Corresponding author: Virginia Buechner-Maxwell, DVM, MS, Diplomate ACVIM, Professor, Virginia Polytechnic Institute and State University, Duck Pond Drive, Blacksburg, VA 24061-0342; e-mail: email@example.com.
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 reproductive and respiratory syndrome virus
U.S. Department of Agriculture
Porcine circovirus type 2 (PCV2) is the primary causative agent of several syndromes collectively known as porcine circovirus-associated disease (PCVAD). Although many other common organisms contribute to the clinical signs associated with PCVAD, PCV2 is the common link among the diseases and therefore it is vital to understand the biology of the virus for control of the disease. PCVAD is a globally emerging disease that is having a huge impact on swine-producing countries and is arguably the most economically important disease affecting the global swine industry today. The British Pig Executive national herd mortality data for finisher pigs in 2006 found that mortality increased from 3.3 to 6.5% when PCVAD began affecting herds. Fifty percent of affected farms had mortality rates over 9.7%.1
PCV2 infection is present in every major swine-producing country in the world and the number of cases of PCVAD is rapidly increasing. In 1998, University of Guelph's Animal Health Laboratory reported a pathologic diagnosis of <20 cases. In contrast, 350 cases were reported in 2005 by the same laboratory and these numbers continue to increase.2
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.
History of the Porcine Circovirus (PCV)
There exist 2 phenotypically different but genetically related strains of PCVs.3,4 Porcine circovirus type 1 (PCV1) was originally discovered in 1974 as a persistent contaminant of the porcine kidney PK-15 cell line ATCC CCL-33.5 This virus is widespread in the swine population but does not cause clinical disease and is nonpathogenic in swine.6–8 PCV2 was discovered in association with postweaning multisystemic wasting syndrome (PMWS) in Canadian weaning piglets in 1991.7,9–11 It is the smallest known freely replicating virus in vertebrates.12 PCV2 has been recognized as the primary causative agent of PMWS,13 now known as PCVAD since the name was modified in March 2006 by the American Association of Swine Veterinarians (AASV).
After Harding and Clark11 defined the syndrome in 1997, it became clear that PCVAD was the cause of many losses in pig herds in all the major swine-producing countries.3,7,10,13–15 Retrospective studies of pig serum samples found PCV2-specific antibodies as early as 1969 in Belgium,16 1970 in the United Kingdom,15 1973 in Ireland,17 and 1985 in Canada and Spain.18,19 PCV2 antibodies were also identified in 13.6% of tissues collected from Canadian pigs in 1985. Virus-positive samples increased to 72.4% in 1989, and then leveled off at 66.7% in 1997.18 In Northern Ireland, 69.1% of serum samples were antibody positive in 1973, 55% in 1984, 100% in 1988, and 92.1% in 1999.17
Retrospective studies on archived pig tissues in the United Kingdom diagnosed PCVAD in 68 cases between 1970 and 1997.15 Sequence analysis of the virus from those tissues showed a high sequence identity to isolates from a pig diagnosed in the year 2000 with another disease associated with PCV2, porcine dermatitis and nephropathy syndrome (PDNS). This indicated that the virus changed very little during the 30-year period.15 Serology has demonstrated that PCV2 antibodies are present globally in most swine herds and up to 100% of individual pigs within those herds 17,18,20; this includes herds in the United States.20
Current State of PCVAD
PCVAD is considered an emerging disease and the incidence of this disease has increased dramatically over the past years. In Part II of the Reference of Swine Health and Health Management in the United States published in the year 2000 by the U.S. Department of Agriculture (USDA),21 PCVAD prevalence was determined by USDA veterinarians who collected data from commercial herds of 100 or more pigs accounting for nearly 94% of the U.S. pig inventory (Table 1).
Table 1. Prevalence of PCVAD by type of operation, farm size, and year.21,22
Grower and finisher
PCVAD, porcine circovirus-associated disease; PDNS, porcine dermatitis and nephropathy syndrome.
Grower and finisher
Grower and finisher
Of nursery age pigs, the percentage of sites where PCVAD was known or suspected to have caused sickness or mortality in 1 or more pigs during the previous 12 months in farms with <2,000 animals was 4.4%, farms with 2,000–10,000 animals had 10.4%, and >10,000 animals reported 20.9% for an overall of 5.7%. Approximately 30% of animals were diagnosed by a veterinarian or diagnostic laboratory. In grower and finisher pigs, small farms had 2.3%, medium farms had 8.8%, and large farms had 12.4% affected pigs for an overall prevalence of 3.6%. Approximately 54% of the pigs reported to have PCVAD were diagnosed by a veterinarian or diagnostic laboratory.
