A sequential study of experimental infection of pigs with porcine circovirus and porcine parvovirus: immunostaining of cryostat sections and virus isolation

Dynamic Changes in Viral Loads during Co-Infection with a Recombinant Turkey Herpesvirus Vector Vaccine and Very Virulent Marek's Disease Virus In Vivo

Marek's disease (MD), caused by the Marek's disease virus (MDV), is a common infectious tumor disease in chickens and was the first neoplastic disease preventable by vaccination. However, the vaccine cannot completely prevent virulent MDV infections, allowing both the vaccine and virulent MDV to coexist in the same chicken for extended periods. This study aims to investigate the changes in viral load of the very virulent strain Md5 and the rHVT-IBD vaccine in different chicken tissues using a real-time PCR assay. The results showed that the rHVT-IBD vaccine significantly reduced the viral load of MDV-Md5 in different organs, while the load of rHVT-IBD was significantly increased when co-infected with Md5. Additionally, co-infection with Md5 and rHVT-IBD in chickens not only changed the original viral load of both viruses but also affected the positive rate of Md5 at 14 days post-vaccination. The positive rate decreased from 100% to 14.29% (feather tips), 0% (skin), 33.33% (liver), 16.67% (spleen), 28.57% (thymus), 33.33% (bursa), and 66.67% (PBL), respectively. This study enhances our understanding of the interactions between HVT vector vaccines and very virulent MDV in chickens and provides valuable insights for the future development of MD vaccines.

A Slaughterhouse Survey for Porcine Circovirus Type 2 in Commercial Pigs in Ibadan, Southwest Nigeria

Porcine circovirus type 2 (PCV2) is recognized as one of the most important agents of reproductive disorders in gilts and sows worldwide. It is associated with considerable economic losses in the swine industry due to the unthriftiness, and variable morbidity and mortality it causes in pigs. In spite of the devastation caused by this virus to the global pig industry, there is little or no report of its occurrence in Nigeria. Hence, a slaughterhouse based survey was conducted to determine the prevalence of PCV2 infections in pigs in Ibadan, southwest Nigeria. Using a commercial ELISA kit, 364 pig sera collected from a major abattoir were screened for IgG antibodies against PCV2. The overall prevalence of anti-PCV2 antibodies in the pigs was 1.4 % (5/364), with more female pigs (4/237, 1.7 %) being seropositive than males (1/127, 0.8 %). Since there is no routine vaccination against this swine disease in Nigeria, thus the antibodies detected in the pig sera indicated a natural exposure to the virus. The absence of clinical disease in the pigs also suggests the possibility of a carrier status for these animals and shows that they could serve as hosts for the perpetuation of the disease. These findings underscore the need for continuous surveillance for PCV2 among pigs in Nigeria in order to determine its contribution to production losses incurred in the Nigerian swine industry and aid the development of prevention and control strategies against the disease.

Evaluation of natural porcine circovirus type 2 (PCV2) subclinical infection and seroconversion dynamics in piglets vaccinated at different ages

This study aimed to determine the porcine circovirus type 2 (PCV2) serological and virological dynamics in piglets vaccinated at different ages in a PCV2 subclinical infection (PCV2-SI) scenario. Six hundred and forty-four 2 week-old healthy piglets were selected and distributed into four treatment groups: vaccination at 3, 6 or 10 weeks of age (3W-VAC, 6W-VAC and 10W-VAC groups, respectively) and unvaccinated pigs (NON-VAC group). Blood (n = 112 pigs) and oral fluid (OF) (n = 40 pens) samples were taken throughout the study to assess PCV2 load, humoral immunity and viral genotyping. Percentage of PCV2-DNA positive sera mainly raised by 10 weeks of age, being maximum at 14 weeks of age, and then started to decrease at 18 and 25 weeks of age. Specifically, PCV2 vaccination at 3 or 6 weeks of age yielded similar results, since they produced an earlier seroconversion and reduced, at different sampling points, the proportion of viremic animals in comparison to the unvaccinated group. In contrast, PCV2 vaccination at 10 weeks of age only achieved such reduction at 25 weeks of age; in this case, vaccination coincided with the increase of the percentage of viremic pigs in the population. Both serological techniques used in sera and OF offered similar results with a high and statistically significant correlation. In contrast, a higher percentage of PCV2 DNA positivity was detected in OF in comparison with sera. In conclusion, under the present study conditions, the optimal time for PCV2 piglet vaccination was at either 3 or 6 weeks of age.

Reproduction of Postweaning Multisystemic Wasting Syndrome in Pigs Experimentally Inoculated with a Swedish Porcine Circovirus 2 Isolate

In recent years, porcine circovirus type 2 (PCV2)—associated postweaning multisystemic wasting syndrome (PMWS) has been reported worldwide. However, to date, PMWS has not been reported in Sweden despite the demonstration of serum antibodies to a PCV2-like virus in Swedish pigs. This communication reports the experimental reproduction of clinical PMWS after inoculation of colostrum-deprived (CD) pigs, derived from a Northern Ireland herd, with an isolate of PCV2 virus recovered from a clinically normal Swedish pig that was necropsied in 1993. The clinical disease and histological lesions observed in CD pigs inoculated with this virus were indistinguishable from those observed in previous studies on CD pigs inoculated with a PCV2 virus isolate recovered from pigs with PMWS. These results highlight the disease potential of PCV2 isolated from regions apparently free of PMWS and suggest that the status of the host and its environment is an important factor in the development of clinical PMWS.

Reproduction of post-weaning multi-systemic wasting syndrome in an animal disease model as a tool for vaccine testing under controlled conditions

Snatch farrowed, colostrum deprived piglets were inoculated with different combinations of porcine circovirus 2, porcine parvovirus and Erysipelothrix rhusiopathiae candidate vaccines. 10 piglets were mock-vaccinated. Following virus challenge with a combined porcine circovirus 2/porcine parvovirus inoculum, all animals were monitored and samples taken for serology, immunohistochemistry and qPCR. At 24 dpc all non-vaccinated animals remaining were exhibiting signs of post-weaning multi-systemic wasting syndrome which was confirmed by laboratory analysis. Details of the study, analysis of samples and performance of the candidate vaccines are described.

