Epidemiology of two human protoparvoviruses, bufavirus and tusavirus

Structural Characterization of Human Bufavirus 1: Receptor Binding and Endosomal pH-Induced Changes

Bufaviruses (BuV) are members of the Parvoviridae of the Protoparvovirus genus. They are non-enveloped, T = 1 icosahedral ssDNA viruses isolated from patients exhibiting acute diarrhea. The lack of treatment options and a limited understanding of their disease mechanisms require studying these viruses on a molecular and structural level. In the present study, we utilize glycan arrays and cell binding assays to demonstrate that BuV1 capsid binds terminal sialic acid (SIA) glycans. Furthermore, using cryo-electron microscopy (cryo-EM), SIA is shown to bind on the 2/5-fold wall of the capsid surface. Interestingly, the capsid residues stabilizing SIA binding are conserved in all human BuVs identified to date. Additionally, biophysical assays illustrate BuV1 capsid stabilization during endo-lysosomal (pH 7.4-pH 4) trafficking and capsid destabilization at pH 3 and less, which correspond to the pH of the stomach. Hence, we determined the cryo-EM structures of BuV1 capsids at pH 7.4, 4.0, and 2.6 to 2.8 Å, 3.2 Å, and 2.7 Å, respectively. These structures reveal capsid structural rearrangements during endo-lysosomal escape and provide a potential mechanism for this process. The structural insights gained from this study will add to the general knowledge of human pathogenic parvoviruses. Furthermore, the identification of the conserved SIA receptor binding site among BuVs provides a possible targetable surface-accessible pocket for the design of small molecules to be developed as anti-virals for these viruses.

Incidental finding of a human-like tusavirus in a lamb with lip lesions and fatal pneumonia

Tusaviruses in the genus Protoparvovirus of family Parvoviridae were first identified in a diarrhoeic Tunisian child in 2014. Thereafter, high prevalence of a genetically similar virus was demonstrated in faeces from caprine and ovine species in Hungary. Here, we describe an investigation into the cause of scabby lip lesions in a 6 month-old lamb, submitted from a farm experiencing weight loss and scouring in lambs in England. Transmission electron microscopy visualised small circular particles of 18 and 22 nm in diameter in lip lesions identified as tusavirus and flumine parvovirus by Next Generation Sequencing. Liver, kidney, lung, small intestine content and faeces were also strongly positive for the tusavirus DNA as well as 10 % of faecal samples of the flock collected 2 months after the initial lip sampling. NS1 and VP1 amino acid sequences of this tusavirus displayed 99.5 and 92.89 % identity to those of a human tusavirus, respectively. These amino acid identities were at 95.5 and 89.68 % when compared to those of a goat tusavirus. Phylogenetic analysis of the NS1 and VP1 also grouped the virus in the genus Protoparvovirus and close to tusaviruses detected in human, ovine and caprine species. Wider surveillance of the virus indicated a broader geographical distribution for the virus in England. Histology of the lip tissue revealed localised areas of epidermal hyperplasia and hyperkeratosis affecting haired skin, with mild leucocyte infiltration of the subjacent dermis, but no changes to implicate virus involvement. Flumine parvovirus was concluded to be an environment contaminant. Broader studies in prevalence of these virus in UK sheep flocks and human population, animal models and experimental infections could provide insights into the pathogenesis of these novel viruses and their zoonotic potential.

Canine bufavirus (Carnivore protoparvovirus-3) infection in dogs with respiratory disease

