Effects of Primary Viruses (PCV2, PPV1, and PRRSV) Involved in Porcine Reproductive Failure as Mono- and Coinfections with Each Other and with Emerging Viruses (PCV3 and nPPVs).

Porcine reproductive failure (PRF) is a complex that affects reproductive parameters, leading to significant economic losses for intensive swine farms worldwide. The causes of PRF involve multiple infectious agents, classified into two main groups: primary or putative viruses, which include PCV2, PPV1, and PRRSV, and secondary or occasional viruses, such as PCV3, PCV4, and the new parvoviruses (nPPVs, PPV2 through PPV8). This review provides an updated overview of both viral groups, detailing their unique characteristics and the most commonly reported clinical signs and lesions linked to the putative viruses. While the impact of primary viruses on PRF is well established, the role of secondary viruses in PRF is still under investigation. PCV3 has been directly associated with PRF, characterized by proposed histopathological lesions. Although PCV4 has been identified in reproductive samples, its role in PRF remains unclear. Additionally, nPPVs have been found in reproductive tissues; however, a clear causal relationship with PRF has not been established. The sporadic presence of nPPVs raises questions about their direct impact on PRF and whether they may have synergistic effects when combined with other viruses. This review highlights the growing importance of viral coinfections in the context of PRF. To date, the most frequently reported coinfections are PCV2/PRRSV and PCV2/PPV1, along with emerging pairings such as PCV2/PCV3 and combinations of these two PCVs with nPPVs. Based on the existing literature and our recent findings, we propose a subclinical presentation of PRF, characterized by the presence of both primary and secondary viruses in asymptomatic sows with low viral loads. Furthermore, the synergistic effects of these viruses could contribute to a clinical form of the disease.