In Vitro Coinfection and Replication of Classical Swine Fever Virus and Porcine Circovirus Type 2 in PK15 Cells

Increasing clinical lines of evidence have shown the coinfection/superinfection of porcine circovirus type 2 (PCV2) and classical swine fever virus (CSFV). Here, we investigated whether PCV2 and CSFV could infect the same cell productively by constructing an in vitro coinfection model. Our results indicated that PCV2-free PK15 cells but not ST cells were more sensitive to PCV2, and the PK15 cell line could stably harbor replicating CSFV (PK15-CSFV cells) with a high infection rate. Confocal and super-resolution microscopic analysis showed that PCV2 and CSFV colocalized in the same PK15-CSFV cell, and the CSFV E2 protein translocated from the cytoplasm to the nucleus in PK15-CSFV cells infected with PCV2. Moreover, PCV2-CSFV dual-positive cells increased gradually in PK15-CSFV cells in a PCV2 dose-dependent manner. In PK15-CSFV cells, PCV2 replicated well, and the production of PCV2 progeny was not influenced by CSFV infection. However, CSFV reproduction decreased in a PCV2 dose-dependent manner. In addition, cellular apoptosis was not strengthened in PK15-CSFV cells infected with PCV2 in comparison with PCV2-infected PK15 cells. Moreover, using this coinfection model we further demonstrated PCV2-induced apoptosis might contribute to the impairment of CSFV HCLV strain replication in coinfected cells. Taken together, our results demonstrate for the first time the coinfection/superinfection of PCV2 and CSFV within the same cell, providing an in vitro model to facilitate further investigation of the underlying mechanism of CSFV and PCV2 coinfection.

Porcine circovirus: a historical perspective

Porcine circoviruses (PCVs) belong to the genus Circovirus and the family Circoviridae, and they are the smallest known viruses that replicate autonomously in mammalian cells. They are nonenveloped, and they have characteristic single-stranded, negative-sense, circular DNA. Two types of divergent PCVs are recognized: PCV1 and PCV2. About 20 years ago, PCV2 began to emerge as a major pathogen of swine around the world, leading to burgeoning knowledge about the virus and porcine circovirus-associated diseases. However, much of the history of its discovery, including the controversy related to its importance, is not recorded. This review examines current issues related to the biology of PCV2 in the context of the original studies related to determining its causal association with disease and to the evolving understanding of the complex pathogenesis of PCV2 infections.

Bioreduction of sheep carcasses effectively contains and reduces pathogen levels under operational and simulated breakdown conditions

Options for the storage and disposal of animal carcasses are extremely limited in the EU after the introduction of the EU Animal By-products Regulations (ABPR; EC/1774/2002), leading to animosity within the livestock sector and the call for alternative methods to be validated. Novel storage technologies such as bioreduction may be approved under the ABPR provided that they can be shown to prevent pathogen proliferation. We studied the survival of Enterococcus faecalis, Salmonella spp., E. coli O157 and porcine parvovirus in bioreduction vessels containing sheep carcasses for approximately 4 months. The vessels were operated under two different scenarios: (A) where the water within was aerated and heated to 40 °C, and (B) with no aeration or heating, to simulate vessel failure. Microbial analysis verified that pathogens were contained within the bioreduction vessel and indeed reduced in numbers with time under both scenarios. This study shows that bioreduction can provide an effective and safe on-farm storage system for livestock carcasses prior to ultimate disposal. The findings support a review of the current regulatory framework so that bioreduction is considered for approval for industry use within the EU.

Outcome of experimental porcine circovirus type 1 infections in mid-gestational porcine foetuses

BACKGROUND

Porcine circovirus type 1 (PCV1) has been described as a non-cytopathic contaminant of the PK-15 cell line. Several experimental infections with PCV1 failed to reproduce disease in pigs. Therefore, PCV1 is generally accepted as non-pathogenic to pigs. To our knowledge, nothing is known about the outcome of PCV1 infections in porcine foetuses. This was examined in the present study.

RESULTS

Nine foetuses from three sows were inoculated at 55 days of gestation: three with 10(4.3) TCID(50) of the PCV1 cell culture strain ATCC-CCL33, three with 10(4.3) TCID(50) of the PCV1 field strain 3384 and three with cell culture medium (mock-inoculated). At 21 days post-inoculation, all 6 PCV1-inoculated and all 3 mock-inoculated foetuses had a normal external appearance. Microscopic lesions characterized by severe haemorrhages were observed in the lungs of two foetuses inoculated with CCL33. High PCV1 titres (up to 10(4.7) TCID(50)/g tissue) were found in the lungs of the CCL33-inoculated foetuses. All other organs of the CCL33-inoculated foetuses and all the organs of the 3384-inoculated foetuses were negative (< 10(1.7) TCID(50)/g tissue) by virus titration. PCV1-positive cells (up to 121 cells/10 mm(2) in CCL33-inoculated foetuses and up to 13 cells/10 mm(2) in 3384-inoculated foetuses) were found in the heart, lungs, spleen, liver, thymus and tonsils. PCR and DNA sequencing of Rep recovered CCL33 or 3384 sequences from CCL33- or 3384-inoculated foetuses, respectively.

CONCLUSIONS

From this study, it can be concluded that cell culture PCV1 can replicate efficiently and produce pathology in the lungs of porcine foetuses inoculated at 55 days of foetal life.

Concurrent infections are important for expression of porcine circovirus associated disease

Porcine circovirus type 2 (PCV2) is the essential component of porcine circovirus disease (PCVD) as the disease syndrome is referred to in Europe and porcine circovirus associated disease (PCVAD) as it is referred to in North America. Singular PCV2 infection rarely results in clinical disease; however, PCVAD is often accelerated in onset, enhanced in severity and prolonged in duration by concurrent viral or bacterial infections. Due to its effect on the immune system, PCV2 has also been shown to enhance protozoal, metazoal, and fungal infections. Several retrospective or cross-sectional studies have investigated the presence and prevalence of various infectious agents associated with PCVAD under field conditions. Experimental models confirm that PCV2 replication and associated lesions can be enhanced by concurrent infection with other viruses or bacteria. The exact mechanisms by which concurrent pathogens upregulate PCV2 are unknown. Co-infections may promote PCV2 infection by increasing immune host cell replication and accumulation in tissues thereby enhancing targets for PCV2 replication. It has also been proposed that co-infections interfere with PCV2 clearance by alteration of cytokine production and profiles. The outcome of differences in timing of co-infections in PCV2-infected pigs is also likely very important and is an area where more research is needed. Given the current knowledge base, it is important that veterinarians do a thorough diagnostic investigation on herds where PCVAD is a recurrent problem in order to implement the most appropriate and cost effective intervention strategies.

