Application of pigeon circovirus recombinant capsid protein for detecting anti-PiCV antibodies in the sera of asymptomatic domestic pigeons and the potential use of a combination of serological and molecular tests for controlling circovirus infections in pigeon breeding flocks

The course of pigeon circovirus infection in young pigeons experimentally kept under conditions mimicking the One Loft Race rearing system

Introduction

Racing pigeon competitions are a popular sport where success depends on birds' ability to return fast to their loft of origin. However, many additional factors like differences in feeding, training, everyday care and even geographical loft location influence race outcomes, which has led to the development of the One Loft Race (OLR) system. The OLR system aims to eliminate these factors by housing pigeons from various lofts in equal conditions in one facility. This in turn, however, fosters inter-individual transmission of pathogens.

Material and Methods

Fifteen young racing pigeons from five different lofts, naturally infected with pigeon circovirus (PiCV) were reared in one unit for six weeks. Four uninfected birds were kept in a separate unit and were treated as controls for flow cytometry analyses (background establishment). Blood samples were collected every seven days to extract DNA for PiCV quantification using droplet digital PCR and to isolate the mononuclear cells for flow cytometry analyses. On day 42, all birds were euthanised for spleen samples to be collected for further analyses.

Results

The viraemia peak was noted on day 14 of the experiment and subsequently decreased afterwards, with a remarkable decrease noted on day 35. The percentage of IgM+ B lymphocytes, including apoptotic cells, in the blood was very similar throughout the experiment. The percentage of apoptotic splenic IgM+ B cells was approximately 40% higher in the experimental group than in the control group.

Conclusion

Study results showed that the birds' adaptation period and the specific immunity they had probably developed hindered PiCV replication. Mild PiCV infection led to a slight increase of B lymphocyte apoptosis in the spleen.

The pigeon circovirus evolution, epidemiology and interaction with the host immune system under One Loft Race rearing conditions

This study was aimed to investigate the frequency of PiCV recombination, the kinetics of PiCV viremia and shedding and the correlation between viral replication and host immune response in young pigeons subclinically infected with various PiCV variants and kept under conditions mimicking the OLR system. Fifteen racing pigeons originating from five breeding facilities were housed together for six weeks. Blood and cloacal swab samples were collected from birds every seven days to recover complete PiCV genomes and determine PiCV genetic diversity and recombination dynamics, as well as to assess virus shedding rate, level of viremia, expression of selected genes and level of anti-PiCV antibodies. Three hundred and eighty-eight complete PiCV genomes were obtained and thirteen genotypes were distinguished. Twenty-five recombination events were detected. Recombinants emerged during the first three weeks of the experiment which was consistent with the peak level of viremia and viral shedding. A further decrease in viremia and shedding partially corresponded with IFN-γ and MX1 gene expression and antibody dynamics. Considering the role of OLR pigeon rearing system in spreading infectious agents and allowing their recombination, it would be reasonable to reflect on the relevance of pigeon racing from both an animal welfare and epidemiological perspective.

Investigating pigeon circovirus infection in a pigeon farm: molecular detection, phylogenetic analysis and complete genome analysis

BACKGROUND

Pigeon circovirus infections in pigeons (Columba livia domestica) have been reported worldwide. Pigeons should be PiCV-free when utilized as qualified experimental animals. However, pigeons can be freely purchased as experimental animals without any clear guidelines to follow. Herein, we investigated the status quo of PiCV infections on a pigeon farm in Beijing, China, which provides pigeons for experimental use.

RESULTS

PiCV infection was verified in at least three types of tissues in all forty pigeons tested. A total of 29 full-length genomes were obtained and deposited in GenBank. The whole genome sequence comparison among the 29 identified PiCV strains revealed nucleotide homologies of 85.8-100%, and these sequences exhibited nucleotide homologies of 82.7-98.9% as compared with those of the reference sequences. The cap gene displayed genetic diversity, with a wide range of amino acid homologies ranging from 64.5% to 100%. Phylogenetic analysis of the 29 full-genome sequences revealed that the PiCV strains in this study could be further divided into four clades: A (17.2%), B (10.4%), C (37.9%) and D (34.5%). Thirteen recombination events were also detected in 18 out of the 29 PiCV genomes obtained in this study. Phylogenetic research using the rep and cap genes verified the recombination events, which occurred between clades A/F, A/B, C/D, and B/D among the 18 PiCV strains studied.