When the same report was published in the year 2006, which again accounted for 94% of the U.S. pig population,22 prevalence in nursery pigs on small farms (<2,000 animals) was 21.5%, medium farms (2,000–5,000 animals) was 12.5%, and large farms (>5,000 animals) was 39.6% for an overall of 22.3%. Approximately 60% were diagnosed by a veterinarian. In growers and finishers, small farms reported 25.0%, medium farms 35.4%, and large farms 59.9% for an overall of 31.3%. Approximately 70% were diagnosed by a veterinarian or diagnostic laboratory.
The prevalence of porcine dermatologic and nephropathy syndrome was also described for the 1st time in the 2006 report, and in nursery pigs of small farms the prevalence was 3.3%, medium farms 0.0%, and large farms 3.4% for an overall of 2.9%. In growers and finishers the numbers were 1.6% for small, 10.4% for medium, and 23.9% for large farms, overall prevalence was 6.0%. The age of onset for PCVAD affected pigs ranged from 8.9 to 16.3 weeks (Table 1).
PCV1 and PCV2 belong to the family Circoviridae7,10,13,23 and to the genus Circovirus.24 Other known viruses in this genus are canary circovirus, goose circovirus, pigeon circovirus, and psittacine beak and feather disease virus. Another genus in the circoviridae family, Gyrovirus, includes chicken anemia virus.24 The Gyrovirus genus is distinct in that it has a negative sense genome and larger virions than is typical for circovirus.25 Circoviruses are host specific, most of which are avian, or have a relatively narrow host range. Several species produce lymphoid depletion in infected hosts whereas others cause subclinical infection.24 PCV1 is most closely related to the beak and feather disease virus.26 Several human circoviruses also exist and include the torque teno virus (TTV), which is related to the swine TTV, TTV-like mini virus, and the SEN virus. The human circoviruses have not been definitively linked to any disease in humans.27–30
Circoviridae is most closely related to the family Nanoviridae, which includes plant viruses. These viruses share a step loop structure at the origin of replication and show similarities in the replication proteins.31 These similarities are also shared by the plant Geminiviruses, and it is speculated that circovirus may be the genetic link between the 2 plant virus families.26 It has been proposed that an ancestor of PCV1 may have been a plant nanovirus that infected a vertebrate host and recombined with a vertebrate-infecting RNA virus, which was most likely a calicivirus.32
PCV2 has been divided into 2 distinct genotypes. They have been named PCV2-group 1 and PCV2-group 2.4 There is no difference in pathogenesis between the 2 genotypes, but the viruses differ in size with PCV2-group 1 being 1,767 nucleotides (nt) and PCV2-group 2 being 1,768 nt.4 PCV2-group 1 is further divided into 3 clades and PCV2-group 2 divided into 5 clades.4 Historically, only PCV2-group 2 isolates were found in the United States; but in late 2005 several outbreaks of higher than normal mortality (5–50%) were reported in Kansas, North Carolina, and Iowa. These outbreaks were found to be associated with PCV2-group 1 isolates.33
About the same time as PCV2 was being grouped into group 1 and group 2 isolates, North American laboratories proposed grouping PCV2 into North American isolates, or PCV2a, and European-like isolates, or PCV2b.34 PCV2b falls into PCV2-group 1 and PCV2a falls into PCV2-group 2.34
PCV1 and PCV2 have a small, nonenveloped icosahedral virion5 with a single-stranded, circular DNA genome of 1,759 nt and 1,767–1,768 nt in size, respectively.3,31 They contain 2 major open reading frames (ORFs) encoded in an antisense direction24 (Fig 1). ORF1 encodes for viral replication proteins (Rep), and ORF2 encodes for the capsid protein (Cap), which contains the immunodominant antigenic epitopes.31,35–38 ORF1 is very similar between PCV1 and PCV2 with 83% nucleotide and 86% amino acid identity between them.39 ORF1 is alternatively spliced into 8 RNA strands in PCV1 and 5 RNA strands in PCV2. Only 2 RNA strands, Rep and Rep′, are essential for virus replication.36,40,41 PCV1 and PCV2 share 67% nucleotide and 65% amino acid sequence identity in ORF2.39 A 3rd ORF has been described in PCV2 and it has been suggested that ORF3 is involved with apoptosis,42 but this report has not been able to be verified by independent laboratories. Overall, PCV1 and PCV2 share 76% nucleotide sequence homology and are similarly organized.3,43
Virus Life Cycle
The mechanisms of PCV2 cell recognition, attachment, and entry are currently being researched and not well understood. It is believed that PCV2 uses a relatively common cell receptor, because viral replication and PCV2 antigen has been found in many different cell types.44 PCV2 binds to heparin sulfate and chondroitin sulfate, which are glycosaminoglycans (GAGs), as a 1st step of attachment.45 However, as PCV2 is found in cells that lack GAGs, it is thought that another coreceptor is also used for viral entry.45
The hallmark lesion of PCV2 infection is lymphoid depletion with histiocytic replacement. In affected lymph organs, dendritic cells, and macrophages that replace the lymphocytes contain large amounts of PCV2 virus.7,46,47 There is no viral degradation in these cells, and because dendritic cells are highly mobile, it is thought that dendritic mobility may be a method of viral dissemination in tissues.48 It is still unknown how PCV2 causes a reduction in lymphocytes. Hypotheses include induced apoptosis, decreased lymphocyte production in the bone marrow, or reduced lymphocyte proliferation in secondary lymphoid tissue.23
Because PCV2 encodes for only 2 major proteins, it is thought that the virus relies on its host cell for protein expression and for replication. The virus requires an actively replicating cell, specifically in the S-phase,49 for DNA replication via a DNA polymerase. PCV2 replication is speculated to involve a rolling-circle method.50,51 It was determined in porcine kidney (PK-15) cells that the 1st detectable protein produced postinfection is the Cap protein, which was localized in the perinuclear area of the cell.52 At 12 hours postinoculation, both Cap and Rep were detectable in the nucleus. By 36 hours postinoculation, the viral titer had stabilized indicating that the virus had completed replication.52 There is currently no information on how the capsid is made, how the virus assembles or how it is released from infected cells.