Farm level risk factors associated with severity of post-weaning multi-systemic wasting syndrome

A cross-sectional study involving 147 pig farms across England was conducted in 2008-2009. Farm severity of post-weaning multi-systemic wasting syndrome (PMWS) was estimated through the use of an algorithm that combined data on post-weaning mortality, PMWS morbidity and proportion of porcine circovirus type 2 PCR positive pigs. Farms were classified as non/slightly, moderately or highly affected by PMWS. Data on potential PMWS risk factors were collected through interviews, on-farm assessment and serological sampling. Risk factors were identified using multivariable ordinal logistic regression and multivariable linear regression. Factors associated with increased PMWS severity were rearing growers indoors (OR=23.7), requiring a higher number of veterinarian visits per year (OR=9.6), having poorly isolated hospital pens (OR=6.4), buying replacement boars (OR=4.8) and seropositivity to Mycoplasma hyopneumoniae (OR=4.29); factors associated with decreased PMWS severity were low stocking density for growers (OR=0.07), adjusting diets at least three times between weaning and 14 weeks of age (OR=0.12), and requiring visitors to be at least 2 days pig free (OR=0.14). This study provides evidence of the association between environmental and management factors and PMWS severity, and suggests that other pathogens may be important co-factors for the disease. In addition, this study highlights the potential efficacy of biosecurity measures in the reduction/prevention of within-farm PMWS severity.

Intestinal gene expression in pigs experimentally co-infected with PCV2 and PPV

The objective of the present study was to characterize the local immune reaction in the intestine of pigs experimentally infected with PCV2 and PPV. Archived intestinal material from an experimental study in which pigs were co-infected with a Swedish isolate of PCV2 (S-PCV2) and PPV, or a reference isolate of PCV2 (PCV2-1010) and PPV, were used. The intestinal samples were analysed by qPCR for expression of a number of selected cytokines and the overall gene expression in the intestine was screened by cDNA microarray. Analyses by qPCR showed that pigs infected with PCV2-1010/PPV displayed a significantly increased mRNA expression for IL-6 (p<0.05), IL-10 (p<0.05) and IFN-γ (p<0.05). The microarray screening revealed a strong up-regulation of IFITM3 along with several other interferon-stimulated genes (ISGs) in pigs infected with PCV2/PPV. The analyses also indicated differences between the two isolates. Fewer pigs infected with S-PCV2/PPV expressed the cytokines detected by qPCR, compared to pigs infected with PCV2-1010/PPV, and pigs infected with S-PCV2/PPV displayed a higher proportion of down-regulated genes than PCV2-1010/PPV-infected pigs.

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.

Development and evaluation of an indirect in situ polymerase chain reaction for the detection of porcine circovirus type 2 in formalin-fixed and paraffin-embedded tissue specimens

Taking advantage of the high sensitivity of polymerase chain reaction (PCR) and the cell-localizing ability of in situ hybridization (ISH), an indirect in situ PCR (ISPCR) method was developed for detecting the distribution of porcine circovirus type 2 (PCV2) in formalin-fixed and paraffin-embedded inguinal lymph nodes obtained from clinically healthy PCV2-carrier pigs and postweaning multisystemic wasting syndrome (PMWS)-affected pigs. Comparisons of the relative sensitivity of indirect ISPCR with other routinely used diagnostic methods for PCV2 indicated that nested PCR was the most sensitive method followed by indirect ISPCR, conventional PCR, ISH, and immunohistochemical (IHC) staining. Although indirect ISPCR, ISH, and IHC staining all revealed a similar signal distribution pattern of PCV2, using indirect ISPCR allowed specific amplification and detection of previously uneasily detected PCV2 signal than by routine ISH or IHC staining, particularly in those cells within the germinal center in clinically healthy PCV2-carrier pigs. Furthermore, six different PCV2 signal expression patterns in conjunction with the correlated lymphoid lesion stages were classified to describe the tissue morphological changes and viral infection. The result indicates that indirect ISPCR is a more effective, cell-based diagnostic tool with good specificity to detect limited PCV2 infection in formalin-fixed and paraffin-embedded tissue specimens and it would be a useful tool for further exploring the pathogenesis of PCV2 infection.

The index herd with PMWS in Sweden: presence of serum amyloid A, circovirus 2 viral load and antibody levels in healthy and PMWS-affected pigs

BACKGROUND

Postweaning Multisystemic Wasting Syndrome (PMWS) is an emerging disease in pigs of multifactorial origin, but associated to porcine circovirus type 2 (PCV2) infection. PMWS was first diagnosed in Sweden at a progeny test station that received pigs aged five weeks from 19 different nucleus herds on the day after weaning. The objective of this study was to examine, for the first time in an index outbreak of PMWS, the relationship between PCV2 virus, antibodies to PCV2 and serum amyloid a (SAA) in sequentially collected serum samples from pigs with and without signs of PMWS.

METHODS

Forty pigs of the last batch that entered the station at a mean age of 37.5 days were monitored for signs of PMWS during the first 55 days after arrival. Serum was collected on six occasions and analysed for presence of PCV2 DNA and antibodies to PCV2, as well as for levels of SAA.

RESULTS

Four of the pigs (10%) were concluded to have developed PMWS, with necropsy confirmation in three of them. These pigs displayed low levels of maternal antibodies to PCV2, more than 107 PCV2 viral DNA copies per ml serum and failed to mount a serological response to the virus. Starting between day 23 and 34 after arrival, an increase in PCV2 viral load was seen in all pigs, but PCV2 did not induce any SAA-response. Pigs that remained healthy seroconverted to PCV2 as the viral load was increased, regardless of initially having low or high levels of PCV2-antibodies.

CONCLUSION

In this index case of PMWS in Sweden, pigs affected by PMWS were not able to mount a relevant serum antibody response which contributed to the disease progression. The maximal PCV2 virus load was significantly higher and was also detected at an earlier stage in PMWS-affected pigs than in healthy pigs. However, a viral load above 107 PCV2 DNA copies per ml serum was also recorded in 18 out of 34 pigs without any clinical signs of PMWS, suggesting that these pigs were able to initiate a protective immune response to PCV2.

