Porcine circovirus type 2 associated disease: update on current terminology, clinical manifestations, pathogenesis, diagnosis, and intervention strategies.

Porcine circovirus type 2 (PCV2)-associated disease (PCVAD) continues to be an important differential diagnosis on pig farms in the United States and worldwide. Case trend analyses indicate that the incidence of PCVAD is on the rise in the United States. Accurate diagnosis is important in order to implement appropriate intervention strategies. PCVAD can manifest as a systemic disease, as part of the respiratory disease complex, as an enteric disease, as porcine dermatitis and nephropathy syndrome, or as reproductive problems. PCVAD may be only a sporadic individual animal diagnosis; however, PCVAD may also manifest as a severe herd problem accelerated and enhanced by concurrent virus or bacterial infections. This article is intended to discuss the most common disease manifestations, pathogenesis, diagnostic approaches, and intervention strategies associated with PCVAD in North America.     

Co-infection of porcine dendritic cells with porcine circovirus type 2a (PCV2a) and genotype II porcine reproductive and respiratory syndrome virus (PRRSV) induces CD4(+)CD25(+)FoxP3(+) T cells in vitro

Porcine circovirus associated disease (PCVAD) is currently one of the most economically important diseases in the global swine industry. Porcine circovirus type 2 (PCV2) is the primary causative agent, however co-infection with other swine pathogens such as porcine reproductive and respiratory syndrome virus (PRRSV) is often required to induce the full spectrum of clinical PCVAD. While the specific mechanisms of viral co-infection that lead to clinical disease are not fully understood, immune modulation by the co-infecting viruses likely plays a critical role. We evaluated the ability of dendritic cells (DC) infected with PRRSV, PCV2, or both to induce regulatory T cells (T(regs)) in vitro. DCs infected with PCV2 significantly increased CD4(+)CD25(+)FoxP3(+) T(regs) (p<0.05) and DCs co-infected with PRRSV and PCV2 induced significantly higher numbers of T(regs) than with PCV2 alone (p<0.05). Cytokine analysis indicated that the induction of T(regs) by co-infected DCs may be dependent on TGF-β and not IL-10. Our data support the immunomodulatory role of PCV2/PRRSV co-infection in the pathogenesis of PCVAD, specifically via T(reg)-mediated immunosuppression.     

Porcine Circovirus Type 2 and Porcine Circovirus-Associated Disease

Porcine circovirus type 2 (PCV2) belongs to the viral family Circoviridae and to the genus Circovirus . Circoviruses are small, single-stranded nonenveloped DNA viruses that have an unsegmented circular genome. PCV2 is the primary causative agent of several syndromes collectively known as porcine circovirus-associated disease (PCVAD). Many of the syndromes associated with PCVAD are a result of coinfection with PCV2 virus and other agents such as Mycoplasma and porcine reproductive and respiratory syndrome virus. PCV2 infection is present in every major swine-producing country in the world, and the number of identified cases of PCVAD is rapidly increasing. In the United States, the disease has cost producers an average of 3–4 dollars per pig with peak losses ranging up to 20 dollars per pig. The importance of this disease has stimulated investigations aimed at identifying risk factors associated with infection and minimizing these risks through modified management practices and development of vaccination strategies. This paper provides an overview of current knowledge relating to PCV2 and PCVAD with an emphasis on information relevant to the swine veterinarian.     

Porcine circoviruses: a minuscule yet mammoth paradox

Porcine circovirus type 2 (PCV2) is the primary causative agent for porcine circovirus-associated disease (PCVAD). PCVAD has been the cause of considerable economic losses to the pork industry worldwide. The disease is primarily characterized by wasting, enlarged lymph nodes, jaundice and weight loss in affected weanling pigs. Several other complex syndromes involving reproductive failure, enteritis, pneumonia and necrotizing dermatitis have also been associated with PCV2 infection. Lymphoid depletion, which is the hallmark lesion of PCVAD, predisposes the host to immunosuppression. Disease progression is further complicated by co-infections with other bacterial and viral pathogens. Despite the availability of effective vaccines for the last 2 years, newly emerging strains of the virus have been reported to cause more severe outbreaks in parts of the USA and Canada. While knowledge of the biology and pathogenesis of PCV2 has progressed considerably over the last 12 years since the disease was recognized, many questions still remain to be answered.     

