PCV2-associated PMWS was first identified in Canadian high health herds in 1991 (Harding, 1996; Harding and Clark, 1997). Affected herds were free of major enteric and respiratory pathogens including Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae (APP), salmonellosis, swine dysentery, transmissible gastroenteritis virus (TGEV), pseudorabies virus, and PRRSV. The herd sizes varied from small (50 sows) to large (1,200 sows) and genetics also varied widely. In 1996, interstitial pneumonia and lymphadenopathy was observed in a 6-week-old pig in California (Daft et al., 1996). Also in 1996, workers in France observed a piglet wasting disease (LeCann et al., 1997). Subsequently, PCV2 was isolated from material of affected French and U.S. pigs (Allan et al., 1998b).

The pathogenesis of PCV2 infection and the major cell types that support PCV2 replication are poorly understood. It has been suggested that PCV2 initially replicates in the tonsil. Allan et al. (2000b) reported that in porcine parvovirus (PPV) and PCV2 coinfected pigs, PPV antigen predominates in the tissues of the pigs killed between 3 and 14 DPI with a maximum observed between 6 and 14 DPI. PCV2 antigen was first observed in minimal amounts in mesenteric lymph nodes at 10 DPI with increasing density and distribution of PCV2 antigen at 14, 17, 21, and 26 DPI (Allan et al., 2000b).

Large numbers of PCV2 antigen or nucleic acids are often detected in the cytoplasm of macrophages and dendritic cells by IHC or ISH (Allan and Ellis, 2000; Sorden, 2000). A recent study found that antigen presenting cells in general, and not only macrophages stained positive by IHC for PCV2 antigen (Chianini et al., 2003). In contrast, PCV2 antigen in lymphocytes was only sporadically detected. In thymus, PCV2 was only detected in few histiocytic cells in the medulla suggesting that thymocytes and T cells might be more resistant to PCV2 infection (Chianini et al., 2003).To a lesser extent, PCV2 antigen is also found in epithelial cells in lungs and kidneys, in smooth muscle cells, and in endothelial cells in several tissues in pigs experimentally-infected with PCV2 (Kennedy et al., 2000) as well as in pigs with naturally occurring PCV2-associated PMWS (McNeilly et al., 1999; Rosell et al., 1999). Kennedy et al (2000) demonstrated PCV2-antigen in infiltrating macrophages in the tunica albuginea, in interstitial macrophages and in germinal epithelial cells in the testes, and in infiltrating macrophages in the epidiymides of boars 24 to 29 days after they had been coinfected with PCV2 and PPV at 3 days of age. PCV2 was also found in the parenchyma of the secondary sex glands in a naturally infected boar (Opriessnig et al., 2006). PCV2 targets mainly cardiomyocytes, hepatocytes, and macrophages during fetal life, and mainly monocytes in early post-natal life (Sanchez et al., 2003).

The mean PCV2-antibody half-life in weanling pigs is estimated to be 19.0 days and the window for PCV2-passive antibody decay within a population is relatively wide (Opriessnig et al., 2004). Serological studies have demonstrated that passively-acquired PCV2 antibodies decay during lactation and early postweaning periods to negative or near negative levels at the end of the nursery period (7 weeks of age) followed by active seroconversion of the majority of the population starting around 12 weeks of age (Roríguez-Arrioja et al., 2002). Allan et al. (1994) showed a similar trend for PCV1 passive antibody decay. Larochelle et al. (2003) did a comparative serologic and virologic study in 5 PMWS-affected herds and 2 herds without PMWS in Quebec. Sixty blood samples were collected in each herd in 4-week-intervals from 3-to 23-week-old pigs and it was found that all herds had similar PCV2 profiles: low PCV2-antibody levels were present at 3 weeks of age and reached very low-to-negative levels by 11 weeks of age. PCV2-infection as determined by PCR took place from 11 to 15 weeks of age at which time PCV2 seroconversion occurred. All pigs were seropositive at 23 week of age.