A meta-analysis on experimental infections with porcine circovirus type 2 (PCV2)

A meta-analysis was performed with the aim to identify factors with a relevant influence on the expression of clinical postweaning multisystemic wasting syndrome (PMWS) under experimental conditions. Data from 44 studies were included in the analysis. Several variables were studied: number of pigs in the experiment, intake of colostrum, serological status against porcine circovirus type 2 (PCV2), strain of PCV2 used for inoculation, the route and dose of inoculation, and use of potential triggering factors (such as co-infections, vaccinations, or immunomodulator products). Multiple correspondence analysis and log-linear regression methods were used to establish the relationships between the studied variables and the number of PCV2 infected pigs that developed PMWS. Based on the results of the meta-analysis, the most successful animal experiment aimed to develop PMWS should include: (1) colostrum-deprived pigs, (2) age of inoculation below 3 weeks, (3) high doses of PCV2 inoculum, (4) PCV2 strain from genotype 1, and (5) co-infection with another swine pathogen as a triggering factor.

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.

Porcine circovirus type 2 infection decreases the efficacy of a modified live porcine reproductive and respiratory syndrome virus vaccine

Porcine reproductive and respiratory syndrome virus (PRRSV)-induced pneumonia is a major problem, and vaccination is used to reduce losses associated with PRRSV. Porcine circovirus type 2 (PCV2) causes lymphoid depletion, and there is concern that this adversely affects the immune response. The objective of this study was to investigate the effect of PCV2 infection on the efficacy of modified live virus (MLV) PRRSV vaccine. Sixty-nine 2-week-old pigs were randomly assigned to one of seven groups of 9 to 10 pigs each. At 6 weeks of age, pigs in groups 4, 5, and 6 were inoculated intranasally with PCV2 ISU-40895. At 8 weeks of age, groups 3, 4, 6, and 7 were vaccinated with a PRRSV MLV vaccine. At 12 weeks of age, groups 2, 3, and 4 were challenged with PRRSV SDSU73. All pigs were necropsied 14 days after PRRSV challenge. PCV2-infected, PRRSV-vaccinated, and PRRSV-challenged pigs had significantly (P < 0.05) more-severe macroscopic lung lesions than did the PRRSV-vaccinated and PRRSV-challenged pigs that were not exposed to PCV2 prior to PRRSV vaccination. Nonvaccinated PRRSV-infected pigs had a significantly (P < 0.001) higher incidence of PRRSV antigen in lungs than did all other groups except the group infected with PCV2 prior to PRRSV vaccination and challenge. The nonvaccinated PRRSV-challenged group and the group challenged with PCV2 prior to PRRSV vaccination and challenge had significantly (P < 0.001) lower average daily weight gain than did the control and the vaccinated groups. This work suggests that PCV2 infection has an adverse effect on the development of protective immunity induced by PRRSV vaccine.

A slide latex agglutination test for the rapid detection of antibodies in serum against porcine parvovirus

A slide latex agglutination test (LAT) was developed and evaluated to detect serum antibodies against porcine parvovirus. Porcine parvovirus antigen was obtained by 10% PEG-6000 and 0.5 mol/l sodium chloride precipitation, and inactivated by 0.1% methanal. Two per cent suspensions of latex particles (0.5-0.8 microm) were coated by adding an equal volume of porcine parvovirus antigen at 0.34 microg/ml. Repeatability of latex agglutination test was evaluated with a panel of 100 sera using the same and different antigen lots. A good agreement between LAT and haemagglutination inhibit assay was observed. Because of convenience and speed of performance, this method would be used widely in clinic examination.

Correlation between the presence of neutralizing antibodies against porcine circovirus 2 (PCV2) and protection against replication of the virus and development of PCV2-associated disease

Background

In a previous study, it was demonstrated that high replication of Porcine circovirus 2 (PCV2) in a gnotobiotic pig was correlated with the absence of PCV2-neutralizing antibodies. The aim of the present study was to investigate if this correlation could also be found in SPF pigs in which PMWS was experimentally reproduced and in naturally PMWS-affected pigs.

Results

When looking at the total anti-PCV2 antibody titres, PMWS-affected and healthy animals seroconverted at the same time point, and titres in PMWS-affected animals were only slightly lower compared to those in healthy animals. In healthy animals, the evolution of PCV2-neutralizing antibodies coincided with that of total antibodies. In PMWS-affected animals, neutralizing antibodies could either not be found (sera from field studies) or were detected in low titres between 7 and 14 DPI only (sera from experimentally inoculated SPF pigs). Differences were also found in the evolution of specific antibody isotypes titres against PCV2. In healthy pigs, IgM antibodies persisted until the end of the study, whereas in PMWS-affected pigs they quickly decreased or remained present at low titres. The mean titres of other antibody isotypes (IgG1, IgG2 and IgA), were slightly lower in PMWS-affected pigs compared to their healthy group mates at the end of each study.

Conclusion

This study describes important differences in the development of the humoral immune response between pigs that get subclinically infected with PCV2 and pigs that experience a high level of PCV2-replication which in 3 of 4 experiments led to the development of PMWS. These observations may contribute to a better understanding of the pathogenesis of a PCV2-infection.

Tissue distribution of two field isolates and two vaccine strains of porcine parvovirus in foetal organs after experimental infection of pregnant sows as determined by real-time PCR

The aim of this study was to investigate the tissue distribution of two different field isolates and two vaccine strains of porcine parvoviruses (PPV) in infected piglets after transplacental infection. The viral load in 10 different foetal organs was determined by real-time polymerase chain reaction assays with SYBR Green targeting the viral VP2 gene and the genomic c-myc gene in 12 foetuses. The viral load in foetal tissues differed greatly among the different parvoviruses. Between one virulent field isolate compared with the other field isolate and the vaccine strains, the detected viral copy number differed in an order of magnitude of 10(9). The virulent isolate contained PPV in all 10 organs with viral loads varying between 10(11) and 10(15) per 10(6) cells. Concerning the other field isolate and the two vaccine strains, if PPV was detected, in most of the cases the highest viral load was found in foetal kidneys with a maximum viral load of 10(3) per 10(6) cells. Additionally, PPV was found in the heart of one foetus, in the liver and duodenum of one foetus and in the thymus of one foetus with viral loads varying between 10(2.1) and 10(3.5) per 10(6) cells. In completely mummified foetuses with no discriminable organs of foetuses infected with the vaccine strains and the less virulent isolate, PPV was present in very low amounts or even below the detection limit.

