Retrospective Study Reveals Decades of PCV4 Circulation in Spain

Area of Science:

• Veterinary Virology
• Molecular Epidemiology
• Animal Health

Background:

• Porcine circovirus 4 (PCV4) was first identified in China in 2019, with retrospective cases dating to 2008.
• PCV4's distribution was thought to be limited to Asia until recent detections in Spain and the United States.
• Previous findings indicated a significant prevalence of PCV4 in Spanish wild boars.

Purpose of the Study:

• To investigate the historical and current circulation of PCV4 in non-Asian countries, focusing on Spain and wild boars.
• To genetically characterize PCV4 strains and place them within national and international epidemiological contexts.
• To understand the introduction and spread dynamics of PCV4 in Spain.

Main Methods:

• Testing of 302 lymph node samples using quantitative real-time polymerase chain reaction (qPCR).
• Genetic sequencing of the complete open reading frame 2 (ORF2) from 10 positive PCV4 strains.
• Phylogenetic and phylodynamic analyses to determine genetic relationships and estimate introduction times.

Main Results:

• 62 out of 302 samples tested positive for PCV4, primarily from 2011-2015, with five positive samples from 2022-2024.
• Phylogenetic analysis revealed two main clusters: one exclusively Chinese and another with strains from multiple continents, including Spain and the US.
• Spanish strains formed a distinct clade, with introduction estimated around 2000, suggesting long-term undetected circulation and spread between wild boars and outdoor domestic pigs.

Conclusions:

• PCV4 has a longer history in Spain than previously thought, challenging the paradigm of recent international spread.
• Evidence suggests ongoing circulation and exchange between wild boar and domestic pig populations in Spain.
• The reasons for PCV4's absence in intensively farmed pigs in Europe require further investigation, emphasizing the need for continued surveillance and data sharing.