PCV2-passively-acquired antibodies present at 1-2 weeks of age were found to decay below ELISA cutoff level by approximately 4.9 ± 1.2 weeks of age in piglets with low levels of antibodies at weaning, by approximately 8.1 ± 1.9 weeks of age in piglets with moderate levels of antibodies at weaning, and by approximately 11.1 ± 2.5 weeks of age in piglets with high levels of antibodies at weaning (Opriessnig et al., 2004).

Serological surveys found that PCV-antibodies are present globally in almost all swine herds tested and in up to 100% of individual pigs within herds. There is a high prevalence of PCV infection in the global swine populations of Canada, France, Germany, New Zealand, Northern Ireland, United Kingdom, and the United States (Allan et al., 1994; Dulac and Afshar,1989; Edwards and Sands, 1994; Hines and Lukert, 1995; Horner, 1991; Magar et al., 2000; Tischer et al., 1982, 1986, 1995a; 1995b; Walker et al., 2000). Magar et al. (2000) found that PCV2 appeared to be the main PCV2 type circulating in Canadian pig herds and that serological evaluation using PCV1 underestimated the seroprevalence of PCV2. Most U.S. breeding herds and the majority of the sows within those herds were found to be seropositive for PCV2 (Opriessnig et al., 2004). Sibila et al. (2004) determined the presence of PCV2-antibodies by ELISA in 5 farms without a history of PMWS and in 4 farms with PMWS. Serum antibodies were detected in a higher percentage of pigs from PMWS farms but overall a high prevalence of PCV2 infection was found (Sibila et al., 2004)

It has been demonstrated that derivation of PCV2-free pigs can be achieved by cesarean-section and colostrum deprivation (Tucker et al., 2003). Segregated early weaning (SEW) techniques are also effective for derivation of PCV2-free pigs from PCV2-positive breeding herds (Opriessnig et al., 2004).

There is evidence from experimental inoculations that persistent infections may be a feature of PCV2 (Bolin et al., 2001). Viral DNA was detected in some cesarian-derived colostrum-deprived (CDCD) PCV2-inoculated pigs up to 125 days post infection. To confirm the presence of infectious virus, viral isolation was done on homogenates of tissues that were PCR positive. Virus was isolated from all tissues in which viral DNA was detected (Bolin et al., 2001). Proof of persistent infection in the field is contradictory. PCV2 viremia was detected in the same animals for at least 8 weeks by PCR confirming persistence of PCV2 in pigs after natural exposure (Larochelle et al., 2003). Rodríguez-Arrioja et al. (2002) found a long duration of PCV2 viremia (up to 28 weeks of age) in a high percentage of naturally-PCV2-infected pigs on a PMWS-affected farm in Spain. PCV2 nucleic acids were detected in sera from 52.6% of 386 healthy slaughter-age pigs (Liu et al., 2002). In another survey however, researchers were unable to demonstrate microscopic lymphoid lesions or PCV2 nucleic acids at slaughter suggesting that pigs typically clear the virus (Quintana et al., 2001).

Transmission of PCV2 is thought to occur through direct contact via oronasal, fecal, and urinary routes (Bolin et al., 2001; Magar et al., 2000a). Shibata et al. (2003) investigated PCV2 shedding in experimentally-infected CDCD pigs and in pigs with naturally-acquired PMWS by PCR. Sixteen pigs were inoculated with PCV2 and oropharyngeal and nasal swabs, feces, whole blood, and serum became PCV2-DNA-positive immediately after inoculation and all samples with the exception of the oropharyngeal swabs (1/2 positive) which remained positive up to 70 DPI. In field samples collected from 313 pigs, PCV2 was detected in 95 (30.4%) of the whole blood samples, 60 (19.2%) of the nasal swabs, and 64 (20.4%) of the feces implying that whole blood and serum are the best samples for detection of PCV2 by PCR (Shibata et al., 2003). Segalés et al. (2005) quantified PCV2 DNA in tonsillar, nasal, tracheo-bronchial, urinary and fecal swabs of pigs with and without PMWS. The authors were able to detect PCV2 DNA in a high percentage of the samples and concluded that PCV2 is most likely excreted through respiratory secretions, oral secretions, urine, and feces of both PMWS-affected and clinically-healthy pigs, with higher viral loads in the PMWS-affected pigs. Yang et al. (2003) detected PCV2 nucleic acids in feces in pigs with (14/54 intestines, and 4/9 feces) and without (3/14 intestines and 16/20 feces) enteric disease by PCR suggesting fecal-oral transmission of PCV2 in feces.

