Porcine circovirus type 2 in China: an update on and insights to its prevalence and control
© Zhai et al.; licensee BioMed Central Ltd. 2014
Received: 25 January 2014
Accepted: 1 May 2014
Published: 14 May 2014
Currently, porcine circovirus type 2 (PCV2) is considered the major pathogen of porcine circovirus associated-diseases (PCVAD) that causes large economic losses for the swine industry in the world annually, including China. Since the first report of PCV2 in 1998, it has been drawing tremendous attention for the government, farming enterprises, farmers, and veterinary practitioners. Chinese researchers have conducted a number of molecular epidemiological work on PCV2 by molecular approaches in the past several years, which has resulted in the identification of novel PCV2 genotypes and PCV2-like agents as well as the description of new prevalence patterns. Since late 2009, commercial PCV2 vaccines, including the subunit vaccines and inactivated vaccines, have already been used in Chinese swine farms. The aim of this review is to update the insights into the prevalence and control of PCV2 in China, which would contribute to understanding the epidemiology, control measures and design of novel vaccines for PCV2.
KeywordsPorcine circovirus type 2 Prevalence Control PCV2 vaccines Update China
Porcine circoviruses (PCVs), the smallest known animal viruses, including PCV1 and PCV2, are members of the genus Circovirus in the Circoviridae family. PCV1 was first discovered in 1974 as a contaminant of the porcine kidney cell line PK-15 (ATCC CCL-33) and was considered non-pathogenic [1, 2]; Whereas PCV2 was isolated from pigs suffering from post-weaning multisystemic wasting syndrome (PMWS) over 20 years later .
PCVs, especially PCV2, cause huge hazards to swine industry. At present, PCV2 is associated with a number of diseases, collectively known as porcine circovirus diseases (PCVD) in Europe , or porcine circovirus-associated diseases (PCVAD) in North America . Apart from PMWS, PCVD/PCVAD also includes reproductive disorders , porcine dermatitis and nephropathy syndrome (PDNS) , nervous system lesions , porcine respiratory disease complex (PRDC) , enteritis [15, 16], and proliferative and necrotizing pneumonia (PNP) [17, 18]. Recently, a new PCV2-related disease syndrome called acute pulmonary edema (APE) that is different from the PCVD/PCVAD syndromes mentioned above was described in U.S. swine herds vaccinated with PCV2 .
A number of field and experimental studies that were performed in some countries of North America, Europe and Asia indicated that commercial vaccines (based on genotype PCV2a) against PCV2 were effective in many aspects, including reducing the incidence of PMWS, the number of co-infections, the severity of lesions in lymphoid tissues [20–25], the level of PCV2 viraemia and the severity of microscopically-visible lesions [26–29] as well as improving average daily weight gain and feed conversion ratios [20–25]. However, due to the subsequent possible vaccination pressure, novel variant strains or genotypes (such as PCV1/2, PCV2d) emerged in Canadian and U.S. swine herds [30–32].
In recent years, due to continuous losses caused by PCVD, the government at home and abroad supports scientists’ great deal of work on molecular epidemiology and vaccine development of PCV2. The aim of the review is to update the insights on the prevalence and control of PCV2 in China, which would help in understanding of the epidemiology, adjusting control measures, and the design of novel vaccines against PCV2.
PCV2 Genotypes in China
Co-Infections of Different PCV2 Genotypes in the Field in China
In 2009, Hesse et al.  described natural co-infections of two different PCV2 genotypes in pigs in the field for the first time. A similar molecular epidemiological survey using 118 PCV2-positive samples between 2008 and 2009 was also conducted by our group. Our result indicated that the co-infection rates of PCV2 were 32.2% (38/118) in diseased pigs and 0% (0/41) in asymptomatic pigs, respectively. Sequencing results of the 38 co-infected samples showed that the co-existent genotypes were PCV2a-PCV2b (12/38), PCV2a-PCV2d (15/38), and PCV2e-PCV2d (11/38), respectively. This was the first study demonstrating the co-existence of different PCV2 genotypes or strains in China that could help better understand the molecular epidemiology and the new infection patterns of PCV2 . We believe that the presence of co-infections of different PCV2 genotypes is a more accurate description with regard to the molecular epidemiology, supported by common observations in the field.
Co-Infections of PCV2 with Other Swine Pathogens in China
In addition to co-infections of different PCV2 genotypes, a number of studies demonstrated that PCV2 could co-infect along with by other traditional pathogens (such as porcine reproductive and respiratory syndrome virus, classical swine fever virus, porcine parvovirus, porcine pseudorabies virus, bovine viral diarrhea virus) [53–59] and emerging pathogens (porcine bocavirus, Torque teno sus virus, porcine hokovirus) [60–63].
