PCV2 has been highly prevalent in the pig population for decades, prior to the emergence of associated clinical disease manifestations that severely affected the pig production worldwide in the late 90s. PCV2 can be further subdivided into several genotypes. From descriptive epidemiologic data, there is evidence of a global shift of the main PCV2 genotypes in different countries from PCV2a to PCV2b, which is generally associated with more severe disease. In addition, from analytic epidemiologic studies, the modified within-herd PCV2 dynamics of infection is strongly related to the increased incidence of clinical disorders associated with PCV2 infection. Because PCV2 is shed for a long time by an extremely large variety of routes, it easily spreads within the population both through horizontal and vertical transmission. Even if airborne transmission cannot be formally excluded, direct contact is certainly the most efficient infectious route due to the simultaneous exposure of susceptible pigs to contaminated respiratory, digestive, and urinary secretions since the probability of transmission is strongly limited by the distance between infectious and susceptible animals. Consequently, farm to farm transmission is restricted to the introduction of infected animals or infected animal products such as semen. More information would be required to assess the risk of other vehicles such as vaccines or feed ingredients since the probability of these products to be contaminated by PCV2 is unknown. However, owing to its transmission characteristics, PCV2 is able to be maintained within pig farms for years without any further need for re-introduction due to the population dynamics of modern pig operations, which continually renew the pool of the susceptible population through replacements and pig movements between compartments.
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