Porcine circoviruses: current status, knowledge gaps and challenges

Highlights

• Porcine circoviruses (PCVs) are widespread and expanding.

• The pathogenic potential and impact on pig production is different for each PCV.

• Current knowledge on PCVs is summarized and contrasted.

Abstract

Circoviruses (CV) include some of the smallest viruses known. They were named after their circularly arranged single-stranded DNA genome with a gene encoding a conserved replicase protein on the sense strand. Circoviruses are widely distributed in mammals, fish, avian species and even insects. In pigs, four different CVs have been identified and named with consecutive numbers based on the order of their discovery: Porcine circovirus 1 (PCV1), Porcine circovirus 2 (PCV2), Porcine circovirus 3 (PCV3) and most recently Porcine circovirus 4 (PCV4). PCVs are ubiquitous in global pig populations and uninfected herds are rarely found. It is generally accepted that PCV1 is non-pathogenic. In contrast, PCV2 is considered an important, economically challenging pathogen on a global scale with comprehensive vaccination schemes in place. The role of PCV3 is still controversial several years after its discovery. Propagation of PCV3 appears to be challenging and only one successful experimental infection model has been published to date. Similarly to PCV2, PCV3 is widespread and found in many pigs regardless of their health history, including high health herds. PCV4 has only recently been discovered and further information on this virus is required to understand its potential impact. This review summarizes current knowledge on CVs in pigs and aims to contrast and compare known facts on PCVs.

Introduction

In the early 70s, a German research group investigated the permanent pig kidney (PK) cell line 15 (ATCC-CCL31) by electron microscopy and recognized small, spherical virus-like particles (Tischer et al., 1974). In a follow-up study, the same research group further characterized these particles as small icosahedral viruses with a diameter of only 17 nm, containing a covalently closed circular single-stranded DNA genome and one main capsid protein (Tischer et al., 1982). The newly identified virus was designated “porcine circovirus (PCV)” as antibodies against it could only be identified in pigs but not in rabbits, mice, calves, and humans (Tischer et al., 1982). Since then, additional PCVs have been identified and there are currently four recognized types: Porcine circovirus 1 (PCV1), Porcine circovirus 2 (PCV2), Porcine circovirus 3 (PCV3) and Porcine circovirus 4 (PCV4) (Fig. 1). All four PCVs are similar in structure: they contain two main open reading frames (ORFs) oriented in opposite directions in the circular genome; the ORF1 or rep gene encodes for proteins associated with replication, Rep and Rep’ (not characterized for PCV3 and PCV4 at this point), and the ORF2 or cap gene encodes for the capsid or Cap protein (Finsterbusch et al., 2005). Basic information on genome length, protein sizes, cell lines used for propagation and information on experimental success for each PCV is summarized in Table 1.

PCV1 corresponds to the virus reported in 1974 in PK-15 cells (Tischer et al., 1974). Although PCV1 is found in both wild and farmed pigs, it is generally considered non-pathogenic as no association with disease was observed after natural or experimental infection of pigs (Allan et al., 1995; Tischer et al., 1986). A new disease in pigs described as postweaning multisystemic wasting syndrome (PMWS) was first observed in Canada in 1995 (Harding, 2004). During 1997-1998, PCV2 was discovered and associated with PMWS by multiple laboratories (Allan et al., 1998; Ellis et al., 1998; Kiupel et al., 1998; Morozov et al., 1998; Nayar et al., 1997). The first identification of PCV2 in PMWS pigs was made possible by restriction enzyme cleavage analysis, leading to identification of a novel restriction pattern different from that of PCV1 (Nayar et al., 1997). The association of PCV2 with pathogenicity was established by traditional methods of virus isolation from PMWS affected pigs, experimental infections, histopathology, in addition to immunohistochemistry (IHC) and in situ hybridization (ISH) to detect PCV2 antigen or genome (Allan et al., 1998; Ellis et al., 1998; Kiupel et al., 1998; Morozov et al., 1998). Since its discovery, PCV2 has become one of the most important pig viruses and is associated with several disease syndromes in pigs.

One of the first viral metagenomics studies performed using the shotgun sequencing technology was published in 2002, describing the viral metagenome of seawater (Breitbart et al., 2002). Since then, the developments in high throughput sequencing and associated bioinformatics computational capacities have accelerated the discovery of many previously unknown viral genomes. While traditional virology methods were used to discover PCV2, metagenomic sequencing on tissue samples from pigs with PDNS, reproductive failure, myocarditis and multi-systemic inflammation was used to discover PCV3 (Palinski et al., 2017; Phan et al., 2016). Although PCV3 has attracted a lot of attention and research, PCV3 isolation in cell culture has only recently been successful (Oh and Chae, 2020) and its role as a pathogen is still controversial. Similarly to PCV3, PCV4 was identified very recently, with the aid of sequencing, in pigs with various health conditions from two farms in Hunan province, China (Zhang et al., 2019). Analysis by PCR with primers complementary to the conserved regions of the rep gene for the genera Circovirus and Cyclovirus allowed the sequencing of PCV4 (Zhang et al., 2019). This review aims to summarize recent findings on circoviruses (CV) currently known to circulate in the global pig population.