Abstract
Porcine circovirus type 2 (PCV2) has a major economic impact on pig production. Vaccination against the virus appears to keep PCV2 disease (PCVD) manifestations in check. Nevertheless, PCV2 has not been eliminated. Several reports indicate that cattle also seem to be susceptible to PCV2 infection. In this study, we detected increased PCV2 cross-species transmission and suggest a strategy for identifying hidden PCV2 infection in cattle. We compared a postweaning multisystemic wasting syndrome (PMWS) distressed-farm with a subclinically-infected farm and investigated how differences in virus concentration would affect PCV2 transmission to a susceptible host such as cattle. The pig population of farm 1 had recurrent PMWS cases among weaners. The 34 cattle in proximity to the PMWS-affected pigs included 14 cattle (41%) that carried a PCV2 load of on average 2.5 x 105 virus genomes per ml blood. Among these was a 7 years old cow with chronic mastitis that was infected with the highest PCV2 load of 1.3 x 106 virus genomes per ml blood. These numbers contrasted with farm 2 that housed PCV2-subclinically infected pigs in proximity to 31 cattle of which six (19%) were infected with an average of 1.7 x 105 virus genome per ml blood. No PCV2-specific humoral response was measured in these cattle. Additionally, we encountered the problem of how to measure PCV2 latent/persistent infections. In pigs, after a one-shot vaccination, we have previously observed a direct anti-PCV2 IgG response instead of the expected IgM response. This indicated that the pig immune system was already primed with PCV2. We therefore hypothesized that cattle may also be PCV2- primed. Although the cattle’s humoral immune system hardly responded to vaccination, we detected PCV2-specific IgG antibody in one cow after the first vaccination shot. This, taken together with the PCV2 transmission rate in hybrid farms indicated sporadic infections in the cattle population. Thus, it can be concluded that PCV2 infections were not well established in cattle in comparison to the situation in swine.
Highlights
Porcine circovirus type 2 (PCV2) is the main etiological agent for porcine circovirus diseases (PCVD) including post weaning multisystemic wasting syndrome (PMWS) [1,2,3]
We have shown that pig foetuses already carry the virus in the thymus even before immune competence, which is central to the virus complex pathogenicity [5]
On farm 1, we found in close proximity to the pigs, 14 cattle (41%) out of 34 carrying virus loads with an average of 2.5 x 105 virus genomes per ml blood (Figure 1)
Summary
Porcine circovirus type 2 (PCV2) is the main etiological agent for porcine circovirus diseases (PCVD) including post weaning multisystemic wasting syndrome (PMWS) [1,2,3]. PCV2 is endemic in the pig population and can be found both in healthy as well as in diseased pigs [3,4]. The virus latent/ persistent presence [3,5] is a constant threat for the host immune system. PCV2 uses a measurable amount of host metabolism reflected by a reduction in pig’s daily weight gain [3,6]. Detection of anti-PCV2 IgG antibodies is not a reliable indicator of individual infection status [7]. In disease the anti-PCV2 IgG antibodies are often absent [8]. This, and the lack of a meaningful T-cell response against the PCV2 capsid protein [9] makes more sense in the light of our proposed pathogenicity mechanism [3]. We have shown that pig foetuses already carry the virus in the thymus even before immune competence, which is central to the virus complex pathogenicity [5]
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