Enhancement of Porcine Circovirus 2 Replication in Porcine Cell Lines by IFN-γ Before and After Treatment and by IFN-α After Treatment

Stimulation of the porcine immune system causes increased replication of porcine circovirus 2 (PCV2) in vivo. In the present study, we investigated whether various cytokines (interleukin-1 [IL-1], IL-6, IL-10, tumor necrosis factor-alpha [TNF-alpha], interferon-alpha [IFN-alpha], and IFN-gamma) are able to influence PCV2 infection in vitro. No changes were observed in IL-1, IL-6, TNF-alpha, or IL-10-treated cells. However, it was demonstrated that IFN- alpha and IFN-gamma influenced PCV2 infection in porcine kidney cells (PK-15) and porcine monocytic cells (3D4/31). IFN-gamma added to the culture medium before, during, or after inoculation increased the number of PCV2 antigen- positive cells, respectively, by 418%, 171%, and 691% in PK-15 cells and by 706%, 114%, and 423% in 3D4/31 cells. IFN-alpha pretreatment decreased the number of infected PK-15 cells. When it was added after inoculation, IFN-alpha enhanced PCV2 infection by 529% in PK-15 cells and by 308% in 3D4/31 cells. The effect of both IFNs on PCV2 infection was dose dependent and could be blocked with IFN-alpha or IFN-gamma neutralizing antibodies. Leukocyte-derived porcine IFN-gamma induced a similar effect on PCV2 infection. Treatment of PK- 15 cultures with IFN-gamma caused a 20 times higher production of progeny virus. Confocal microscopy studies showed that the enhancing effect of IFN-gamma on PCV2 infection was achieved by increased internalization of PCV2 virionlike particles (VLPs). Binding of the VLPs to the cell or expression kinetics of PCV2 proteins in infected cells were not altered by IFN-gamma treatment. To our knowledge, this study reports the first enhancement of a viral infection by treatment with type I or type II IFNs.

Correlation Between Type of Adaptive Immune Response Against Porcine Circovirus Type 2 and Level of Virus Replication

Porcine circovirus 2 (PCV2) replication is characterized by high variation among infected pigs. This study investigated the role of immunologic responses in causing this variation. Twelve gnotobiotic pigs were inoculated with PCV2. Four of these pigs were treated with cyclosporin A (CysA) to monitor the effect of the adaptive immunity on the development of the PCV2 infection. Through lymph node biopsies at 10, 15, and 21 days postinoculation (DPI), PCV2 replication in lymphoid tissues was monitored. The production of total PCV2-specific and PCV2-neutralizing antibodies was followed, together with interferon-gamma (IFN-gamma) mRNA expression levels in peripheral blood monocytes as a marker for cellular immunity. In general, the CysA-treated pigs showed the highest PCV2 titers, indicating that the adaptive immunity is necessary to restrain PCV2 replication. Three different PCV2 replication patterns were observed in non-CysA-treated pigs. Pattern 1: In two pigs, PCV2 was not detected. They had the highest neutralizing antibody titers, appearing from 15 DPI. In these pigs a good cellular response was indicated by a peak in IFN-gamma mRNA at 15 DPI. Pattern 2: Five pigs contained low to moderate PCV2 titers at 15 DPI, remaining constant or decreasing towards 21 DPI. Lower neutralizing antibody titers were observed and no rise in IFN-gamma was detected. Pattern 3: In one pig, a low PCV2 titer at 15 DPI dramatically increased toward 21 DPI. Although an antibody response against PCV2 was mounted, no PCV2-neutralizing antibodies were detected. This pig also showed no rise in IFN-gamma. The study findings indicate that variation in the onset of the adaptive immunity may account for variation in PCV2 replication among pigs. Absence of PCV2-neutralizing antibodies may be an important factor in the development of an increased virus replication.

Experimental reproduction of postweaning multisystemic wasting syndrome (PMWS) in pigs in Sweden and Denmark with a Swedish isolate of porcine circovirus type 2

An experimental model using 3-day-old snatch-farrowed colostrum-deprived piglets co-infected with porcine circovirus type 2 (PCV2) and porcine parvovirus (PPV) is at present one of the best methods to study factors affecting development of postweaning multisystemic wasting syndrome (PMWS). A Swedish isolate of PCV2 (S-PCV2) retrieved in 1993 from a healthy pig has been used in this model to reproduce PMWS in pigs from Northern Ireland. This virus has been present in the Swedish pig population for at least a decade without causing any known PMWS disease problems, despite its potential pathogenicity. The reasons for this are unknown, but could be related to genetics, absence of triggers for PCV2 upregulation (infectious agent and/or management forms) within Swedish pig husbandry. In order to confirm the pathogenicity of S-PCV2, Swedish and Danish pigs were experimentally infected with this isolate according to the established model. Swedish pigs were also infected with a reference isolate of PCV2 (PCV2-1010) to compare the severity of disease caused by the two isolates in Swedish pigs. Both Danish and Swedish pigs developed PMWS after the experimental infection with S-PCV2. Antibodies to PCV2 developed later and reached lower levels in serum from pigs infected with S-PCV2 than in pigs inoculated with PCV2-1010. In general, pigs infected with S-PCV2 showed more severe clinical signs of disease than pigs infected with PCV2-1010, but pigs from all PCV2-inoculated groups displayed gross and histological lesions consistent with PMWS. All pigs inoculated with PPV, alone or in combination with PCV2, displayed interleukin-10 responses in serum while only pigs infected with PPV in combination with PCV2 showed interferon-alpha in serum on repeated occasions. Thus, the pathogenicity of S-PCV2 was confirmed and a role for cytokines in the etiology of PMWS was indicated.

Quantitation of porcine circovirus type 2 isolated from serum/plasma and tissue samples of healthy pigs and pigs with postweaning multisystemic wasting syndrome using a TaqMan-based real-time PCR

Porcine circovirus type 2 (PCV2) has been linked to several disease syndromes during the last decade. In this context, postweaning multisystemic wasting syndrome (PMWS) has emerged as a significant disease. As most pig herds are infected with PCV2, the determination of viral load in animals may be useful in discriminating between healthy and PMWS pigs. A TaqMan-based real-time PCR for quantitation of PCV2 in serum/plasma and tissue samples was established. A standard curve was created from serial dilutions of a plasmid encoding ORF2 (cap gene) of PCV2 and used to estimate the number of viral DNA copies in the analyzed samples. Comparison of viral load in mesenteric lymph nodes and serum/plasma from healthy animals and PMWS animals showed statistical significant difference between the two groups (p < 0.01). No healthy pigs had viral load greater than 10(6) PCV2 genomes per ml serum or 500 ng tissue sample, while all clinically sick PMWS pigs had PCV2 loads above 10(7) in both serum/plasma and in tissue samples. Furthermore, the estimated viral load in tissue samples from PMWS pigs was related to the immunohistochemical findings, with especially lymph nodes, ileum, and tonsil giving both high viral load, and a high degree of staining by immunohistochemistry.