Genomic comparison of foot-and-mouth disease virus R strain and its chick-passaged attenuated strain

Porcine circovirus type 2 (PCV2) is a single-stranded circular DNA virus that is the causative agent of porcine circovirus associated disease (PCVAD), a disease complex affecting swine around the world. Although this virus is believed to negatively affect the host's immune system, the mechanism by which PCV2 induces disease is not completely understood. This report describes a series of PCV2 experiments using the gnotobiotic pig model in which a relationship was demonstrated between abnormal leukograms and development of clinical disease in PCV2-infected pigs. When compared to control pigs the leukogram was characterized by a decrease in lymphocytes within 14 days post inoculation (dpi) followed by an increase in neutrophils 7-14 days later. No significant changes in the circulating monocyte, basophil, and eosinophil cell populations were detected. The combination of an absolute neutrophilia and lymphopenia produced a neutrophil/lymphocyte ratio that was predictive of clinical disease and was inversely correlated with the presence of neutralizing antibodies. Based on previous reports, the lymphopenia may be attributed to a direct cytolytic effect of the virus and could negatively affect the pig's immune response. The role of the neutrophilia in the pathogenesis of PCVAD in gnotobiotic pigs is unknown.     

Porcine circovirus-associated disease in weaner pigs in Western Australia

Objective To report the occurrence and pathology of porcine circovirus type 2 (PCV2)‐associated disease (PCVAD) of postweaning pigs in two Australian pig herds. Methods Mortality data from two commercial piggeries that experienced higher than normal postweaning illthrift and mortalities were examined. Gross and histopathological examinations were performed on the index cases, and at weekly intervals thereafter for a period of 10 weeks. Specimens were submitted to the laboratory for routine diagnostic testing and for exclusion of porcine reproductive and respiratory syndrome virus (PRRSV). The genomes of two strains of PCV2 isolated during testing were sequenced. Results Mortality rates in weaned, 5–12‐week‐old pigs spiked significantly during mid to late 2007. This increase in the mortalities was mainly attributed to salmonella‐associated diarrhoea and illthrift. Salmonellosis was diagnosed in 73/110 cases inclusive of both piggeries. Many pigs also had chronic granulomatous lymphadenitis and diffuse histiocytic interstitial pneumonia consistent with PCVAD and associated with varying amounts of PCV2 antigen and inclusion bodies. All samples tested for PRRSV were negative. Sequence analysis of the PCV2 isolates showed strain differences between piggeries. Conclusion This report describes the first outbreaks of PCVAD in growing pigs in Western Australia (WA) and describes lesions not previously seen in this laboratory. It also describes the first isolation of a PCV2 group 1 virus in WA associated with PCVAD. Although the outbreaks of PCVAD occurred with concurrent salmonellosis, the two diseases were unrelated. Neither of the outbreaks met the Australian case definition for the diagnosis of postweaning multisystemic wasting syndrome.     

猪圆环病毒2型靶细胞及其受体的研究进展

猪圆环病毒2型(porcine circovirus type 2,PCV2)是统称为猪圆环病毒相关疾病(porcine circovirus-associated disease,PCVAD)的几个综合征的主要致病因子。目前,对PCV2配体表位和细胞受体在病毒入侵过程中的致病机理还不清楚,受体识别是病毒黏附到宿主细胞表面的第一步,在病毒入侵和致病机理方面起到重要的作用。作者对近几年来PCV2侵染的靶细胞及其靶细胞受体的研究进展作一综述。     

