The natural history of porcine circovirus type 2: from an inoffensive virus to a devastating swine disease?

The present review summarizes the current knowledge on the natural history of porcine circovirus type 2 (PCV2) infection and its related diseases. The perception about PCV2 as a significant pathogen has markedly changed in the last 15 years. The ubiquitous nature of the virus, the retrospective evidence of this infection long before disease association, the multifactorial aetio-pathogenesis of PCV2-systemic disease (SD) and the lack of consistent demonstration of Koch's postulates caused great controversy about the real causal capabilities of this virus. The advent of vaccines against PCV2 radically changed such perception and this virus is nowadays regarded as a very important pig pathogen. Moreover, the current PCV2 vaccines are ones of the most widely used in pig producing countries. On the other hand, how the virus causes disease is still a not fully solved complex scientific question, but host, infection timing and the virus itself are pivotal factors to consider explaining disease presentation at an individual level. The appearance of PCV2-SD as an epidemic problem at the end of 1990 s or early-mid 2000s might be related with a number of known and unknown variables. Based on available data, the international trade of pigs may have played a major role in the dissemination of more susceptible swine genetic lines as well as in the global PCV2 genotype replacement (PCV2b over PCV2a) during such period.