), and reproductive associated disorder.Vaccination against PCV2 is nowdays the most effective tool for preventing the PCVAD. Since the first PCV2 vaccine was introduced in 2004, the six commercial PCV2 vaccine have been registered worldwide up to date. These vaccines were largely classified in inactivated whole virus vaccine, capsid protein recombinant vaccine, and chimeric PCV1/PCV2 vaccine according to antigen types. The objectives of these studies were to evaluate the PCV2 vaccines based on clinical, virological, immunological, and pathological analyses and investigate the difference of efficacy among the vaccines. The experimental challenge study with a reformulated inactivated chimeric PCV1-2 vaccine was perfomed for 42 days to investigate the vaccine efficacy about humoral and cell-mediated immunity. Vaccinated challenged animals had a significantly lower number of genomic copies of PCV2 in the blood than non-vaccinated challenged animals at 14 and 28 days post challenge (dpc

P < 0.001). The percentage of viremic pigs was significantly lower in vaccinated challenged animals compared to non-vaccinated challenged animals (P < 0.05). The Neutralizing antibody (NA) titers were significantly higher in vaccinated challenged and vaccinated non-challenged animals than in non-vaccinated challenged animals at 0, 14, and 28 dpc. The mean numbers of PCV2-specific IFN-r-SCs were significantly higher in vaccinated challenged and vaccinated non-challenged animals compared to non-vaccinated challenged animals at 0 and 14 dpc(P < 0.05). The vaccinated animals displayed significantly greater PCV2-specific DTH responses than the non-vaccinated animals (P < 0.01). The results of the present study demonstrated the protective immunity induced by the reformulated inactivated chimeric PCV1-2 vaccine correlated with the reduction of PCV2 viremia. The efficacy of the reformulated inactivated chimeric PCV 1-2 vaccine was evaluated under field conditions. Three farms were selected based on their history of postweaning multisystemic wasting syndrome (PMWS). On each farm, a total of 50 3-week-old pigs were randomly allocated to one of two treatment groups: (i) vaccinated at 3 weeks of age and (ii) non-vaccinated. In case of field trial, it is most important to investigate for vaccine efficacy of economical aspects. Clinical examination indicated that vaccinated animals displayed an improved average daily weight gain (ADWG) (+47.3 grams/day

P < 0.05) and a reduced time to market (-6 days

P < 0.05). Virological and pathological examinations indicated that vaccinated animals displayed a reduced PCV2 viremia and associated lesion compared to non-vaccinated animals. Immunological examination indicated that vaccinated animals induced PCV2-specific NAs and IFN-r-SCs. The vaccine not only successfully induced the humoral and cell-mediated immune response but also improved the average daily weight gain which is criteria for vaccine efficacy of production improvement. Studies for comparion of PCV2 vaccines efficacy were performed to investigate different level of immune responses induced by 4 commercial vaccines. These studies were divided by two experiments. The objective of first experiment was to compare the vaccines without challenge for investigating a tendancy of vaccines such as duration of persistent efficacy and time to indicate the peak level of efficacy. Inactivated chimeric PCV1-2 vaccines induced higher levels of PCV2-specific NAs and IFN-r-SCs in pigs than did the other 3 commercial PCV2 vaccines. The proportions of CD4+ cells were significantly higher in animals vaccinated with inactivated whole virus and chimeric PCV 1-2 vaccines than in animals vaccinated with the 2 subunit vaccines. Maximal induction of cell-mediated immunity is reached at 21 days post vaccination (dpv), whereas maximal induction of humoral immunity is reached at 42 dpv in all four vaccines. In second experiment, protective efficacy induced by pcv2 vaccines were compared through a challenge of single PCV2. The groups vaccinated with inactivated whole virus and chimeric PCV 1-2 vaccines exhibited significantly higher NA titers, number of IFN-r-SCs, and proportion of CD4+ lymphocytes than the other 2 vaccinated groups (P < 0.05). The groups vaccinated with inactivated whole virus and chimeric PCV 1-2 vaccines reduced significantly more viral load in blood than another subunit vaccine group (P < 0.05). These studies demonstrated that the different types of antigens in the 4 commercial single-dose PCV2 vaccines induce different levels of protective immune responses. The effects of PCV2 vaccines on PCV2 virus in experimentally infected boars were evaluated and compared based on virus shedding in semen. In the first experiment, the vaccinated group were immunized with an inactivated PCV2 vaccine at a 3-week interval, and then, 3 weeks after the second vaccination, the boars were intranasally inoculated with PCV2b. Serum and semen samples were collected for 60 dpc. The number of PCV2 genome copies of PCV2 in the serum and semen were significantly lower in vaccinated challenged boars than in non-vaccinated challenged boars at 7 to 60 dpc (P < 0.05). In the second experiment, efficacy of three commercial vaccines were compared in boars for 70 dpc. Serum and semen samples from the group vaccinated with chimeric PCV 1-2 vaccine had significantly decreased PCV2 genomic copy numbers compared with the goup vaccinated with subunit vaccine at 21 and 28 dpc (P < 0.05). Consequently, the vaccination protocol reduced the amount of PCV2 DNA shed in the semen. However, there was a significantly different amount of PCV2 DNA shed in semen among the 3 vaccinated groups.