猪圆环病毒2型分子致病机理及相关疾病研究进展

猪圆环病毒(porcine circovirus,PCV)有两个基因型,分别为PCV1和PCV2。PCV2有致病性,它可以引发多种相关的疾病(PCVAD),如断奶后多系统衰竭综合征(PMWS)等。该病毒主要通过攻击猪的淋巴细胞,产生免疫抑制,诱发细胞凋亡,还可以和其他病毒协同感染,大大增加了动物的发病率及死亡率。文章对PCV2分子致病机理及引起的相关疾病和防治作一综述。     

Porcine circovirus type 2 (PCV2)-infection and re-inoculation with homologous or heterologous strains: virological, serological, pathological and clinical effects in growing pigs

Long-term PCV2 infection and/or concurrent infection with genotypes PCV2a and PCV2b may play a role in the development of clinical porcine circovirus-associated disease (PCVAD). To evaluate this premise, 24 11-week-old specific pathogen-free (SPF) pigs were randomly assigned to 1 of 4 treatments: negative controls, a single inoculation with PCV2a, single inoculation followed by re-inoculation with a homologous PCV2a strain, or repeated inoculations with heterologous strains (PCV2a, PCV2b). Pigs were evaluated for clinical signs daily through 140 days post inoculation (dpi). Serum samples were collected every other day from dpi 0 through 14 and weekly thereafter. PCV2-inoculated pigs were viremic by dpi 2 and 13 of 18 pigs remained viremic at 140 dpi. No statistical differences in the onset, level, or duration of PCV2 viremia were detected among treatment groups. Anti-PCV2 antibodies were detected between 14 and 28 dpi and were present through 140 dpi without statistical differences in antibody response among treatment groups. In the current study, pigs had extended viremia combined with detectable tissue PCV2 antigen levels despite the presence of high levels of anti-PCV2 antibody; however, no clinical disease was observed.     

猪圆环病毒感染及动物模型研究进展

1974年德国学者Tischer从多株连续传代的猪肾细胞(PK-15)中发现了猪圆环病毒(PCV),其致病性及其疾病直到1991年才由Clark报道,称为断奶仔猪多系统衰竭综合征(PMWS).随后,许多学者对PCV-2感染以及PMWS的动物模型建立进行了广泛研究,证实了免疫激发及PCV-2与其他病原混合感染对PMWS的...     

Diagnostic performance measures of ELISA and quantitative PCR tests for porcine circovirus type 2 exposure using Bayesian latent class analysis

Porcine circovirus 2 (PCV2) is believed to be a necessary but not sufficient underlying cause of porcine circovirus associated disease (PCVAD) in swine (Opriessnig et al., 2007). Since the potential threat of PCVAD is dependent on the prevalence of PCV2 in swine populations, accurate diagnostic tests are important for epidemiologic surveillance. Therefore, we evaluated the diagnostic sensitivity (Se) and specificity (Sp) of a new indirect ELISA and two quantitative PCR tests for PCV2 in a series of latent class models that used Bayesian estimation procedures. A total of 4140 samples from finisher pigs were tested for evidence of PCV2 by the ELISA and a TaqMan (TM) quantitative PCR, 995 by the ELISA and a SYBR Green (SG) dye-binding PCR, 998 by both PCRs and 993 by all three tests. Overall, the median (95% probability interval) ELISA Se and Sp was 0.85 (0.83-0.87) and 0.74 (0.68-0.79), respectively, when all three tests were analyzed together at an ELISA absorbance (optical density or OD) cutoff of ≥0.3. The TM PCR Se and Sp was 0.86 (0.84-0.88) and 0.94 (0.87-0.97), respectively, and the SG PCR Se and Sp was 0.83 (0.81-0.85) and 0.98 (0.94-1.00), respectively when all three tests were analyzed together at an ELISA OD cutoff of ≥0.3. Sensitivity analysis revealed that Sp estimates in general had less stability than Se estimates, but the SG PCR(Sp) was the most stable. Limited conditional dependence between the two PCR tests was detected. We conclude that the ELISA had the highest diagnostic Se at an absorbance cutoff of ≥0.3, while the SG PCR had the highest diagnostic Sp. The prevalence levels for exposure to PCV2 in finishing swine populations across all analyses ranged from 58 to 100%.     