Features of cell degeneration and death in hepatic failure and systemic lymphoid depletion characteristic of porcine circovirus-2-associated postweaning multisystemic wasting disease

Tissue section replicates from lymphoid tissues and livers of gnotobiotic swine were examined by immunohistochemistry for the colocalization of porcine circovirus-2 (PCV-2) nucleocapsid and terminal deoxynucleotidyl transferase (TdT)-mediated incorporation of biotinylated nucleotides (UTP) onto the 3'-exposed hydroxyl groups (nick end labeling) nuclear deoxyribonucleic acid (TUNEL), a marker for apoptosis. Single- and dually stained replicates from uninfected controls, subclinically affected PCV-2-infected gnotobiotic pigs, PCV-2-infected piglets immunosuppressed with cyclosporine (Cys), and PCV-2-infected piglets with post-weaning multisystemic wasting syndrome (PMWS) were evaluated. Thymuses were used as positive controls for apoptosis absent PCV-2, tissue sections from dogs given hyperthermic stress were examined as positive controls for induced TUNEL. Tissues from heat-stressed dogs contained TUNEL-positive cell nuclei in both lymphoid tissues and liver, TUNEL was greatest shortly after the delivery of the hyperthermic insult. In uninfected control and subclinically affected PCV-2-infected gnotobiotic pigs, rare hepatocytes and lymphoid cells were TUNEL positive, the frequency of these was similar to that seen in uninfected controls. In PMWS-affected and Cys-treated PCV-2 piglets, the only consistent strongly positive TUNEL signal was contained within the cytoplasm of virus-positive phagocytic mononuclear cells. In phagocytes, some PCV-2 inclusions were TUNEL positive. Collectively, these data indicate that apoptosis is not the primary mechanism of lymphoid depletion and hepatocyte loss in PMWS. Apoptosis associated with systemic viral diseases may be attributable to pyrexia rather than direct or indirect effects of viruses on target cells.

In vitro and in vivo characterization of an infectious clone of a European strain of porcine circovirus type 2

The aim of this study was to describe the generation of a PCV2 (porcine circovirus type 2) infectious clone (pIC-PCV2) and its infectivity under in vitro and in vivo conditions. The constructed pIC-PCV2 contained the whole PCV2 genome from a German isolate together with a partial duplication of 467 bp. PK-15 cells were transfected with pIC-PCV2 and an indirect immune fluorescence assay (IFA) was performed 7 days post-transfection. The PCV2 Cap gene was expressed in approximately 20 % of the cultured cells, and only the recombination product, and not pIC-PCV2, was subsequently detected by PCR and Southern blot. This result indicated that infection by pIC-PCV2 delivered genomic PCV2 DNA specifically into susceptible cells and led to the expression of a functional virus genome. Eighteen 30- to 40-day-old conventional pigs were distributed into three groups. Group 1 pigs (n=6) were inoculated intranasally (i.n.) with a Spanish isolate of PCV2 propagated in cell culture; pigs from group 2 (n=6) were inoculated with pIC-PCV2 intramuscularly (i.m.), and the last group of pigs (n=6) was inoculated with pIC-PCV2 intraperitoneally (i.p.). All pigs remained clinically healthy during the whole experimental period (35 days). Pigs that received pIC-PCV2 i.p. and i.m., as well as those PCV2 i.n. inoculated, became infected based on an in situ hybridization (ISH), PCR, TaqMan PCR and serological results. The results of this study confirm that cloned PCV2 genomic DNA is infectious both in vitro and in vivo, and is able to cause PMWS-like lesions in i.p. and i.m. experimentally inoculated pigs.

Effect of porcine parvovirus vaccination on the development of PMWS in segregated early weaned pigs coinfected with type 2 porcine circovirus and porcine parvovirus

The objectives of this study were to determine if coinfection of segregated early weaned (SEW) pigs with porcine circovirus type 2 (PCV2) and porcine parvovirus (PPV) induces an increase in the incidence of post-weaning multisystemic wasting syndrome (PMWS) compared to singular PCV2 infection, and to determine if vaccination against PPV protects pigs against PMWS associated with PCV2/PPV coinfection in SEW pigs. Seventy, 3-week-old, SEW pigs were randomly assigned to one of the five groups. Pigs in group 1 (n = 14) served as the negative controls, group 2 pigs (n = 14) were inoculated with PCV2, group 3 pigs (n = 12) were inoculated with PPV, groups 4 (n = 16) and 5 (n = 14) pigs were inoculated with both PCV2 and PPV. Pigs in groups 1-3 and 5 were vaccinated with two doses of a killed parvovirus-leptospira-erysipelothrix (PLE) vaccine prior to inoculation. The PCV2/PPV-coinfected pigs (groups 4 and 5) had significantly (P < 0.05) higher and more persistent fevers than the singular PCV2-infected pigs. One pig in each of the coinfected groups developed clinical disease (fever, respiratory disease, jaundice, weight loss) consistent with PMWS. Lymphoid depletion was significantly (P < 0.05) more severe in the dually-infected pigs at 42 days post-inoculation (DPI). Vaccinated, coinfected pigs (group 5) remained viremic significantly (P < 0.05) longer and had higher copy numbers of genomic PCV2 DNA in sera at 28, 35, and 42 DPI compared to the unvaccinated coinfected pigs (group 4). PPV-viremia was detected only in the unvaccinated group 4 pigs. PLE-vaccination prevented PPV-viremia but did not prevent clinical PMWS or reduce the severity of lymphoid depletion in PCV2/PPV-coinfected pigs. Evidence of increased incidence of clinical PMWS due to vaccination was not observed in this model.

