Persistent infections can occur with many pathogens, but reappearance is generally linked to waning immunity or antigenic shifts. Interestingly with PCV2, it is possible for circulating virus and neutralizing antibodies to co-exist in individual pigs. Vaccine-induced immunity may provide greater protection against subsequent viral challenges than existing immunity.

Herd level PCV2 control must consider the behavior of the virus in individual pigs. While there is still much to learn about PCV2, research has offered insights into the persistence of infection and the incompleteness of some aspects of immune protection.

Persistent PCV2 infections

Growing pigs. Long persistence has been reported from 69 days post-infection (confirmed by bioassay) to 209 days following natural exposure. Previously infected pigs can also test negative for viremia for up to 7 weeks before returning to positivity. These same pigs were also tested by pen-based oral fluids and remained consistently positive up to the end of monitoring 98 days post-infection, despite concurrent presence of antibody.

Sows. Sows infected at 56 days of gestation had PCV2 DNA in their colostrum after farrowing, including animals that had been vaccinated pre-challenge and that had mounted an immune response. Other research indicates that despite the presence of immunity, a high level of sow infection and vertical transmission has been found, suggesting PCV2 infection is lifelong. This could indeed be true, at least in some animals. The fact that viremia (or at least detection of viremia) becomes intermittent makes it practically impossible to confirm elimination of infection and explains why viremic piglets can be born to, by all appearances, non-viremic sows.

The persistence of infection is linked to the fact that immunity to PCV2 is not sterilizing. Antibody does not always prevent infection, nor does it eliminate existing infection. Persistent infections can occur with many pathogens, but reappearance is usually linked to waning immunity or some sort of antigenic shift. With PCV2, circulating virus and neutralizing antibody can co-exist, which is unusual but not unique.

When PCV2a/PCV2b co-infected pigs were inoculated with PCV2a at 11 weeks of age, PCV2b at 16 weeks, PCV2a again at 21 weeks, and PCV2b again at 26 weeks, low levels of PCV2b DNA were found in serum samples from two of six pigs at the final sampling at 31 weeks of age but not in any previous samples. One possible explanation is that the immune response generated by the initial infection, while not able to clear that infection, was more successful in preventing a second infection by a new strain.