Preliminary study of Porcine circovirus type 2 and Torque teno sus virus coinfection frequencies in Brazilian pig herds

Torque teno sus virus (TTSuV) is emergent in swine herds. Recent studies have shown an increased frequency of TTSuV2 in Porcine circovirus type 2 (PCV2)-associated diseases (PCVAD), which are endemic in many swine-producing countries, including Brazil. Coinfection with several other viral and bacterial agents results in an increased incidence of more severe PCVAD. Given the limited information on TTSuV and PCV2 coinfection, especially in Brazilian swine herds, this study made a preliminary estimation of the occurrence of coinfection in swine herds by testing samples from different categories. Between 2008 and 2009, 111 samples of feces and 23 serum samples from 5 swine herds were tested for PCV2 and TTSuVs and the results analyzed for associations between these agents. No significant differences in coinfection frequency were observed for PCV2 + TTSuV1 or for PCV2 + TTSuV2 between nursery piglets (P = 0.730), growing pigs (P = 0.331), or sows (P = 0.472). However, a significant difference was observed for PCV2 + TTSuV1 + TTSuV2 between nursery piglets and growing pigs (P = 0.004; Fisher’s exact test). Phylogenetic studies agreed with the grouping of TTSuV1 and TTSuV2 into 2 different clades, with no distinct pattern of clustering of these isolates with the animal categories.     

Detection and molecular characterization of porcine circovirus type 2 (PCV2) from piglets with exudative epidermitis in Uruguay

Porcine circovirus type 2 (PCV2) is an economically important emerging pathogen associated with distinct syndromes and diseases in swine, collectively known as porcine circovirus associated diseases (PCVAD). The main purpose of this study was to investigate the presence of PCV2 in piglets affected with exudative epidermitis (EE) in Uruguay. In addition we aimed to analyze the phylogenetic relationships of the isolated strains. In June 2011 an outbreak of EE detected in a small herd was reported. Piglets presented skin lesions compatible with EE and symptoms associated with postweaning multisystemic wasting syndrome (PMWS) were also observed. Sera from affected and healthy animals were tested for the presence of viral DNA. Exclusively, diseased piglets were infected with PCV2. Phylogenetic analysis showed that PCV2 isolates belonged to PCV2b genotype. We report the detection and molecular characterization of PCV2 strains for the first time in Uruguay.     

Investigation of Co-infection of PPV7 and PCV2 and Genetic Variation Analysis for PPV7 Cap Gene in Fujian and Guangdong

【Objective】 To investigate the co-infection situation of Porcine parvovirus (PPV7) and Porcine circovirus type 2 (PCV2)in Fujian and Guangdong, and to understand the molecular genetic characteristics of PPV7 Cap gene.【Method】 The blood sample of 432 infected pigs from 69 pig farms in Fujian and Guangdong were collected to detect PPV7 and PCV2 by PCR.The PPV7 Cap gene of positive samples was cloned and sequenced.The DNAStar software was used to analyze the nucleotide and amino acid sequences of PPV7 Cap gene, and Mega 7.0 software was used to draw the genetic evolution tree.【Result】 The results showed that the positive rate of PPV7 was 21.99% (96/432), the positive rate of farms was 53.62%(37/69), and the positive rate of PCV2 was 54.17% (234/432), and the co-infection rate of PCV2 and PPV7 was 13.43% (58/432).The 17 PPV7 Cap gene sequences were amplified using PCR.Nucleotide homology analysis revealed that the homology of the 17 PPV7 Cap gene sequences was 85.6%-100%, and the homology with reference strains was 85.8%-99.0%.Amino acid sequence comparison analysis revealed that the amino acid homology of the 17 PPV7 Cap protein sequences was 87.6%-100%, and the amino acid homology with reference strains was 82.6%-98.7%.Phylogenetic analysis of Cap gene showed that PPV7 could be divided into five main evolutionary branches of PPV7a-PPV7e, among which 9 isolates belonged to PPV7a subtype, 3 isolates belonged to PPV7b subtype, 4 isolates belong to PPV7c subtype, and only 1 isolates isolates belonged to PPV7e subtype.【Conclusion】 This study indicated that PPV7 was widely prevalent in Fujian and Guangdong regions, and had a high co-infection rate with PCV2, which might be the pathogenic factor of Porcine circovirus associated disease(PCVAD).The genetic diversity of PPV7 isolates was abudant in both regions, and PPV7a was the dominant strain at present.The findings of this study provided theoretical basis and data reference for PPV7 prevention and control and vaccine research.     