Novel PCV2 strains in China
PCV2 Strains with nucleotide deletion
PCV2 Strains with nucleotide insertion
In 2004, a novel PCV2 strain, named Fh16 (AY321993), with gene insertion (11 nt, GGCAGCACCTC) at position 42 (non-coding region from position 1 to 50) was identified in non-PMWS herds in France. However, those herds presented with wasting, necrosis, and proliferous ileitis associated with Lawsonia and Pasteurella multocida. Further genetic analysis showed that Fh16 was belonged to the genotype PCV2b . Furthermore, a Danish PCV2 strain DK475case (EF565360) isolated from non-PMWS herds had the identical 11-nt nucleotide insertion to the Fh16 strain .
PCV2 strains with nucleotide insertion also exist in China. Among them, the 10JS-2 strain (JQ806749) had 11-nt insertion (AGCAGCACCTC) at position 42 (Figure 4), which was similar to the strains Fh16 and DK475case . The strain JF2 (HQ402903) had one additional nucleotide insertion (T) at position 1043 comparing to the genotype PCV2a, which led to the changes of ORF2 size (711 bp) (Figure 4) comparing to the previous strains . Notably, phylogenetic analysis suggested that the strains 10JS-2 and JF2 were classified into the genotype of PCV2e and PCV2a, respectively (Figure 3).
PCV2 Recombinant strains
PCV2-Like viral agents
PCV2 DNA in Chinese commercial swine vaccines
PCV2 DNA in mosquitoes collected from China
PCV2 has a high prevalence in swine herds in the world, implying that there might be other transmission routes (Figure 7). Recently, extensive PCV2 DNA was detected from water samples in Brazil, farm air in Canada, house flies in UK, and even in Culex (a kind of mosquito) (6.78%, 4/59 sampling) collected in China [94–97]. In general, mosquito is considered as an important vector for many zoonotic infectious diseases, such as Japanese encephalitis B and dengue fever. The control and killing of this vector (such as Culex and house flies) is usually overlooked under the poor swine farm conditions in China, which might exacerbate the spread of PCV2 in pig farms.
Control of PCV2 in China
PCV2 vaccines used in China since 2009
GenBank accession No.
ORF2 protein (PCV2a)
1 mL IM single dose
Piglets (>2 weeks of age)
Inactivated virus (PCV2d)
1 mL IM single dose
Piglets (14–21 days);
2 mL IM 2 doses
Gilts (45 days before mating/3 weeks later; Prenatal 30–40 days); Productive sows (Prenatal about 45 days/3 weeks later)
Inactivated virus (PCV2a)
1 mL IM single dose
Piglets (3–4 weeks);
2 mL IM 2 doses
Gilts (before mating/3 weeks later; Prenatal 30 days); Productive sows (Pregnancy/ Prenatal 30 days); Normal vaccination for others
Inactivated virus (PCV2b)
1 mL IM single dose
Piglets (14–21 days/14 days later)
Inactivated virus (PCV2b)
2 mL IM single dose
Piglets (21–28 days)
China Animal Husbandry
Inactivated virus (PCV2b)
2 mL IM single dose
Piglets (>14 days)
In addition to vaccination, other comprehensive measures have been taken in China, which mainly followed the recommendations in the developed countries, such as “Madec’s 20-point plan”, including “all-in”–“all-out” procedures, disinfection, limiting animal contact, preventing mixing of batches and cross-fostering and the isolation or euthanasia of diseased pigs; It also included the environmental factors such as the maintenance of appropriate temperature, air-flow, and space within pens; and furthermore, appropriate use of anti-parasitic treatments and vaccination , co-infections control [105–107], breeding and semen quality control [108–110], and herd nutrition improvement [111–113] are included as well. Nevertheless, autogenous vaccines were also considered as an effective method to prevent and control PCV2 on some pig farms from our experience.
In summary, PCV2 infection is very common in Chinese swine herds. There are various PCV2 genotypes, PCV2-like strains, or mutants circulating in China. Together with the potential cross-species transmission of PCV2 (Figure 7), these factors lead to great challenges for the control of PCV2 despite the availability of vaccines. Furthermore, although cross-protection within inter-genotypes of PCV2 is present, NA only neutralizes certain PCV2 genotype but not all PCV2 genotypes . At present, PCV2 infection status can be quite complex clinically, if it is not mastered well by veterinarians or practitioners. It might hinder choosing the right choice of PCV2 vaccines and thus not conducive to the prevention of PCV2. To overcome this problem, it is necessary to perform the detection and sequencing of PCV2 antigen. In addition to effective comprehensive measures, the development of polyvalent vaccines (such as PCV2a + PCV2b + PCV2d) or universal vaccines against all PCV2 genotypes are needed in the future.
This review is part of the work supported by grants (No. 2011A090200117, No. 2012A020602075 and No. 2012224–26) funded by Guangdong Provincial Department of Science and Technology and Guangzhou Science and Technology and Information Bureau, respectively. We also thank Dr. Susanna KP. Lau (Department of Microbiology, The University of Hong Kong), Prof. Yao-Wei Huang (College of Animal Sciences, Zhejiang University & College of Veterinary Medicine, Virginia Polytechnic Institute and State University), Dr. Shanzhong Gong (The University of Texas at Austin) for improving the language quality of the manuscript.
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