Joachim Denner

Background As sequencing is becoming more broadly available, virus discovery continues. Small DNA viruses contribute to up to 60% of the overall virus load in pigs. Porcine circoviruses (PCVs) are small DNA viruses with a single‐stranded circular genome. They are common in pig breeds and have not been properly addressed for their potential risk in xenotransplantation. Whereas PCV1 is non‐pathogenic in pigs, PCV2 has been associated with various disease manifestations. Recently two new circoviruses have been described, PCV3 and PCV4. While PCV4 is currently present mainly in Asia, PCV3 is widely distributed, and has been identified in commercial pigs, wild boars, and pigs generated for xenotransplantation. In one case PCV3 was transmitted by pigs to baboons via heart transplantation. PCV3 pathogenicity in pigs was controversial initially, however, the virus was found to be associated with porcine dermatitis and nephropathy syndrome (PDNS), reproductive failure, and multisystemic inflammation. Inoculation studies with PCV3 infectious clones confirmed that PCV3 is pathogenic. Most importantly, recently discovered human circoviruses (CV) are closely related to PCV3. Methods Literature was evaluated and summarized. A dendrogram of existing circoviruses in pigs, humans, and other animal species was created and assessed at the species level. Results We found that human circoviruses can be divided into three species, human CV1, CV2, and CV3. Human CV2 and CV3 are closest to PCV3. Conclusions Circoviruses are ubiquitous. This communication should create awareness of PCV3 and the newly discovered human circoviruses, which may be a problem for blood transfusions and xenotransplantation in immune suppressed individuals.