Canine bufavirus (CBuV) or Carnivore protoparvovirus-3, a nonenveloped DNA virus belonging to the genus Protoparvovirus, family Parvoviridae, has been identified in dogs with respiratory and enteric diseases. Although CBuV detection has been reported in multiple countries, descriptions of pathologic findings associated with infection have not yet been provided. In this study, the authors necropsied 14 dogs (12 puppies and 2 adult dogs) from a breeding colony that died during multiple outbreaks of respiratory diseases. Postmortem investigations revealed extensive bronchointerstitial pneumonia with segmental type II pneumocyte hyperplasia in all necropsied puppies but less severe lesions in adults. With negative results of common pathogen detection by ancillary testing, CBuV DNA was identified in all investigated dogs using a polymerase chain reaction (PCR). Quantitative PCR demonstrated CBuV DNA in several tissues, and in situ hybridization (ISH) indicated CBuV tissue localization in the lung, tracheobronchial lymph node, and spinal cord, suggesting hematogenous spread. Dual CBuV ISH and cellular-specific immunohistochemistry were used to determine the cellular tropism of the virus in the lung and tracheobronchial lymph node, demonstrating viral localization in various cell types, including B-cells, macrophages, and type II pneumocytes, but not T-cells. Three complete CBuV sequences were successfully characterized and revealed that they clustered with the CBuV sequences obtained from dogs with respiratory disease in Hungary. No additional cases were identified in small numbers of healthy dogs. Although association of the bufavirus with enteric disease remains to be determined, a contributory role of CBuV in canine respiratory disease is possible.

First detection and genetic characterization of canine bufavirus in domestic dogs, Thailand

Canine bufavirus (CBuV) was reported in domestic dogs worldwide. We conducted a survey of canine bufavirus in domestic dogs in Thailand from September 2016 to October 2022. Rectal swab samples (n = 531) were collected from asymptomatic dogs and dogs with gastroenteritis signs. The samples were tested for CBuV using PCR with specific primers to the VP1/VP2 gene, and 9.42% (50/531) was CBuV positive. Our findings showed that CBuVs could be detected in both symptomatic and healthy dogs. The Thai CBuVs were found in dogs from different age groups, with a significant presence in those under 1 year (12.60%) and dogs aged 1-5 years (7.34%) (p < 0.05), suggesting a high prevalence of Thai CBuVs in dogs under 5 years of age. We performed complete genome sequencing (n = 15) and partial VP1/VP2 sequencing (n = 5) of Thai CBuVs. Genetic and phylogenetic analyses showed that whole genomes of Thai CBuVs were closely related to Chinese and Italian CBuVs, suggesting the possible origin of Thai CBuVs. The analysis of VP1 and VP2 genes in Thai CBuVs showed that 18 of them were placed in subgroup A, while only 2 belonged to subgroup B. This study is the first to report the detection and genetic characterization of CBuVs in domestic dogs in Thailand. Additionally, surveillance and genetic characterization of CBuVs in domestic animals should be further investigated on a larger scale to elucidate the dynamic, evolution, and distribution of CBuVs.

Molecular Surveillance for Bocaparvoviruses and Bufaviruses in the European Hedgehog (Erinaceus europaeus)

The presence of bocaparvoviruses (BoVs) and bufaviruses (BuVs) in the European hedgehog (Erinaceus europaeus) was investigated by screening duodenal and liver samples collected from 183 carcasses, delivered to wildlife rescue centers located in northwestern Italy. BoV DNA was detected in 15 animals (8.2%), with prevalences of 7.1% (13/183) and 2.7% (5/183) in intestine and liver samples, respectively. Upon the sequence analyses of the NS1 gene, two highly divergent BoVs (65.5-67.8% nt identities) were identified. Fourteen strains showed the highest identity (98.3-99.4% nt) to the hedgehog BoV strains recently detected in China in Amur hedgehogs (Erinaceus amurensis), whilst four strains were genetically related (98.9-99.4% nt identities) to the porcine BoVs identified in pigs and classified in the species Bocaparvovirus ungulate 4, which included related viruses also found in rats, minks, shrews, and mice. BuV DNA was detected in the duodenal samples of two hedgehogs, with a prevalence rate of 1.1%. The nearly full-length genome of two BuV strains, Hedgehog/331DU-2022/ITA and Hedgehog/1278DU/2019/ITA, was reconstructed. Upon phylogenetic analysis based on the NS and VP aa sequences, the Italian hedgehog BuVs tightly clustered with the BuVs recently identified in the Chinese Amur hedgehogs, within a potential novel candidate species of the genus Protoparvovirus.