Immunogenicity and pathogenicity of chimeric infectious DNA clones of pathogenic porcine circovirus type 2 (PCV2) and nonpathogenic PCV1 in weanling pigs

Porcine circovirus type 2 (PCV2) is the primary causative agent of postweaning multisystemic wasting syndrome (PMWS), whereas the ubiquitous porcine circovirus type 1 (PCV1) is nonpathogenic for pigs. We report here the construction and characterization of two chimeric infectious DNA clones of PCV1 and PCV2. The chimeric PCV1-2 clone contains the PCV2 capsid gene cloned in the backbone of the nonpathogenic PCV1 genome. A reciprocal chimeric PCV2-1 DNA clone was also constructed by replacing the PCV2 capsid gene with that of PCV1 in the backbone of the PCV2 genome. The PCV1, PCV2, and chimeric PCV1-2 and PCV2-1 DNA clones were all shown to be infectious in PK-15 cells, and their growth characteristics in vitro were determined and compared. To evaluate the immunogenicity and pathogenicity of the chimeric infectious DNA clones, 40 specific-pathogen-free (SPF) pigs were randomly assigned into five groups of eight pigs each. Group 1 pigs received phosphate-buffered saline as the negative control. Group 2 pigs were each injected in the superficial inguinal lymph nodes with 200 micro g of the PCV1 infectious DNA clone. Group 3 pigs were each similarly injected with 200 micro g of the PCV2 infectious DNA clone, group 4 pigs were each injected with 200 micro g of the chimeric PCV1-2 infectious DNA clone, and group 5 pigs were each injected with 200 micro g of the reciprocal chimeric PCV2-1 infectious DNA clone. As expected, seroconversion to antibodies to the PCV2 capsid antigen was detected in group 3 and group 4 pigs. Group 2 and 5 pigs all seroconverted to PCV1 antibody. Gross and microscopic lesions in various tissues of animals inoculated with the PCV2 infectious DNA clone were significantly more severe than those found in pigs inoculated with PCV1, chimeric PCV1-2, and reciprocal chimeric PCV2-1 infectious DNA clones. These data indicated that the chimeric PCV1-2 virus with the immunogenic ORF2 capsid gene of pathogenic PCV2 cloned into the nonpathogenic PCV1 genomic backbone induces a specific antibody response to the pathogenic PCV2 capsid antigen but is attenuated in pigs. Future studies are warranted to evaluate the usefulness of the chimeric PCV1-2 infectious DNA clone as a genetically engineered live-attenuated vaccine against PCV2 infection and PMWS.

Change of porcine circovirus 2 target cells in pigs during development from fetal to early postnatal life

Change of porcine circovirus 2 (PCV2) target cells during development from fetal to postnatal life in pigs was examined. PCV2 inoculation was performed in fetuses in utero at either 57, 75 or 92 gestational days and in piglets at 1 day of age. Twenty-one days after virus inoculation, PCV2-infected cells in the heart, lungs, liver, spleen and inguinal lymph nodes were localized and immuno-phenotyped by double-immunofluorescence labeling using different cell markers and PCV2-antibodies. During fetal life, viral antigens were detected in cardiomyocytes, hepatocytes and macrophages and infected cell numbers decreased with increasing fetal age at inoculation. The heart contained the highest number of infected cells and cardiomyocytes were the main target cell. Postnatally, macrophages were the only target cell type in different organs and infected cell numbers were similar to those of fetuses inoculated at 92 days of gestation. One piglet showed exceptionally high number of infected cells in different organs with values 13-513-fold higher compared to littermates. In this piglet, the majority of infected cells in lymphoid tissues could not be typed. This study reveals that PCV2 target cells change from cardiomyocytes, hepatocytes and macrophages during fetal life to only macrophages postnatally.

In vitro studies on the infection and replication of porcine circovirus type 2 in cells of the porcine immune system

Porcine circovirus type 2 (PCV2) nucleic acid and/or antigens are consistently observed in cells of monocytic morphology in lesions of pigs affected by post-weaning multisystemic wasting syndrome (PMWS). In this study, PCV2 antigen was detected in the cytoplasm of monocytes, pulmonary macrophages (PMs) and monocyte-derived macrophages exposed to the virus in vitro, by immunofluorescence analysis (IFA) and the phenotype of these cells confirmed by detection of monocytic cell surface markers using flow cytometry. Viral antigen was not observed in lymphocytic cells. Replication of the virus in PMs was investigated further by comparison to that observed in the continuous pig kidney cell line (PK15A) using quantitative virus titration, quantitative PCR and by the detection of double stranded DNA intermediates of viral replication by Southern blotting analyses. Although increases in viral DNA and levels of infectious virus progeny and the presence of replicative intermediates, indicative of viral replication, were observed in PK15A cells, no such changes were observed in PMs in spite of the fact that infectious virus, viral antigen and viral DNA persisted in the cells for at least the duration of the experiment. These results suggest that in vivo, monocytic cells may not represent the primary target for PCV2 replication.

Absence of evidence of porcine circovirus infection in piglets with congenital tremors

Porcine circovirus types 1 (PCV1) and 2 (PCV2) have been associated with congenital tremors (CTs) in piglets in the United States. In this study, central nervous system and nonneural tissues of 40 CT piglets from Spain, the United Kingdom, Ireland, and Sweden were investigated for the presence of PCV1 and PCV2 using in situ hybridization and immunohistochemical labeling on paraffin sections. The polymerase chain reaction for PCV2 was also carried out on sera from the Spanish CT cases. No evidence of circovirus nucleic acid or antigen was found in any CT piglet. Although these results do not support the hypothesis that PCV1 or PCV2 are linked to porcine CT, they cannot disprove it.

Immunologic features of porcine circovirus type 2 infection

Clinical expression of porcine circovirus 2 (PCV-2) infection in swine may result in two distinct high mortality disease syndromes. In North America, postweaning multisystemic wasting syndrome (PMWS), while still sporadic in incidence, predominates. In Europe and elsewhere, both PMWS and a second syndrome, porcine dermatitis and nephropathy syndrome (PDNS), occur in endemic and epidemic forms. PMWS but not PDNS has been reproduced in piglets by inoculations with PCV-2 alone or in PCV-2-infected swine co-infected with porcine parvovirus (PPV) or porcine respiratory and reproductive syndrome (PRRS) virus and also if PCV-2-infected piglets are immunostimulated by injections with an immunogen emulsified in an oil-based macrophage-targeted adjuvant. Subclinical but active infection has been achieved by direct inoculation of piglets with cloned PCV-2 DNA and/or progeny virus derived from cloned DNA. Morphologic changes in lymphoid tissues and preliminary functional data suggest that immunosuppression may occur in PMWS-affected swine. This phenomenon appears to be mediated by generalized lymphoid depletion and replacement by infiltrating and proliferating histiocytes and macrophages. Accumulation of virus in both mononuclear phagocytes and follicular dendritic cells is characteristic of PCV-2 infection. Exogenous immunosuppression of PCV-2-infected gnotobiotic piglets with cyclosporine (Cys), but not corticosteroid (St), potentiates PCV-2 replication and promotes productive virus infection of hepatocytes in Cys-treated piglets, a tropism not previously apparent in experimentally induced PMWS in gnotobiotic swine. In the Cys-treated piglets, inflammatory lesions characteristic of PMWS are absent, even though tissues contain high titers of infectious virus, a finding which suggests that the granulomatous inflammatory lesions characteristic of PMWS are immune mediated.