PCV2 can be transmitted in several ways. The main route is by oro-nasal contact with infected feces,13 contact with infected urine, or directly with infected pigs.53,54 PCV2 is shed in respiratory secretions, oral secretions, urinary secretions, and feces in both clinically affected as well as in infected but apparently healthy pigs. Clinically affected pigs shed virus in higher quantity compared with infected but clinically healthy pigs.55 PCV2 can also be transmitted vertically (from the sow to the piglets) through the placenta causing persistently infected piglets at birth, but this method of transmission appears to be rare.56–59 PCV2 has also been shown to be shed in colostrums,60 but whether this can result in an infection is still being investigated. Recent work has shown that PCV2-infected pork products (lymphoid tissue, skeletal muscle, and bone marrow), when fed to naive piglets for 3 days, resulted in viremia and seroconversion to PCV2 in all of the piglets.61
PCV2 is also shed in semen and in experimental studies seminal virus shedding was detected as early as 5 days postinoculation.62–64 Shedding in naturally infected boars appears to be low and sporadic.62 The greatest amount of virus appears in the seminal fluid and nonsperm fraction.65 Boars that are persistently infected may continue to shed the virus in semen and semen samples were found to be positive in boars up to 71 weeks of age. Samples collected from boars ranging from 71 to 149 weeks of age were not found to shed virus in semen.62 The virus did not appear to affect the percentages of live and morphologically normal sperm.62 Recent evidence has shown that PCV2 virus present in the semen is infectious when injected IP into pigs, but failed to seroconvert gilts that were artificially inseminated. In addition, all pigs born of the gilts were negative for PCV2 antibodies.66,67
The incubation period in experimentally infected pigs ranges from 2 to 4 weeks.54,68–70 Once inside the host, PCV2 1st infects the tonsils and lymph nodes of the head and begins replicating.71 PCV2 also infects B cells72 that likely causes dissemination throughout the body via the lymphatic system. PCV2 has been detected in the spleen, Peyer's patches, and many lymph nodes.73 PCV2 then starts replicating in T cells and peripheral blood mononuclear cells.72,74 Viremia in pigs is detectable between 7 and 14 days postinoculation. PCV2 has the ability to cause a prolonged infection, with viral DNA detectable in pigs up to 125 days postinoculation in experimental infections.53,54,68–70,75–77
Because most breeding age sows are seropositive for PCV2, most piglets are born with maternal antibodies against PCV2.78 In weaned piglets, the mean half life of antibodies is 19 days. Antibody levels will wane at 4–6 weeks in pigs with initially low levels of antibody, at 6–10 weeks with moderate antibody levels, and by 8.5–13.5 weeks in pigs with high antibody levels.20 Piglets do not typically demonstrate clinical signs of disease before 4 weeks of age, suggesting that maternally derived antibodies are protective.78–81 Experimental studies found that maternal antibody protection is dependent on the level of maternal antibodies present. High levels of maternal antibodies are more protective than low levels, but do not completely prevent infection, whereas low levels of antibodies did not provide any protection against infection.78
Two- to 3-month-old pigs are capable of producing an antibody response to PCV2 infection, but this response is not completely protective as these pigs can still develop viremia.82–84 Experimental infections show that pigs seroconvert between 14 and 28 days postinfection (DPI).68,69,76,85 By 10 DPI, pigs can develop neutralizing antibodies that increase in titer up to 21 DPI.86 Neutralizing antibodies were detected in naturally infected Belgian pigs by 10 weeks of age, and in Danish pigs at 3 weeks of age. Pigs that develop PCVAD have low or undetectable levels of neutralizing antibodies.86
PCV2 pathogenesis appears to be related to the immunomodulatory effects of the virus. PCV2 infection results in a decreased expression of B-cell growth factor IL-4 and the cytotoxic T cell and macrophage-activating cytokine IL-2.87 This results in a decreased proliferation of lymphocytes and the interferon antiviral response88 although causing an increase in expression of proinflammatory cytokines IL-1B and IL-8.87
PCV is believed to be a species-specific virus; however, antibodies to PCV1 have been detected in mice, cattle, and humans,89 but currently PCV2 is not considered to be a zoonotic disease. However, with the advent of xenotransplantation using porcine organs, the risk of implanting PCV2 infected organs into immunocompromised xenograft recipients should be investigated.