Direct contact with pigs inoculated with virus 42 days previously resulted in transmission of virus to 3 of 3 control CDCD pigs (Bolin et al., 2001). Vertical transmission has been demonstrated to occur in individual sows in the field (Ladekjær-Mikkelsen et al., 2001; O’Conner et al., 2001) and experimentally (Johnson et al., 2002). There are reports of vertical intrauterine transfer of PCV2 resulting in viremic or persistently-infected piglets at birth (West et al., 1999). Vertical infection with PCV2 may not always cause fetal death, and virus, antibody, or both were detected in clinically normal fetuses (Sanches 2003).

Larochelle et al. (2000) infected four 7-month-old boars intranasally with PCV2. Serum samples were collected at 4, 7, 11, 13, 18, 21, 25, 28, 35, 55, and 90 DPI and were tested for the presence of PCV2 DNA by nested PCR. PCV2 DNA was detected as early as 4 DPI in 3 of the 4 boars and serum samples were positive up to 35 DPI but negative by 90 DPI. Semen was collected at 5, 8, 11, 13, 18, 21, 25, 28, 33, and 47 DPI and analyzed by nested PCR. PCV2 was detected as soon as 5 DPI in semen of two of the boars and intermittently through 47 DPI in all 4 boars. The semen of 2/4 infected boars was positive for PCV2 DNA at 47 DPI (Larochelle et al., 2000). Kim et al. (2003) tested semen from ninety-eight 1-year-old boars from 49 herds in Korea and found 13 of 98 semen samples to be positive for PCV2 by first round (conventional) PCR, 26 of 98 semen samples were positive by semi nested PCR, and 11 of 98 semen samples were positive by virus isolation. The same study also investigated prevalence of PCV2 in seminal fluid, non-sperm cells, and sperm heads, and detected the greatest amount of PCV2 DNA in the seminal fluid and nonsperm fraction (Kim et al., 2003). The frequency of PCV2 DNA in semen from naturally infected boars was found to be low and sporadic (McIntosh et al., 2005). The authors concluded that boars seropositive for PCV2 may have persistent shedding of the virus in semen. PCV2 DNA in semen didn’t appear to affect the percent of morphologically-normal or live sperm cells in PCV2-shedding boars and boars older than 17.5 month of age did not appear to shed PCV2 DNA in semen (McIntosh et al., 2005).

PCV1 and PCV2 DNA were detected in porcine-derived commercial pepsin (Fenaux et al., 2004). It was found that the PCV-contaminated pepsin lacks infectivity in vitro and also in vivo as determined by culture in PK-15 cells and experimental inoculation of pigs (Fenaux et al., 2004).

Clinical Manifestation and Diagnosis of PCV2 Associated Disease

PCV2 Associated Lesions

Diagnostic Submission

Available Tests

Detection of PCV2 Nucleic Acids

Polymerase Chain Reaction (PCR) 

In-Situ-Hybridization (ISH) 

Detection of Anti-PCV2 Antibodies

Serum-virus neutralization (SVN) assay

Indirect immunoperoxidase monolayer assay (IPMA)

Indirect immunofluorescence (IFA) assay

Enzyme-linked immunosorbent assay (ELISA)

Detection of PCV2 Virus or Viral Antigen

Virus Isolation (VI)

Immunohistochemistry (IHC)

Electron Microscopy (EM)

Indirect and Direct Fluorescent Antibody Assays (IFA/FA) on Tissue Sections

Antigen-Capture ELISA

Further Characterization of PCV2 Isolate

Restriction Fragment Length Polymorphism (RFLP)

Sequencing

Successful treatment and control of PCVAD has primarily focused on assuring good production practices that minimize stress, eliminating coinfections or minimizing their effect and eliminating potential triggering factors that induce immune stimulation. The focus on control of PCV2-associated diseases remains on improving pig comfort and minimizing the effect of those coinfections or other circumstances that trigger PCV2 infection to progress to PCV2-associated disease.

