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Are PCV2-Cattle Infections at an Early Evolutionary Virus Adaption Stage?

Xaver Sidler, Titus Sydler, Michael Hässig and Enrico Brugnera

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.