Factors that Modulate Diseases Caused by PCV2
PCV2 infection is characterized by having a high prevalence of infection but low morbidity, and thus not all animals infected with PCV2 will develop clinical signs of PCVAD.90–92 Seroprevalence in commercial herds in some countries is near 100%.92,93 Although most pigs in a herd will become viremic, only 5–30% of susceptible pigs will show clinical signs of PCV2.90,92 There are 4 main factors essential in the expression of PCV2-related diseases: viral effects, host effects, and the effects of coinfection and immunomodulation.23
Although PCV2 is capable of causing several distinct disease syndromes, there are no significant differences in the virus genomes recovered from the different syndromes. Sequence analysis of PCV2 from PCVAD-affected pigs, and pigs with clinically unapparent infection, showed 95.6–100% sequence homology and no distinct patterns of sequence variations were evident between the 2 groups. This has led to the belief that there are other factors affecting the expression of disease.94 It has been demonstrated that PCV2 isolated from pigs without disease can cause PCVAD under experimental conditions.95,96
It was shown that 2 amino acid mutations in the PCV2 genome significantly altered the gross and histopathologic lesions seen in pigs, indicating that only minor alterations in the viral genome are required to alter the function of the virus.97 In the Canadian outbreak of 2004, a change in virus type was demonstrated that caused a much more severe disease characterized by pulmonary edema, granulomatous enteritis, more severe lymphoid depletion, and lymphoid necrosis.98 Subsequent reports of the introduction of PCV2b into the United States were associated with severe outbreaks in Kansas, Iowa, and North Carolina in 2006.33 It is unknown if the increased prevalence of PCV2b is associated with a change in virulence, new introduction to the area, or other factors that allowed an increase in replication of this viral type.
All breeds of pigs appear to be susceptible to infection, and clinical disease has been observed in many purebred and crossbred pigs (PG Halbur, unpublished data). However, studies have shown differences in susceptibility in different breeds of pigs.99,100 Differences in the type of adaptive immune response against PCV2 in different pigs may explain the host variation in the outcome of infection.101 There are significant differences in the replication patterns of PCV2 in alveolar macrophages from different conventionally crossbred pigs.52
Although PCV2 is required to cause the characteristic lymphoid depletion of PCVAD, many strains likely require a cofactor to cause the full spectrum of clinical signs associated with PCVAD. Coinfection with several other viral and bacterial pathogens has been shown to cause an increase in incidence and a markedly more severe clinical course of disease. The agent implicated as creating the greatest risk is porcine reproductive and respiratory syndrome virus (PRRSV).102 Other agents include porcine parvovirus (PPV),68,75,103–105Mycoplasma hyopneumoniae,106 and very recently the TTV, which singly is not associated with disease but present in many pig populations.107,108 A retrospective analysis of the number of PCVAD cases in which there were coinfections was performed by the Iowa State Veterinary Diagnostic Laboratory. The results showed that more than 98% of pigs had coinfections. Specifically, 52% were coinfected with PRRSV, 36% with M. hyopneumoniae, 15% with PPV, 14% with bacterial septicemia, 7.6% with bacterial pneumonia, and 5.4% with swine influenza virus. A single PCV2 infection occurred in only 1.9% of the cases.109
Part of the pathogenesis of coinfection causing more severe disease may be associated with immunostimulation before PCV2 infection. One study showed that pigs that were immunostimulated with keyhole limpet hemocyanin developed clinical PCVAD when infected with PCV2.85 There is also mounting evidence that common adjuvanted vaccination regimens may actually enhance the development of PCVAD.110–112 In pigs vaccinated with the same antigen, but different adjuvants, the oil-in-water adjuvant was shown to cause a longer length of viremia, increased amounts of PCV2 in serum and tissue, and more severe lymphoid depletion when compared with pigs vaccinated with aqueous and aluminum hydroxide products.113 Similarly, vaccination with M. hyopneumoniae or Actinobaccilus pleuropneumoniae vaccines followed by immediate infection with PCV2 in specific pathogen-free pigs caused a significant increase in viremia duration and more severe histopathologic lesions than in nonvaccinated pigs.112