CONCLUSIONS

In conclusion, PiCV infection, which is highly genetically varied, is extremely widespread on pigeon farms in Beijing. These findings indicate that if pigeons are to be used as experimental animals, it is necessary to evaluate the impact of PiCV infection on the results.

Pigeon Circovirus over Three Decades of Research: Bibliometrics, Scoping Review, and Perspectives

The pigeon circovirus (PiCV), first described in the literature in the early 1990s, is considered one of the most important infectious agents affecting pigeon health. Thirty years after its discovery, the current review has employed bibliometric strategies to map the entire accessible PiCV-related research corpus with the aim of understanding its present research landscape, particularly in consideration of its historical context. Subsequently, developments, current knowledge, and important updates were provided. Additionally, this review also provides a textual analysis examining the relationship between PiCV and the young pigeon disease syndrome (YPDS), as described and propagated in the literature. Our examination revealed that usages of the term 'YPDS' in the literature are characterizations that are diverse in range, and neither standard nor equivalent. Guided by our understanding of the PiCV research corpus, a conceptualization of PiCV diseases was also presented in this review. Proposed definitions and diagnostic criteria for PiCV subclinical infection (PiCV-SI) and PiCV systemic disease (PiCV-SD) were also provided. Lastly, knowledge gaps and open research questions relevant to future PiCV-related studies were identified and discussed.

Molecular detection and phylogenetic analysis of pigeon circovirus from racing pigeons in Northern China

Background

Pigeon circovirus (PiCV) infections in pigeons (Columba livia) have been reported worldwide. Currently, pigeon racing is becoming increasingly popular and considered to be a national sport in China, and even, the greatest competitions of racing pigeons are taking place in China. However, there are still no epidemiologic data regarding PiCV infections among racing pigeons in China. The purpose of our study was to provide information of prevalence, genetic variation and evolution of PiCV from racing pigeons in China.

Results

To trace the prevalence, genetic variation and evolution of PiCV in sick and healthy racing pigeons, 622 samples were collected from 11 provinces or municipalities in China from 2016 to 2019. The results showed that the positive rate of PiCV was 19.3% (120/622) at the sample level and 59.0% (23/39) at the club level, thus suggesting that the virus was prevalent in Chinese racing pigeons. A sequence analysis revealed that the cap genes of the PiCV strains identified in our study displayed a high genetic diversity and shared nucleotide homologies of 71.9%–100% and amino acid homologies of 71.7%–100%. 28 and 36 unique amino acid substitutions were observed in the Cap and Rep proteins derived from our PiCV strains, respectively. A cladogram representation of PiCV strains phylogeny based on 90 cap gene sequences showed that the strains in this study could be further divided into seven clades (A, B, C, E, G, H, and I) and some of them were closely related to worldwide strains from different types of pigeons. A large number of recombination events (31 events) were also detected in the PiCV genomes from Chinese racing pigeons.

Conclusions

These findings indicate that PiCV strains circulating in China exhibit a high genetic diversity and also contribute to information of prevalence, genetic variation and evolution of PiCV from racing pigeons in China.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08425-8.