Madec et al. (1999, 2000) proposed a 20-point plan to minimize the impact of PCVAD in severely affected farms.

Application of strict all-in and all-out production with thorough cleaning and disinfecting between batches.

Dams should be washed and treated for parasites before farrowing.

Cross-fostering should be limited.

Post-weaning Facilities

Post-weaning pens should be small and separated by solid partitions. 

Pits should be emptied, cleaned and disinfected on a regular basis. 

The stocking density should be lowered to 0.33 m2 per pig.

 The feeder space should be increased to more than 7 cm per pig. 

The air quality should be improved so that ammonia is less than 10 ppm, carbon dioxide is less than 0.1%, and the relative humidity is less than 85%.

The temperature should be controlled.

There shouldn’t be any mixing of batches

Grow/Finisher Facilities

Grow/finish pens should be small and separated by solid partitions. 

The pits should be emptied, cleaned and disinfected on a regular basis and strict all-in, all-out rules should be applied. 

There should be no mixing of pigs from the post-weaning pens.

There should be no remixing between finishing pens.

The stocking density should be lowered to more than 0.75 m2 per pig.

The air quality and temperature should be improved.  

In Addition the Following should be considered

The vaccination program should be appropriate.

The air and animal flow within buildings should be carefully controlled.

Strict hygiene should be applied (tail and teeth clipping, injections).

Sick pigs should be removed as soon as possible to a hospital room or should be euthanized. 

It is recommended that at least 16 of the points should be adopted in order for the plan to be effective (Madec et al., 1999). Usage of disinfectants in buildings and transport vehicles that have been demonstrated to be efficacious against PCV2 (Royer et al., 2001) is also recommended.

Rose et al. (2003) reported on the risk factors for PCVAD in French farrow-to-finish herds and found that things such as PPV or PRRSV coinfection of finishers, large pen size versus small pen size for weaners, and increased levels of cross-fostering increased the risk for PCVAD; whereas long empty periods in the pig flow, regular treatment against external parasites, pen versus crated gestation, and internal versus external gilt replacement decreased the risk for PMWS. López-Soria et al. (2005) did an exploratory study on risk factors for PDVAD involving 62 Spanish farms and found that vaccination of gilts against PRRSV increased the odds of PMWS expression and vaccination of sows against atrophic rhinitis decreased odds of the disease.

Chlortetracycline (CTC)-treatment used in the M. hyopneumoniae-PCV2-coinfection model provided evidence that the CTC treatment is highly efficacious in reducing lesions associated with PCV2 and M. hyopneumoniae coinfection (Halbur et al., 2005). At 6 weeks of age, pigs were inoculated intratracheally with M. hyopneumoniae, followed by intranasal inoculation with PCV2 at 8 weeks of age. At 8 weeks of age, half of the pigs received a CTC feed additive at an approximate dose of 22 mg/kg. The CTC-treated and coinfected pigs had significantly less severe clinical disease, macroscopic and microscopic lung lesions compared to the non-treated coinfected pigs.

A recent study evaluated the losses or gains associated with the use of three different commercially-available M. hyopneumoniae bacterins in pigs experimentally co-infected with M. hyopneumoniae and PCV2. Two hundred ninety-six M. hyopneumoniae-negative pigs were randomly assigned to one of four treatment groups. Three commercial vaccines, administered as per label direction, were tested: two bacterins containing an oil-based adjuvant and one bacterin containing an aqueous-based adjuvant. The M. hyopneumoniae challenge resulted in severe macroscopic and microscopic lesions in the non-vaccinated pigs. Pigs in all vaccine treatment groups had significantly higher mean body weight and average daily gain on 100 and 131 DPI, compared to the unvaccinated controls (Halbur et al., 2005).