The effects of immunosuppression on disease caused by PCV2 have also been studied. Infection of pigs with PCV2 after injection with cyclosporine caused an increase in PCV2 replication, and a higher titer of virus compared with controls, but the pigs did not develop clinical PCVAD.114 In another study, pigs treated with dexamethasone before PCV2 infection developed a granulomatous lymphadenitis that was not observed in pigs inoculated with PCV2 alone.115 In addition, a series of studies indicate that cell-mediated immunity plays an important role in protection.45,78,101,116 A proportion of pigs vaccinated with a live PCV1-2 chimeric vaccine developed only low levels of antibody and yet the vaccinated pigs were fully protected against subsequent challenges with PCV2.116,117
PCVAD recently replaced the older name of PMWS. The name PCVAD was adopted to be inclusive of all the recognized syndromes associated with PCV2 infection.118 According to the AASV, PCVAD can be subclinical or include 1 or more clinical manifestations including multisystemic disease with weight loss and high mortality, respiratory disease, porcine dermatologic and nephropathy syndrome, enteric signs including diarrhea, and reproductive disorders on an individual or herd basis.119 Distinguishing the different forms of PCVAD can be accomplished by observation of gross or histopathologic characteristic lesions in the intestines, lungs, and lymphoid tissue.23
The most significant manifestation of PCVAD is the multisystemic syndrome. This syndrome has been recognized in wild boars, but the source of the infection is believed to be the domestic pig.120 This disease affects pigs between 7 and 16 weeks old in the United States and 5–12 weeks old in Europe.7,121,122 This age difference is most likely related to variation in management practices and vaccination timing between producers in the United States and Europe.112 Morbidity is associated with the development of viremia and lymphopenia in piglets followed by the clinical manifestations of disease. Mortality is usually around 10%7,11,121 (range 4–20%),121 but can reach 50%.7,11 Because the clinical course of wasting and decreased economic efficiency can be prolonged, 70–80%121 of pigs that develop PCVAD are subsequently euthanized.
Clinical signs of PCVAD include wasting with progressive weight loss, lethargy, dark-colored diarrhea, lymphadenopathy, and paleness or jaundice. The main characteristic histopathologic lesions are lymphoid depletion with histiocyte replacement in lymphoid tissues, and intracytoplasmic inclusion bodies.6,7,11,13,71,123 Early signs of reduced weight gain, ill-thrift, pale skin, and rough hair coat often go unnoticed or are misdiagnosed. Later signs include dyspnea, tachypnea, anemia, diarrhea, and jaundice.11 Pigs can also have coughing and gastric ulceration, which most likely contributes to the anemia. On necropsy, the lungs fail to collapse and are mottled, tan colored, and in chronic cases some kidneys have white streaks or spots.23 Affected pigs also have enlargement of the superficial inguinal, submandibular, mesenteric, and mediastinal lymph nodes.71 Granulomatous lesions can also be found in the lungs, liver, kidney, heart, and intestines.23
A scoring system has been developed that estimates the severity of disease based on the extent of lymphatic tissue involvement. The 7 lymphoid tissues that are evaluated for the purpose of scoring include the tracheobronchial lymph nodes, the mesenteric lymph node, the mediastinal lymph nodes, the superficial inguinal lymph nodes, the external iliac lymph nodes, the tonsils, and the spleen. This system accounts for the severity of lesions, the amount of PCV2 antigen and the distribution of the lesions. Scores are assigned and range from 0 to 9.106 Although this system is useful for classifying the severity of disease, it is impractical for field necropsies.
