Circoviruses are closed, circular, single-stranded DNA viruses belonging to the family Circoviridae and the genus Circovirus. To date, at least four porcine circoviruses (PCVs) have been recognized, including PCV1 to PCV4, respectively. Similar to PCV2 pathogenesis, PCV3 has been reported worldwide with myriad clinical and pathological presentations such as reproductive disorders, respiratory diseases, diarrhea etc. Current understanding of PCV3 pathogenesis is very limited since the majority of studies were mostly field observations. Interpretation of the results from such studies is not always simple. Various confounding factors affect the clinical appearance and pathological changes of the infected pigs. Recently, several experimental PCV3 infection studies have been reported, providing a better understanding of its pathogenesis. In this review, we focused on novel findings regarding PCV3 pathogenesis from both field observation and experimental infection studies. Possible factors involved in the conflicting results among the experimental infection studies are also discussed. This review article provides important insight into the current knowledge on PCV3 pathogenesis which would aid in prioritizing research in order to fill the knowledge gaps.

Porcine circovirus 4 (PCV4) is considered a novel PCV, firstly found in China in 2019 and later discovered in Korea. This present study investigated the prevalence and genetic characteristics of PCV4 from high pig-density areas in Thailand during 2019–2020. From 734 samples, three samples (0.4%) from aborted fetuses and porcine respiratory disease complex (PRDC) cases were found positive for PCV4, two of the PCV4-positive samples were coinfected with both PCV2 and PRRSV, and the other PCV4-positive sample was found coinfected with PCV2. In situ hybridization (ISH) revealed the presence of PCV4 in the bronchial epithelial cells and in lymphocytes and histiocyte-like cells in the lymphoid follicles of the PRDC-affected pig. The complete Thai PCV4 genome had over 98% nucleotide identity with other PCV4 strains and was closely related to the Korean and Chinese PCV4b strains. Importantly, the amino acid residue at position 212 of the Cap gene is recommended for differentiating PCV4a (²¹²L) from PCV4b (²¹²M) based on currently available PCV4 genome sequences. These findings provide important clues for the pathogenesis, epidemiology, and genetic characteristics of PCV4 in Thailand.

Since its first incursion in 2019, African swine fever (ASF) has remained a constant threat to the Vietnamese swine industry. The nationwide commercialization and implementation of live‐attenuated vaccines (LAVs), alongside depopulation‐repopulation efforts, necessitate a critical assessment of whether these strategies have driven inadvertent changes in circulating African swine fever virus (ASFV) strains. In this study, molecular surveillance of ASFV core genes was conducted in pigs from non‐vaccinated farms across Central and Southern Vietnam, comparing strains before and after LAVs’ introduction. The results confirmed spillover and ongoing field circulation of LAV‐related strains and revealed a marked increase in genetic diversity, encompassing both vaccine‐like and novel wild‐type microvariants in the post‐vaccine commercialization phase. Notably, these microvariants harbored non‐synonymous single‐nucleotide polymorphisms (SNPs) and large segmental deletions within the EP153R-EP402R gene, suggesting potential alterations in viral function. Most compellingly, a unique 15‐AGACCAGCAACAAAC‐nucleotide repeat in the E183L gene resulted in a five‐amino‐acid insertion (Arg‐Pro‐Ala‐Thr‐Asn) in the p54 protein, serving as a key marker for Vietnamese strains. Initial in silico analysis indicated that this novel p54 protein contained a longer α‐helix structure, which could influence viral movement and interactions with host cells. Overall, these subtle yet significant genetic shifts imply that ASFV might be rapidly evolving intra‐host under selective pressure from widespread immunization and/or diverse introduction sources during repopulation, potentially enabling it to evade the host immune system. This highlights the essential need for robust biosecurity protocols, stringent regulatory oversight of LAV deployment, and the required genetic surveillance. For diagnosis and management, the urgent development of reliable DIVA (Differentiating Infected and Vaccinated Animals) tools and real‐time, adaptable molecular identification methods, coupled with a vaccination‐tracking system, is necessary for accurate and sustained ASF control.

Porcine circovirus 2 (PCV2) has been recognized as a causative agent of porcine circovirus diseases (PCVDs) affecting the global swine industry. In this study, the genetic diversity of PCV2 strains circulating in Thailand between 2019 and 2020 was investigated using 742 swine clinical samples from 145 farms. The results showed PCV2-positive rates of 54.2% (402/742) and 81.4% (118/145) at the sample and farm levels, respectively. Genetic analysis of 51 Thai PCV2 genomic sequences showed that 84.3% (43/51) was PCV2d, 13.7% (7/51) was PCV2b and 1.9% (1/51) was PCV2b/2d recombinant virus. Surprisingly, the majority of the Thai PCV2d sequences from this study (69.77%, 30/43) formed a novel cluster on a phylogenetic tree and contained a unique ¹³³HDAM¹³⁶ on the ORF2 deduced amino acid sequence, which is in one of the previously identified immunoreactive domains strongly involved in virus neutralization. The PCV2b/2d recombinant virus also carried ¹³³HDAM¹³⁶. The emergence of the novel PCV2d strains predominating in Thailand was discussed. This study highlights the need for further investigations on the spreading of these PCV2d strains in other regions and the efficacy of current commercial vaccines.