Timing of use of adjuvanted bacterins may also be important in herds with recurrent PCVAD. Pigs should be vaccinated two to four weeks prior to expected PCV2 exposure as opposed to at or shortly after exposure, performed best (Opriessnig et al., 2006).

Porcine Parvovirus

Rodibaugh (2002) described changing a lepto-parvo-erysipleas program from weaning to pre-farrowing in a herd with approximately 7-8% mortality due to PCVAD. After intervention, the mortality rate declined to 2-3% (Rodibaugh, 2002).

Conventional pigs were experimentally coinfected with PCV2 and PPV and the effect of PPV vaccination in reducing disease and lesions associated with PCV2/PPV coinfection was investigated (Opriessnig et al., 2004). PPV vaccination was done 24 and 10 days before PCV2/PPV virus challenge with a killed PPV vaccine. Clinical signs consistent with PMWS (fever, respiratory disease, jaundice, weight loss) were seen in vaccinated and non-vaccinated PCV2/PPV coinfected pigs (Opriessnig et al., 2004).

PCV2 Vaccines

There is substantial and increrasing evidence that PCV2 vaccines will be useful in controlling PCVAD. Vaccination of sows with an inactivated oil-adjuvanted PCV2-vaccine (CIRCOVAC®; Merial Inc.) in field conditions in Europe was beneficial in reducing the PCV2 circulation and shedding in the first weeks of life, and also in improving the pig health after experimental PCV2 challenge at 3- 4 weeks of age (Charreyre et al., 2005). Eleven gilts free of PCV2-antibodies were vaccinated with the PCV2-vaccine intramuscularly at 5 and 2 weeks before breeding and again at 2 weeks before farrowing. A group of 22 piglets born to 4 vaccinated gilts and a group of 22 piglets born to unvaccinated control gilts were inoculated with PCV2 intranasally at 3-4 weeks of age. Seroconversion was observed in piglets born to seronegative dams. PCV2 DNA in serum and mesenteric lymph nodes was significantly (P = 0.00002) lower in piglets born to unvaccinated dams. In another field efficacy study, 3 groups of piglets were selected on farm and brought to the research facility. Group 1 pigs (n = 12) were born to unvaccinated sows, group 2 pigs (n = 10) were born to sows that had been vaccinated once with the PCV2-vaccine 2 weeks before farrwowing, and group 3 pigs (n = 11) were from a different farm and free of antibodies to PCV2. All piglets were infected intranasally with PCV2 at 25-47 days of age. Piglets born to non-vaccinated sows had a rise in PCV2 antibodies, whereas the PCV2-antibodies decayed in the pigs born to vaccinated sows. Lymph nodes were grossly unremarkable in this group whereas the lymph nodes were enlarged in the two other groups. During field efficacy studies of the PCV2 vaccine conducted in Germany and France it was found that there was a rise in PCV2 antibody level in the breeder herds concurrently with a decrease in PMWS rates in the pigs originating from the farms (Charreyre et al., 2005).

Pogranichniy et al. (2004) tested two inactivated US-PCV2 isolate preparations (ultraviolet irradiation or chemical inactivation) in the CDCD PCV2 PRRSV coinfection model and in the CDCD PCV2 KLH model using 57 piglets randomly assigned to 6 groups. Vaccination was done at 7 days of age and again 2 weeks later. PRRSV inoculation was done at 7 days of age. KLH with incomplete Freund’s adjuvant was infected at 21 and 27 days of age. At 24 days of age, the pigs were inoculated with PCV2. After PCV2 inoculation, the mortality in the vaccinated pigs was 20% whereas it was 70% in the non-vaccinated pigs implying that vaccination against PCV2 can be effective (Pogranichniy et al., 2004). 