Subclinical PCV2 Infection
PCV2 infection can be limited to 1 or 2 lymph nodes in the absence of evidence of clinical disease.106,124 However, the presence of PCV2 might be associated with a decrease in vaccine efficacy125 and healthy pigs can still exhibit a necrotizing lymphadenitis.124,126 The significance of this finding to the pig is unknown, but can cause the carcass to be condemned at slaughter.23
This syndrome affects piglets from 8 to 16 weeks old and resembles chronic ileitis associated with Lawsonia intracellularis infection. Affected piglets have diarrhea, unthriftiness, retarded growth, and increased mortality. Histopathologic lesions include a granulomatous enteritis and characteristic PCV2 lesions in Peyer's patches but not in other lymphoid tissues. At necropsy, mesenteric lymph nodes are enlarged and the intestinal mucosa is grossly thickened. Histopathology is able to easily distinguish between Lawsonia versus PCV2 infections.127
This syndrome can play a role in porcine respiratory disease complex.128,129 It affects pigs from 8 to 26 weeks old and is associated with multiple pathogens. The clinical signs include decreased rate of growth, decreased feed efficiency, anorexia, fever, cough, and dyspnea. This can be very similar to systemic infection and there is some overlap of the syndromes. The histopathologic lesions include a granulomatous bronchointerstitial pneumonia with mild to severe necrotizing and ulcerative bronchiolitis and bronchiolar fibrosis. Differentials for the bronchiolitis lesions include swine influenza or porcine respiratory coronavirus infections.23
PCV2-Associated Reproductive Failure
This syndrome was first reported in Canada in 1999130 and typically affects gilts and start-up operations.131 The clinical signs include increased abortion, still births, fetal mummies, and preweaning mortalities. The histopathologic lesions include a nonsuppurative to necrotizing or fibrosing myocarditis in still born and neonatal pigs.131 The time of infection determines the clinical course of the disease. Fetuses inoculated at 57 days of gestation had higher viral replication than those infected later in gestation and when killed at 21 days postinoculation had edema, enlarged livers, and congestion. Fetuses inoculated at 75 and 92 days of gestation failed to produce similar lesions or viral loads.132 Late term infections at 86, 92, and 93 days of gestation caused an increase in reproductive abnormalities including still birth, fetal mummies, and weak piglets.133 However, data from field cases indicate that most breeding herds appear to be immune to this disease.23
This syndrome was first described in the United Kingdom in 1993134 and was associated with PCV2 in the year 2000.135 This disease is often fatal within 3 days of development and mostly affects grower pigs, but can affect pigs as young as 5 weeks old. Clinical signs include an acute onset of fever, lethargy, and raised purple skin lesions progressing to multifocal red-purple scabs with black centers being most prominent on the rear legs. At necropsy the kidneys are enlarged, tan, and waxy in appearance with petechial hemorrhages. Histopathologically, there is a systemic vasculitis with dermal and epidermal necrosis and necrotizing and fibrinous glomerulonephritis appearing similar to a type 3 hypersensitivity reaction with deposition of antigen-immune complexes in the vascular and glomerular capillary walls.23 The development of disease is aided by coinfection with PRRSV,136,137Pasteurella multocida, Streptococcus suis types 1 and 2, among others.138,139 Recently, PDNS was experimentally reproduced with PRRSV and TTV in PCV2-free pigs.140 Therefore, PDNS is not always associated with PCV2.
In 2001, PCV2 was associated with pigs born with congenital tremors and a nonsuppurative menigoencaphalitis located in the brain.43,59,141 More recent reports have associated PCV2 infection with cerebellar lymphohistiocytic vasculitis combined with hemorrhages or with lymphohistiocytic meningitis. PCV2 antigen was found with immunohistochemistry (IHC) in the cytoplasm and nuclei from intralesional perivascular machrophages and endothelial-like cells in the brain tissue.142 In addition, naturally occurring neurologic disease characterized by opisthotonus, nystagmus, and convulsions was associated with PCV2 infection in pigs ranging from 6 to 8 weeks old in which cerebellar vasculitis was also present.143 The role of PCV2 in the development of this disease is still under investigation, but may indicate a new spontaneously occurring type of PCV2 disease.
The diagnosis of PCVAD is based on clinical signs and demonstration of PCV2 antigen in more than 1 lymphoid tissue, or 1 lymphoid tissue and 1 other organ system such as the lungs, liver, kidney or intestine, or in 2 organ systems. If antigen is found only in 1 organ system, then the disease is categorized based on that organ system. If only limited PCV2 antigen is found but there are severe lesions, it is classified as chronic severe PCVAD.23 Scoring of lesions and the amount of antigen in tissues allows for staging of infection.23 Diagnosis can be tentatively made based on clinical signs. In a survey of farms experiencing PCVAD disease, the percentage that observed the following clinical signs included wasting (98.1%), diarrhea (77.2%), dyspnea (75.1%), lymphadenopathy (44.8%), central neurologic signs (39.6%), jaundice (37.1%), inappetence (90.4%), and death (96.8%).
