Susceptibility of pig embryos to porcine circovirus type 2 infection

The aim of the present study was to determine if porcine circovirus type 2 (PCV2) is able to infect embryonic cells of in vivo produced porcine embryos with and without zona pellucida (ZP). ZP-intact and ZP-free morulae (6-day post-insemination) and early blastocysts (7-day post-insemination), and hatched blastocysts (8-day post-insemination) were exposed to 10(5.0) TCID50 PCV2 per ml (strain 1121, fifth passage PK15). At 48 h post-incubation, the percentage of infected embryos and the percentage of viral antigen-positive cells per embryo were determined by indirect immunofluorescence (IF). Significantly different percentages of infected embryos were detected: 15% for ZP-free morulae, 50% for ZP-free early blastocysts and 100% for hatched blastocysts. The percentage of cells that expressed viral antigens was similar for the three stages of development. PCV2 exposure did not affect the in vitro development of the embryos during the 48 h study period. All ZP-intact embryos remained negative for viral antigens. In an additional experiment the diameter of the channels in the porcine ZP was determined. After incubation of early blastocysts with fluorescent microspheres of three different sizes, beads with a diameter of 20 nm and beads with a diameter of 26 nm crossed the zona whereas beads with a diameter of 200 nm did not. In conclusion, it can be stated that PCV2 is able to replicate in in vivo produced ZP-free morulae and blastocysts and that the susceptibility increases during development. The ZP forms a barrier to PCV2 infection, but based on the size of the channels in the ZP the possibility that PCV2 particles cross the ZP cannot be excluded.