It has been demonstrated that a chimeric PCV1-2 virus (with the immunogenic capsid gene of PCV2 cloned into the backbone of PCV1) induces an antibody response to PCV2 capsid protein and is attenuated in pigs (Fenaux et al., 2003). The attenuated chimeric PCV1-2 induced protective immunity to wild-type PCV2 challenge in pigs (Fenaux et al., 2004). A total of 48 SPF pigs were randomly and equally assigned to 4 groups of 12 pigs each. Pigs in group 1-3 were vaccinated with the chimeric PCV1-2 and pigs in group 4 were not vaccinated and served as controls. At 42 days post vaccination, all pigs were challenged intranasally and intramuscularly with wild-type pathogenic PCV2. Mild-to-severe lymphoid depletion and histiocytic replacement were detected in lymphoid tissues in the majority of nonvaccinated group 4 pigs but in only a few vaccinated group 1-3 pigs (Fenaux et al., 2004)

Blanchard et al. (2003) found the ORF2 protein to be a major immunogen, inducing protection in a prime-boost protocol. Thirty-five, 25-day-old SPF pigs were divided into 5 groups of seven piglets. Groups 1-4 piglets received an intramuscular injectin of DNA plamid preparation followed by a second injection 2 weeks later. Pigs were challenged intratracheally and intramuscularly with PCV2 10 days after the second injection. As evaluated by growth parameters, clinical signs and seroconversion, the pigs were protected against a PCV2 challenge after vaccination. In a second trial, sixty-four 4-week-old SPF pigs were divided into eight groups with eight pigs in each group. Piglets received either in intramuscular DNA injection or a intramuscular infection of baculovirus-expressed protein followed by a booster injection 2 weeks later. The piglets were inoculated with PCV2 11 days after second injection. The results indicated that protection induced by a subunit vaccine was even better than the one induced by a DNA vaccine, since PCV2 replication was completely inhibited (Blanchard et al. 2003).

Seven week old female Balb/c mice were vaccinated with a DNA-based vaccine against PCV2 structural protein on 0, 30, and 52 days and seroconverted to PCV2 (Kamstrup et al., 2004). The authors cloned a 768 bp fragment of the capsid protein of a Danish PCV2 isolate into the vector pcDNA1.1/V5-His/TOPO. The plasmid DNA was coated onto gold particles and used for particle mediated DNA vaccination. After 2 vaccinations, all mice had seroconverted to PCV2. A PCV2 challenge was not done in this study (Kamstrup et al., 2004).

Serotherapy

“Serotherapy” had been utilized extensively before commercial vaccines became available in Europe by some European practitioners to control and prevent PMWS; however, the literature reports of this technique are limited to abstracts describing uncontrolled field studies. In those trials, serum was typically obtained from healthy pigs in a group of pigs on the farm that went through PMWS 2-3 months prior. Recipient pigs were then treated by subcutaneous (Ferreira et al., 2001) or intraperitoneal (Waddilove and Marco, 2002) injection of convalescent serum. With both protocols, a significant reduction of clinical disease and mortality was observed. Sick animals treated with serum had a significantly increased survival rate in Spain (48%) and in England (58%) compared with normal survival of below 10% on both farms (Waddilove and Marco, 2002). The overall herd mortality rate of growing pigs in three trials was reduced from 15-18% in untreated pigs to 2-5% in treated pigs (Ferreira et al., 2001).

A controlled study tested efficacy of serotherapy at minimizing PCV2-associated disease and lesions (Halbur et al., 2005). Serum from experimentally-infected pigs in the acute and convalescent stages of infection was used. In addition, serum from pigs with high levels of anti-PCV2 maternal antibodies was also used. These three serotherapy treatments were compared to a group that was vaccinated with an experimental PCV1-2 chimeric vaccine. All pigs were necropsied at 21 DPI. The group vaccinated with the chimeric vaccine had significantly (P < 0.05) lower levels of viremia than all other groups and four of the seven pigs in this group remained PCV2 negative by PCR for the duration of the project. Under the conditions of this study, neither the acute- or convalescent-serotherapy were effective practices for preventing PCV2 infection and PCV2-associated lesions. The apparent success of serotherapy reported in European field trials remains unexplained. The experimental PCV1-2 chimeric vaccine appears to be effective, safe, and superior to serotherapy in reducing PCV2 infection and viremia (Halbur et al., 2005).