Detection of PCV2 antigen or nucleic acid is considered the gold standard for the diagnosis of PCVAD. The best tests for this are polymerase chain reaction, in situ hybridization (ISH) and IHC.23 There is currently no information on sensitivity and specificity of these tests, but IHC gives more intense staining and is considered more sensitive, but less specific than ISH. IHC is also cheaper to run and has a faster turn around time. However, many labs do not offer IHC, because one of the required reagents is anti-PCV2 antiserum. Although a monoclonal anti-PCV2 is commercially available, definitive diagnosis can still be difficult with that product. The best way to diagnose PCVAD is the identification of the characteristic lesions of the disease. Microscopic lesions associated with PCVAD include syncytial cells in lymph nodes, Peyer's patches, and the lamina propria of intestinal villa. In addition, macrophages have sharply demarcated, spherical, basophilic cytoplasmic inclusion bodies.71
Serology can also be performed and is a convenient method for detecting exposure to PCV2 for large numbers of pigs. However, it must be remembered that many clinically healthy pigs are seropositive. Other tests that have been developed include immunofluorescence assay, IgM immunoperoxidase monolayer assay, enzyme-linked immunosorbant assay, virus isolation, electron microscopy, and serum virus neutralization assays.9,15,23,39,76,89,101,103,144–149 There is currently no field test for the diagnosis of PCVAD.
Management and Classification of Herd Outbreaks
No specific treatment is available for diseases associated with PCV2 infection. In general, treatment of individually affected pigs is supportive only and will vary greatly depending upon the clinical signs that the animal displays. Because many animals are coinfected, choosing appropriate treatment will also depend upon identification of the other agents infecting the animal. In addition, prognosis is dependent upon animal factors, such as age as well as the syndrome that the animal displays. There is currently no data on the existence of PCVAD in pet potbellied pigs, but some veterinarians are recommending PCV2 vaccination to their clients. During initial PCVAD outbreaks, pigs treated with antibiotics actually suffered a higher mortality rate than those not treated, but it is believed that this was more because of spreading the virus with common use needles than any effect by the antibiotics (Dr RB Baker, personal communication, Iowa State University, Ames, IA).
PVC2-related syndromes are of greatest economic concern when they occur in herd populations. To identify and manage outbreaks of PCV2 and PCVAD diseases within a herd, it is important to determine whether the disease is a significant herd problem or is only sporadically causing a herd problem. A definition has been developed to help with this determination. An important herd problem has been defined as an increase in mortality of equal to or more than the mean of historical mortality levels plus 1.66 times the standard deviation. If there are no historical mortality data, a herd problem can be defined as an increase in mortality that exceeds the national or regional level by 50%. In other words, if 50% or more of the pigs from a representative sample are diagnosed with PCVAD and there has been a significant increase in mortality compared with previous mortality data, it is considered a herd problem. However, if <50% of the pigs are diagnosed with PCVAD, but there has still been an increase in mortality or if more than 50% of the pigs are diagnosed with PCVAD but there has been no increase in mortality, the outbreak may be considered sporadic and not a herd problem.150
Prevention of PCV2-Associated Diseases
Prevention of PCVAD can be difficult. Disease outbreaks are reported to occur on farms even with strict isolation practices. It has been shown that vaccination against M. hyopneumonia or A. pleuropneumoniae 2–4 weeks in advance of infection with PCV2 prevented any lesions associated with PCV2 infection,112 but the practical issues of this method make it nearly impossible to accomplish. PPV vaccination does not reduce the severity of PCVAD in coinfected pigs105 but a combined PPV and swine erysipelas vaccine appeared to protect against PCV2-induced reproductive failure.151
Treatment of bacterial infections and prevention of cofactor-associated diseases is also a good practice in preventing PCV2 diseases. Treating M. hyopneumonia with chlorotetracyclines was highly effective.152 Bleach (3–6% sodium hypochlorite) is an effective chemical in killing PCV2, but has unknown field efficacy. The protocol utilized at Iowa State University to disinfect pens is included in Table 2.23
Table 2. Iowa State University disinfection protocol
Antec International (Sudbury, Suffolk, UK).
b US Pharmacal Com LLC (Erie, CO).
