First detection of PCV4 in swine in the United States: codetection with PCV2 and PCV3 and direct detection within tissues

Abstract

Since PCV4 was first described in 2019, the virus has been identified in several countries in SoutheastAsia and Europe. Most studies have been limited to detecting PCV4 by PCR. Thus, PCV4 has an unclearassociation with clinical disease. This study utilized 512 porcine clinical lung, feces, spleen, serum,lymphoid tissue, and fetus samples submitted to the ISU‑VDL from June–September 2023. PCV4 wasdetected in 8.6% of samples with an average Ct value of 33. While detection rates among sample typeswere variable, lymphoid tissue had the highest detection rate (18.7%). Two ORF2 sequences wereobtained from lymphoid tissue samples and had 96.36–98.98% nucleotide identity with referencesequences. Direct detection of PCV4 by RNAscope revealed viral replication in B lymphocytes andmacrophages in lymph node germinal centers and histiocytic and T lymphocyte infiltration in thelamina propria of the small intestine. PCV4 detection was most commonly observed in nursery tofinishing aged pigs displaying respiratory and enteric disease. Coinfection with PCV2, PCV3, and otherendemic pathogens was frequently observed, highlighting the complex interplay between differentPCVs and their potential roles in disease pathogenesis. This study provides insights into the frequencyof detection, tissue distribution, and genetic characteristics of PCV4 in the US.