Patterns of condemnation rates in swine from a federally inspected abattoir in relation to disease outbreak information in Ontario (2005-2007)

Strong correlations between clinical signs on farms and the presence of lesions at slaughter have been reported. The objective of this study was to determine if changes in condemnation rates provide a data source for surveillance of disease outbreaks in pigs. The data were obtained from 1 abattoir in Ontario (2005-2007). The epidemiological relevance of the results was based on an outbreak of porcine circovirus associated disease (PCVAD) in Ontario in 2005. The total condemnations and condemnations due to arthritis and pneumonia patterns reflected the field infection of PCVAD in 2005 followed by the widespread use of porcine circovirus type 2 (PCV-2) vaccine in 2007. In contrast, increased rates of nephritis and enteritis suggested areas for enhanced surveillance for unexplained changes in disease patterns not identified through traditional passive surveillance. Further studies looking at the benefits of using abattoir data should compare condemnation patterns with multiple sources of swine health data.     

Development and use of a multiplex real-time quantitative polymerase chain reaction assay for detection and differentiation of Porcine circovirus-2 genotypes 2a and 2b in an epidemiological survey

By the end of 2004, the Canadian swine population had experienced a severe increase in the incidence of Porcine circovirus-associated disease (PCVAD), a problem that was associated with the emergence of a new Porcine circovirus-2 genotype (PCV-2b), previously unrecovered in North America. Thus, it became important to develop a diagnostic tool that could differentiate between the old and new circulating genotypes (PCV-2a and PCV-2b, respectively). Consequently, a multiplex real-time quantitative polymerase chain reaction (mrtqPCR) assay that could sensitively and specifically identify and differentiate PCV-2 genotypes was developed. A retrospective epidemiologic survey that used the mrtqPCR assay was performed to determine if cofactors could affect the risk of PCVAD. From 121 PCV-2-positive cases gathered for this study, 4.13%, 92.56%, and 3.31% were positive for PCV-2a, PCV-2b, and both genotypes, respectively. In a data analysis using univariate logistic regressions, the PCVAD-compatible (PCVAD/c) score was significantly associated with the presence of Porcine reproductive and respiratory syndrome virus (PRRSV), PRRSV viral load, PCV-2 viral load, and PCV-2 immunohistochemistry (IHC) results. Polytomous logistic regression analysis revealed that PCVAD/c score was affected by PCV-2 viral load (P = 0.0161) and IHC (P = 0.0128), but not by the PRRSV variables (P > 0.9), which suggests that mrtqPCR in tissue is a reliable alternative to IHC. Logistic regression analyses revealed that PCV-2 increased the odds ratio of isolating 2 major swine pathogens of the respiratory tract, Actinobacillus pleuropneumoniae and Streptococcus suis serotypes 1/2, 1, 2, 3, 4, and 7, which are serotypes commonly associated with clinical diseases.     

Genomic analysis of PCV2 isolates from Danish archives and a current PMWS case-control study supports a shift in genotypes with time

