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.

圆环病毒（CV）包括一些已知的最小的病毒。它们以其环状排列的单链DNA基因组命名，其有义链上编码保守复制酶蛋白的基因。圆环病毒广泛分布于哺乳动物，鱼类，禽类甚至昆虫中。在猪中，已经鉴定出四种不同的CV，并根据其发现顺序用连续的数字命名：猪圆环病毒1（PCV1），猪圆环病毒2（PCV2），猪圆环病毒3（PCV3）和最近的猪圆环病毒4（PCV4）。PCV在全球猪群中无处不在，很少发现未感染的牛群。通常认为PCV1是非致病性的。相比之下，PCV2被认为是重要的，具有经济挑战性的病原体，已在全球范围内实施了全面的疫苗接种计划。自发现PCV3以来，其作用仍存在争议。PCV3的传播似乎具有挑战性，迄今为止，仅公布了一种成功的实验感染模型。与PCV2相似，PCV3广泛存在于许多猪中，无论它们的健康史如何，包括高健康猪群。PCV4直到最近才被发现，并且需要有关此病毒的更多信息以了解其潜在影响。这篇综述总结了有关猪CV的最新知识，旨在对比和比较PCV的已知事实。