Old and Novel Enteric Parvoviruses of Dogs

Parvovirus infections have been well known for around 100 years in domestic carnivores. However, the use of molecular assays and metagenomic approaches for virus discovery and characterization has led to the detection of novel parvovirus species and/or variants in dogs. Although some evidence suggests that these emerging canine parvoviruses may act as primary causative agents or as synergistic pathogens in the diseases of domestic carnivores, several aspects regarding epidemiology and virus-host interaction remain to be elucidated.

Human-stool-associated tusavirus (Parvoviridae) in domestic goats and sheep

In this study, genetic counterparts of the human-stool-associated tusavirus (subfamily Parvovirinae, family Parvoviridae) with >97% and 95-100% amino acid sequence identity in the parvoviral NS1 and VP1 protein were identified in faecal specimens from domestic goats (Capra hircus) and sheep (Ovis aries) in Hungary. Eleven (17.8%) of the 62 faecal specimens from goats and 12 (25.5%) of the 47 from sheep both from less than 12 months old animals were positive for tusavirus DNA by PCR, while none of the specimens collected from cattle and swine were positive. Thus, it cannot be ruled out that tusavirus infection in humans is of zoonotic origin.

First report of canine bufavirus in India

Canine bufavirus (CBuV), a novel protoparvovirus of dogs that is associated with enteric and respiratory symptoms, has been reported only in Italy and China. The enteric prevalence of CBuV in India was investigated, and the nearly complete genome sequence (4292 bp) was amplified and reconstructed for one strain. A nucleotide sequence alignment indicated 93.42–98.81% identity to the other available CBuV sequences and 70.88–73.39% and 54.4–54.8% identity to human bufavirus and canine parvovirus 2 (CPV-2), respectively. The current strain is most closely related to Chinese CBuV strains, which together form an Asian lineage. This first report of the prevalence of CBuV in India emphasizes the need for further epidemiological surveillance.

Global Status of Bufavirus, Cosavirus, and Saffold Virus in Gastroenteritis: A Systematic Review and Meta-Analysis

Background: Bufavirus (BuV), Human Cosavirus (HCoSV), and Saffold (SAFV) virus are three newly discovered viruses and have been suggested as possible causes of gastroenteritis (GE) in some studies. The aim of the present study was to estimate the overall prevalence of viruses and their association with GE. Methods: A comprehensive systematic search was conducted in Scopus, Web of Science, PubMed, and Google scholar between 2007 and 2021 to find studies on the prevalence of BuV, HCoSV, and SAFV viruses. Result: Meta-analysis of the 46 included studies showed the low prevalence of BuV (1.%, 95% CI 0.6-1.5%), HCoSV (0.8%, 95% CI 0.4-1.5%), and SAFV (1.9%, 95% CI 1.1-3.1%) worldwide. Also, no significant association between these viruses and GE was observed. BuV was isolated from patients with GE in Africa, while SAFV was more common in Europe. BuV1 and BuV2 have the same prevalence between the three identified genotypes of BuV. HCoSV-C was the most prevalent genotype of HCoSV, and SAFV2 was the commonest genotype of SAFV. All of these viruses were more prevalent in children older than 5 years of age. Conclusion: This was the first meta-analysis on the prevalence and association of BuV, HCoSV, and SAFV with GE. While no significant association was found between infection with these viruses and GE, we suggest more studies, especially with case-control design and from different geographical regions in order to enhance our knowledge of these viruses.