Porcine circovirus type 2 (PCV2) is the primary cause of Postweaning Multisystemic Wasting Syndrome (PMWS) in pigs. PCV2, however, is found in both PMWS-affected herds and non-affected herds. The objective of this study was to clarify if PCV2 genome nucleotide sequences isolated from pigs from PMWS-affected herds and non-affected herds cluster phylogenetically in two separate groups. All isolates (45) belonged to PCV2 group 1 and shared a nucleotide sequence identity of 99.4-100% indicating a very homogeneous PCV2 population in Denmark. Phylogenetic analysis of the PCV2 isolates revealed no distinctive clustering of case- and control-herds suggesting that there is no link between PCV2 sequences and herd disease status. The appearance of only PCV2 group 1 isolates in this study (isolates from 2003/2004) led us to determine if PCV2 nucleotide sequences had changed in Denmark over time. Interestingly, all PCV2 isolates from before the first outbreak of PMWS (2001) belonged either to a new PCV2 group identified for the first time in this study and named group 3 (isolates from 1980, 1987 and 1990) or PCV2 group 2 (isolates from 1993 and 1996). The shift from PCV2 group 2 to 1 was confirmed on a more global scale by placing all full genome PCV2 sequences submitted to GenBank from 1997 to 2006 in either of the groups by phylogenetic analysis. The analysis showed that the shift happened in 2003 or even earlier. This may indicate that PCV2 group 1 is a more adapted form of PCV2 and possibly could be more pathogenic.     

Proinflammatory cytokine expression in the lung of pigs with porcine circovirus type 2-associated respiratory disease

Porcine circovirus type 2 (PCV2) causes a significant health problem for the swine industry worldwide. In this study, we investigated the cytokine expression profiles (IFN-γ, IL-1α, IL-8, and IL-10) in the lungs of pigs with PCV2-associated respiratory disease. The mRNA expressions of IL-1α and IL-8 were significantly up-regulated in pigs with PCV2-associated respiratory disease, while IL-10 expression was not detected. These results suggest that the increased expressions of proinflammatory cytokines in the lungs may play an important role in the immunopathologic response in pigs with PCV2-associated respiratory disease.     

Singular PCV2a or PCV2b infection results in apoptosis of hepatocytes in clinically affected gnotobiotic pigs

Porcine circovirus type 2 (PCV2) is clinically associated with respiratory disease, failure-to-thrive, hepatitis, and diarrhea; however, the precise pathogenesis of PCV2-associated disease and in particular its involvement in apoptosis is still controversial. The objectives of this study were (1) to determine whether PCV2 is associated with apoptosis by examining and comparing hepatic tissues from clinically affected or unaffected gnotobiotic pigs that were experimentally infected with PCV2, (2) to determine if there are differences between PCV2a and PCV2b in inducing hepatocyte apoptosis, and (3) to determine if there are differences between apoptosis detection systems. Forty-eight gnotobiotic pigs were separated into five groups based on inoculation status and development of clinical disease: (1) sham-inoculated, clinically-unaffected (n=4), (2) inoculated with PCV2a, clinically-unaffected (n=10), (3) inoculated with PCV2a, clinically-affected (n=6), (4) inoculated with PCV2b, clinically-unaffected, (n=13) and (5) inoculated with PCV2b, clinically-affected (n=15). Formalin-fixed, paraffin-embedded sections of liver from all pigs were analyzed for signs of apoptosis [presence of single strand DNA breaks in the nucleus by the terminal transferase dUTP nick end labeling (TUNEL) assay or presence of intra-nuclear cleaved caspase 3 (CCasp3) demonstrated by CCasp3 immunohistochemistry (IHC)]. In addition, the liver tissues were also tested for presence of cytoplasmic and intra-nuclear PCV2 antigen by an IHC assay. Specific CCasp3 and TUNEL labeling was detected in the nucleus of hepatocytes in PCV2a and PCV2b infected pigs with significantly (P<0.05) higher levels of apoptotic cells in clinically-affected pigs. Regardless of PCV2 subtype (PCV2a; PCV2b), there were higher levels of PCV2 antigen in clinically-affected pigs compared to clinically-unaffected pigs. There was no significant difference in detection rate of apoptotic cells between the TUNEL assay and CCasp3 IHC. When high amounts of PCV2 antigen were present, the incidence of CCasp3 and TUNEL staining also increased regardless of the PCV2 genotype. This suggests that PCV2-induced apoptosis of hepatocytes is important in the pathogenesis of PCV2-associated lesions and disease.