The Clinical Infection with Pigeon Circovirus (PiCV) Leads to Lymphocyte B Apoptosis But Has No Effect on Lymphocyte T Subpopulation

The pathology of pigeon circovirus (PiCV) is still unknown, but it is regarded as an immunosuppressant. This study aimed to find a correlation between PiCV natural infection and immunosuppression. The study was conducted with 56 pigeons divided into the following groups: PiCV-positive but showing (group S) or not (group I) non-specific clinical symptoms and asymptomatic pigeons negative for PiCV (group H). The percentage and apoptosis of T CD3⁺ and B IgM⁺ splenocytes; the expression of CD4, CD8, and IFN-γ genes in splenic mononuclear cells; the number of PiCV viral loads in the bursa of Fabricius; and the level of anti-PiCV antibodies were analyzed. The results showed that the percentage of B IgM⁺ cells was almost two-fold lower in group S than in group H, and that ca. 20% of the lymphocytes were apoptotic. No increased apoptosis was detected in TCD3⁺ subpopulation. The PiCV viral loads were approximately one thousand and ten thousand times higher in group S than in groups I and H, respectively. Our results indicate a possible correlation between the number of PiCV viral loads and severity of PiCV infection and confirm that PiCV infection leads to the suppression of humoral immunity by inducing B lymphocyte apoptosis.

Influence of pigeon interferon alpha (PiIFN-α) on pigeon circovirus (PiCV) replication and cytokine expression in Columba livia

Pigeon circovirus (PiCV) is the most diagnosed virus in pigeons (Columba livia) and have been studied and reported globally. PiCV infections can lead to immunosuppression and pigeons infected with PiCV can result to lymphocyte apoptosis and atrophy of immune organs. Young pigeon disease syndrome (YPDS) is a complex disease and believed that PiCV could be one of the agents leading to this syndrome. An effective treatment regimen is needed to control the spread of PiCV in pigeons. In this study pigeon interferon alpha (PiIFN-α) was cloned and expressed and its antiviral effects were tested against fowl adenovirus type 4 (FAdV-4) in vitro and PiCV in vivo. No detectable levels of FAdV-4 viral genome in LMH cells stimulated with 300 μg/mL PiIFN-α were found. Additionally, PiIFN-α was stable at different temperature and pH for 4 h, and no reduction in antiviral activity was observed in untreated and treated cells. In pigeons naturally and experimentally infected by PiCV, no detectable levels of PiCV virus titers were found after treatment with PiIFN-α. Cytokine and ISG expression levels in liver and spleen samples were detected and IFN-γ and Mx1 genes were dominantly up-regulated following PiIFN-α treatment (p < 0.05). This study demonstrated that PiCV can be inhibited by administration of PiIFN-α and PiFN-α can be used as a therapeutic approach to prevent the spread of PiCV in pigeons.

Assembly of pigeon circovirus-like particles using baculovirus expression system

Pigeon circovirus (PiCV) is able to infect racing and meat pigeons of all ages and is a key factor that triggers young pigeon disease syndrome (YPDS). PiCV vaccine research has been impeded because PiCV cannot be grown or propagated in cell cultures. Virus-like particles (VLPs), which can be generated by a wide range of expression systems, have been shown to have outstanding immunogenicity and constitute promising vaccines against a wide range of pathogens. Cap protein, which contains neutralizing antibody epitopes, is the only capsid protein of PiCV. In this study, the baculovirus expression system was utilized to express the PiCV Cap protein, which was self-assembled into VLPs with a spherical morphology and diameters of 15-18 nm. Specific antibodies against the Cap protein were induced after BALB/c mice immunized intramuscularly (i.m.) with VLPs combined with adjuvant. Based on these findings, PiCV VLPs may be a promising candidate vaccine against PiCV.

Comparison of the immune response to vaccination with pigeon circovirus recombinant capsid protein (PiCV rCP) in pigeons uninfected and subclinically infected with PiCV