Apply degreaser detergent with foamer at a 1 : 64 dilution (Iowa uses grease-free PV, but any product should work the same)
Before occupancy, fog with Clindox-Sb at 1 : 5 : 1 dilution and let dry
Rinse with water 6–12 hours after fogging and allow to dry
Good housing management is critical in disease prevention. It has been shown that reducing stress, paying attention to proper hygiene, preventing mixing of ages, and utilizing all in/all out practices are effective in controlling disease. Other options include immunized serum therapy, which has practical limitations, and depopulation, which has been shown to be ineffective because the virus is very resistant in the environment.153 Whether PCV2 can be found in insects or wild animals that could possibly transmit disease to pigs is not known. However, because circoviruses are highly species specific, it is unlikely that these animals, excluding feral boars, would pose a threat of PCV2 transmission to domestic herds.154
Risk factors for development of infection include PPV or PRRSV infection, large pen sizes versus small pen sizes for weaning piglets, increased levels of cross fostering155 and vaccination against PRRSV.156 As such, reduction of these risk factors on farms may be helpful in controlling PCVAD. A study in Canada showed a strong association of increased mortality with M. hyopneumoniae infection, PRRSV, diarrhea caused by Escherichia coli K88, close proximity to other herds, multiple suppliers, large within-group range in age of pigs and not using spray-dried plasma in 1st nursery rations.157 Factors that decreased the risk included long empty times between pig groups, regular treatment of external parasites, pen versus crate gestation, internal versus external gilt replacement,155 and vaccination against atrophic rhinitis.156
Vaccine Development and Vaccination
Initial attempts at vaccine development included using a killed PPV vaccine. Because pigs are often coinfected with PPV and PCV2, it was hoped that vaccination at an early age would prevent PCVAD. This approach empirically appeared promising in the field, but a benefit was not confirmed when the vaccine was tested under more controlled conditions.105,158,159
One of the 1st PCV2 vaccines on the market was Merial's CIRCOVAC (Duluth, GA), an inactivated PCV2 vaccine with oil adjuvant for use in breeding age animals.23 This vaccine was most extensively used in Europe and it was also available in Canada. The vaccine was successful in reducing PCV2 circulation and shedding in the 1st weeks of life of the piglets born to the vaccinated sows.160 This vaccine is designed to be given as 2 injections IM 3–4 weeks apart and completed at least 2 weeks before breeding and once at each subsequent gestation.23
Another vaccine available in the United States is the Boehringer Ingelheim's Ingelvac CircoFLEX vaccine (Petersurg, VA), a capsid-based subunit vaccine expressed in inactivated baculovirus. This vaccine demonstrated significant decreases in mortality in vaccinated pigs versus unvaccinated pigs on 4 different Canadian finishing sites.161 This vaccine is to be given in a single dose IM to piglets >3 weeks of age.23 A field trial for this vaccine recently was reported in the United Kingdom in which 3-week-old pigs were vaccinated. Mortality caused by PCVAD was reduced from 14.3 to 4.6%.162
A 3rd vaccine that is produced by Intervet Inc/Schering-Plough Animal Health (Kenilworth, NJ) is also a capsid-based subunit vaccine. This vaccine, expressed in a baculovirus, is marketed under the name Circumvent PCV in the United States and Canada and Porcilis PCV in Europe and Asia. It is designed for vaccination of piglets 3 weeks and older. Circumvent PCV is given in 2 doses 3 weeks apart at 3 and 6 weeks of age given IM.23 Porcilis PCV only requires 1 dose. Studies including 35,000 pigs on 21 different farms showed that mortality of vaccinated pigs was reduced by 77.5% when compared with unvaccinated pigs.163
The 1st USDA fully licensed PCV2 vaccine was licensed for use in pigs 4 weeks of age or older. This genetically engineered chimeric vaccine was created by inserting the immunogenic capsid protein of PCV2 into the genetic backbone of the nonpathogenic PCV1.97,116,117,164 This chimeric vaccine was shown to be attenuated in pigs and was able to prevent the viremia and lymphopenia associated with PCV2 morbidity.116,117 This vaccine was named PCV1-2 to indicate its chimeric nature, and was released in July 2006 in a killed form labeled as Suvaxyn PCV2 One dose (Fort Dodge and Wyeth Animal Health, Fort Dodge, IA). The vaccine is given as a single dose IM.
Field studies on Suvaxyn PCV2 One dose indicate that it is very effective and safe. The vaccine is able to decrease the mortality from 8–10% in nonvaccinated pigs to 1.0–2.0% in vaccinated pigs. Safety studies have been completed on over 1,000 pigs in 4 different locations. No adverse reactions were recorded in any of the vaccinated pigs. In addition, experiments have proven that the PCV1-2 vaccine is able to break through maternal antibodies to provide protection in piglets against PCV2 infection. This allows piglets to be vaccinated before maternal antibodies wane and before the typical age of infection with PCV2.165 The live version of the chimeric PCV1-2 vaccine has also been demonstrated to be genetically stable in vaccinated pigs,166 and thus could serve as a good candidate for a live vaccine against PCV2.
Since its initial discovery in 1991, PCV2 and PCVAD have had a significant and adverse impact on the economy of the swine industry. There are currently 7 recognized syndromes related to PCV2 infections. Many of these syndromes are a result of coinfection with PCV2 virus and other agents like Mycoplasma and PRRSV. Diagnosis of PCV can occasionally be difficult because of nondescript clinical signs but diagnostic lab tests are available. The current knowledge and research into vaccines is providing relief for the swine producer from the heavy losses associated with the disease. Key points in reducing loses are centered on proper vaccination and management. The reduction in swine loses already being witnessed after the introduction of vaccines in the world market has been significant, and further research to provide even better vaccines will likely continue to reduce economic loss in the world swine market.