No abstract

There are marked differences in the clinical expression of diseases associated with porcine circovirus type 2 (PCV2) in the field. The objective of this study was to compare the sequences and pathogenicity of PCV2 isolates from field cases with and without PCV2-associated lesions. Forty-two specific-pathogen-free (SPF) pigs were assigned randomly to three groups of 14 pigs each. At 7 weeks of age, group 1 pigs were mock-inoculated with saline, group 2 pigs were inoculated with PCV2-4838 (isolated from a pig with no evidence of PCV2-associated lymphoid lesions) and group 3 pigs were inoculated with PCV2-40895 (isolated from a pig with PCV2-associated lymphoid lesions and disease). The PCV2-4838 and PCV2-40895 isolates shared approximately 98?9 % nucleotide sequence identity across the entire genome. A total of nine amino acid changes in ORF2 and two amino acid changes in ORF1 were identified between the two isolates. PCV2-4838-inoculated pigs had significantly more genomic copy numbers of PCV2 in their sera at 7 days post-inoculation (p.i.) (P<0?0001) and significantly fewer genomic copy numbers at 14, 21 and 28 days p.i. (P<0?05) compared with pigs inoculated with the PCV2-40895 isolate. Microscopic lesions in lymphoid tissues were significantly less severe (P<0?05) and the amount of PCV2 antigen associated with these lesions was significantly lower (P<0?05) in pigs inoculated with PCV2-4838. The results of this study suggest that PCV2 isolates from the USA differ in virulence in an SPF pig model.

To determine the effects of porcine circovirus type 2 (PCV2) maternal antibodies on and response to experimental PCV2 infection, 24 piglets were divided into four groups on the basis of the enzyme-linked immunosorbent assay titers of PCV2 maternal antibodies: group A (n ‫؍‬ 6; sample/positive [S/P] ratio, <0.2), group B (n ‫؍‬ 5; S/P ratio, >0.2 to <0.5), and groups C (n ‫؍‬ 8) and D (n ‫؍‬ 5) (S/P ratio, >0.5). Piglets in groups A, B, and C were inoculated with PCV2 at day 0 and challenged with PCV2 at day 42. Group D piglets were not exposed to PCV2 at day 0 but were challenged at day 42. Before challenge, seroconversion to PCV2 antibodies occurred in five of six group A piglets, and the antibody level rose above the cutoff level in one of five group B piglets. Viremia was detected in five of six, four of five, and two of eight pigs in groups A, B, and C, respectively. After challenge, PCV2 DNA was detectable from 7 to 21 days postchallenge in the sera from six of six, four of five, three of eight, and five of five pigs in groups A, B, C, and D, respectively. The results indicated that protection against PCV2 infection conferred by maternal antibodies is titer dependent: higher titers are generally protective, but low titers are not.