Infections with immunosuppressive pigeon circovirus (PiCV) pose the most severe health problem to the global pigeon breeding. The vaccination with immunogenic PiCV recombinant capsid protein (PiCV rCP) is a potential tool for disease control. Because of the high prevalence of PiCV asymptomatic infections, the subclinically infected pigeons will be vaccinated in practice. The aim of this study was to answer a question if vaccination of asymptomatic, infected with PiCV pigeons induces a similar immune response to PiCV rCP as in uninfected birds. One hundred and twenty 6-week-old carrier pigeons were divided into 4 groups (2 groups of naturally infected and uninfected with PiCV individuals). Birds from groups V and V1 were vaccinated twice with PiCV rCP mixed with an adjuvant, whereas pigeons from groups C and C1 were immunized with an adjuvant only. The expression of genes encoding IFN-γ, CD4, and CD8 T lymphocyte receptors; the number of anti-PiCV rCP IgY-secreting B cells (SBC) and anti-PiCV rCP IgY were evaluated 2, 21, 39 and 46 days post vaccination (dpv). Study results showed that the expression of CD8 and IFN-γ genes was higher in both groups of infected pigeons than in the uninfected birds, irrespective of vaccination. In the uninfected birds, the expression of these genes was insignificantly higher in the vaccinated pigeons. The anti-PiCV rCP IgY-SBC were detected on 2 and 23 dpv and seroconversion was noted on 23 and 39 dpv in V and V1 groups, respectively. In the light of the results obtained, it could be concluded that pigeon circovirus recombinant capsid protein elicits the immune response in both naturally infected and uninfected pigeons, but its rate varies depending on PiCV infectious status. The infection with PiCV masks the potential cellular immune response to the vaccination with PiCV rCP and leads to the suppression of humoral immunity.

Immunogenicity of Pigeon Circovirus Recombinant Capsid Protein in Pigeons

Pigeon circovirus (PiCV) is the most frequently diagnosed virus in pigeons and is thought to be one of the causative factors of a complex disease called the young pigeon disease syndrome (YPDS). The development of a vaccine against this virus could be a strategy for YPDS control. Since laboratory culture of PiCV is impossible, its recombinant capsid protein (rCP) can be considered as a potential antigen candidate in sub-unit vaccines. The aim of this basic research was to evaluate the immune response of pigeons to PiCV rCP. Sixty six-week-old carrier pigeons were divided into two groups (experimental immunized with PiCV rCP mixed with an adjuvant, and control immunized with an adjuvant only), and immunized twice in a 21-day interval. On the day of immunization and on two, 23, 39, and 46 days post first immunization (dpv), samples of blood, spleen, and bursa of Fabricius were collected from six birds from each group to examine anti-PiCV rCP IgY, anti-PiCV rCP IgY-secreting B cells (SBC), IFN-γ gene expression, and percentage of T CD3⁺, CD4⁺, CD8⁺, and B IgM⁺ lymphocytes. The results indicated a correct immune response to PiCV rCP both in humoral and cell-mediated immunity, which was manifested by seroconversion since 23 dpv, by a significantly higher anti-PiCV rCP IgY-SBC number on two and 23 dpv, and significantly higher IFN-γ gene expression since two dpv. There were no significant differences or trends noted between particular T and B lymphocyte subpopulations. To conclude, PiCV rCP may be deemed immunogenic and could be considered as an antigen candidate in sub-unit vaccines against PiCV infections in pigeons.

The epidemiology, molecular characterization and clinical pathology of circovirus infections in pigeons - current knowledge

The first cases of circovirus infections in pigeons were documented less than 25 years ago. Since then, circovirus infections have been reported on nearly all continents. The specificity of pigeon breeding defies biosecurity principles, which could be the reason for the high prevalence of PiCV infections. PiCV infections in pigeons lead to atrophy of immune system organs and lymphocyte apoptosis. Infected birds could be more susceptible to infections of the respiratory and digestive tract. PiCV has been associated with the young pigeon disease syndrome (YPDS). PiCVs are characterized by high levels of genetic diversity due to frequent point mutations, recombination processes in the PiCV genome and positive selection. Genetic recombinations and positive selection play the key role in the evolution of PiCV. A protocol for culturing PiCV under laboratory conditions has not yet been developed, and traditional vaccines against the infection are not available. Recombinant capsid proteins for detecting anti-PiCV antibodies have been obtained, and these antigens can be used in the production of diagnostic tests and subunit vaccines against PiCV infections. However, YPDS has complex etiology, and it remains unknown whether immunization against PiCV alone will contribute to effective control of YPDS.