Exudative epidermitis and porcine circovirus-2 infection in a Swedish SPF-herd

An outbreak of exudative epidermitis (EE) among piglets in a Swedish SPF-herd initiated a survey for indications as to the cause of disease. The herd was established by caesarean section and has been closed to all new animal material, with the exception of semen for artificial insemination (AI). The study comprised serum samples from the SPF-herd over a 10-year period (n=109) and a close monitoring of animals in the herd during the period after the EE outbreak. Serum samples from conventional boars at the AI-station servicing the herd were also included (n=9). All serum samples were tested for antibodies to porcine circovirus-2 (PCV-2). In addition, 3-week-old piglets from three litters (n=24) farrowed close after the initial EE outbreak were closely monitored for clinical signs of skin disease, sampled for Staphylococcus hyicus, tested for antibodies to porcine parvovirus and in sequentially collected serum samples tested for interferon-alpha (IFN-alpha) and interleukin-6. The PVC-2 serology showed that animals in the herd were sero-negative at least until 2 months prior to the EE outbreak. During the period close after the EE outbreak the animals showed varying levels of antibodies to PCV-2 but all the tested animals had sero-converted 4 months later. The AI boars were also sero-positive to PCV-2 at the time of the EE outbreak. Animals in the SPF-herd remained sero-positive to PCV-2 during the following 7 years. In the monitored litters, one piglet had clinical EE and 15 piglets displayed defined erythemas on the abdomen. Fourteen of the piglets also had IFN-alpha in serum on one or more occasions during the study, indicating viral activity among the animals. S. hyicus was isolated from all of the piglets from the earliest sampling point (3 days of age) and onwards, irrespective of clinical signs. PCV-2 was isolated from lymph node tissue collected from one of the EE affected pigs.Further, increases in the number of stillborn piglets, small litters (<6 piglets) and repeat breeders could be correlated to the time of PCV-2 sero-conversion. Coincidence of active viral infection and sero-conversion to PCV-2 points to the virus as the cause of the EE outbreak and reproductive disturbances.

Cloned genomic DNA of type 2 porcine circovirus is infectious when injected directly into the liver and lymph nodes of pigs: characterization of clinical disease, virus distribution, and pathologic lesions

Infection of animals with a molecular viral clone is critical to study the genetic determinants of viral replication and virulence in the host. Type 2 porcine circovirus (PCV2) has been incriminated as the cause of postweaning multisystemic wasting syndrome (PMWS), an emerging disease in pigs. We report here for the first time the construction and use of an infectious molecular DNA clone of PCV2 to characterize the disease and pathologic lesions associated with PCV2 infection by direct in vivo transfection of pigs with the molecular clone. The PCV2 molecular clone was generated by ligating two copies of the complete PCV2 genome in tandem into the pBluescript SK (pSK) vector and was shown to be infectious in vitro when transfected into PK-15 cells. Forty specific-pathogen-free pigs at 4 weeks of age were randomly assigned to four groups of 10 each. Group 1 pigs served as uninoculated controls. Pigs in group 2 were each inoculated intranasally with about 1.9 x 10(5) 50% tissue culture infective doses of a homogeneous PCV2 live virus stock derived from the molecular clone. Pigs in group 3 were each injected intrahepatically with 200 microg of the cloned PCV2 plasmid DNA, and pigs in group 4 were each injected into the superficial iliac lymph nodes with 200 microg of the cloned PCV2 plasmid DNA. Animals injected with the cloned PCV2 plasmid DNA developed infection resembling that induced by intranasal inoculation with PCV2 live virus stock. Seroconversion to PCV2-specific antibody was detected in the majority of pigs from the three inoculated groups at 35 days postinoculation (DPI). Viremia, beginning at 14 DPI and lasting 2 to 4 weeks, was detected in the majority of the pigs from all three inoculated groups. There were no remarkable clinical signs of PMWS in control or any of the inoculated pigs. Gross lesions in pigs of the three inoculated groups were similar and were characterized by systemically enlarged, tan lymph nodes and lungs that failed to collapse. Histopathological lesions and PCV2-specific antigen were detected in numerous tissues and organs, including brain, lung, heart, kidney, tonsil, lymph nodes, spleen, ileum, and liver of infected pigs. This study more definitively characterizes the clinical course and pathologic lesions exclusively attributable to PCV2 infection. The data from this study indicate that the cloned PCV2 genomic DNA may replace infectious virus for future PCV2 pathogenesis and immunization studies. The data also suggest that PCV2, although essential for development of PMWS, may require other factors or agents to induce the full spectrum of clinical signs and lesions associated with advanced cases of PMWS.

Activation of the immune system is the pivotal event in the production of wasting disease in pigs infected with porcine circovirus-2 (PCV-2)

Porcine circovirus (PCV)-2, a newly described single-stranded circular DNA virus pathogen of swine is the cause of postweaning multisystemic wasting syndrome (PMWS). In gnotobiotic piglets, PCV-2 infection alone produces asymptomatic infection without evidence of overt PMWS. Gnotobiotic piglets infected with PCV-2 were injected with keyhole limpet hemocyanin in incomplete Freund's adjuvant (KLH/ICFA), and the effects on virus production and development of PMWS were determined. In the first experiment, piglets were injected subcutaneously on the left hip and shoulder, and viral burden was assessed in regional lymph nodes draining the injection sites and in contralateral lymph nodes 13-14 days after infection. Immune activation increased the number of virus antigen-positive cells in draining lymph nodes and increased the amount of infectious virus recovered by 1-4 log10. In a second experiment, the effects of injections of KLH/ICFA with or without concurrent stimulation of peritoneal macrophages by intraperitoneal injections of thioglycollate broth on induction of PMWS was assessed. All immunized piglets developed moderate to severe PMWS, whereas none of the piglets infected with PCV-2 alone developed PMWS. In PMWS-affected piglets, extensive replication of PCV-2 was documented by both immunocytochemistry and quantitative viral titrations. Thus, immune activation is a key component of the pathogenesis of PCV-2-associated PMWS in swine.