[Investigation of viruses harbored by wild animals: toward pre-emptive measures against future zoonotic diseases]

Zoonoses are caused by pathogens transmitted from animals. To prepare mitigating measures against emerging zoonoses, it is imperative to identify animal reservoirs that carry potential pathogens and also elucidate the transmission routes of these pathogens. Under the continuous collaboration with counterparts from Zambia and Indonesia, we have so far identified various viruses in wild animals. Some of the identified viruses were phylogenetically distinct from known virus species and this finding led to approved new virus species by the International Committee on Taxonomy of Viruses (ICTV). Our studies provided new insights into the divergence, natural hosts and lifecycle of viruses. Through the exploration and characterization of viruses in animals, we will endeavor to contribute to the existing knowledge on viral pathogens in wild animals. This is cardinal for evidence-based preemptive measures against future zoonoses.

Investigation of Bufavirus and Parvovirus 4 in Patients with Gastro-Enteritis from the South-East of France

Bufavirus (BuV) and human parvovirus 4 (PARV4) belong to the Parvoviridae family. We assessed BuV and PARV4 DNA presence by real-time PCR analysis in stool, blood and respiratory samples collected in patients from Marseille and Nice, two large cities in the South-East of France. Bu-V DNA was detected in diarrheic stool samples from 92 patients (3.6% of 2583 patients), particularly men and adults, and patients from the nephrology and the infectious disease departments. Among the patients with a BuV-positive stool sample and for whom at least one blood sample was available (n = 30 patients), BuV DNA was detected also in 3 blood samples. In contrast, BuV DNA was not detected in any of the respiratory samples from 23 patients with BuV-positive stool. BuV detection rate was comparable in stool samples from patients with and without diarrhea. We did not detect PARV4 DNA in any of the stool specimens (n = 2583 patients). Our results suggest that PARV4 fecal–oral transmission is rare or non-existent in the South-East of France while BuV circulates with a relatively high rate in this area.

Emerging Parvoviruses in Domestic Cats

Parvovirus infections in cats have been well known for around 100 years. Recently, the use of molecular assays and metagenomic approaches for virus discovery and characterization has led to the detection of novel parvovirus lineages and/or species infecting the feline host. However, the involvement of emerging parvoviruses in the onset of gastroenteritis or other feline diseases is still uncertain.

Human Protoparvovirus DNA and IgG in Children and Adults with and without Respiratory or Gastrointestinal Infections

Three human protoparvoviruses, bufavirus (BuV), tusavirus (TuV) and cutavirus (CuV), have recently been discovered in diarrheal stool. BuV has been associated with diarrhea and CuV with cutaneous T-cell lymphoma, but there are hardly any data for TuV or CuV in stool or respiratory samples. Hence, using qPCR and IgG enzyme immunoassays, we analyzed 1072 stool, 316 respiratory and 445 serum or plasma samples from 1098 patients with and without gastroenteritis (GE) or respiratory-tract infections (RTI) from Finland, Latvia and Malawi. The overall CuV-DNA prevalences in stool samples ranged between 0-6.1% among our six patient cohorts. In Finland, CuV DNA was significantly more prevalent in GE patients above rather than below 60 years of age (5.1% vs 0.2%). CuV DNA was more prevalent in stools among Latvian and Malawian children compared with Finnish children. In 10/11 CuV DNA-positive adults and 4/6 CuV DNA-positive children with GE, no known causal pathogens were detected. Interestingly, for the first time, CuV DNA was observed in two nasopharyngeal aspirates from children with RTI and the rare TuV in diarrheal stools of two adults. Our results provide new insights on the occurrence of human protoparvoviruses in GE and RTI in different countries.

Unmasking viral sequences by metagenomic next-generation sequencing in adult human blood samples during steroid-refractory/dependent graft-versus-host disease

BACKGROUND

Viral infections are common complications following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Allo-HSCT recipients with steroid-refractory/dependent graft-versus-host disease (GvHD) are highly immunosuppressed and are more vulnerable to infections with weakly pathogenic or commensal viruses. Here, twenty-five adult allo-HSCT recipients from 2016 to 2019 with acute or chronic steroid-refractory/dependent GvHD were enrolled in a prospective cohort at Geneva University Hospitals. We performed metagenomics next-generation sequencing (mNGS) analysis using a validated pipeline and de novo analysis on pooled routine plasma samples collected throughout the period of intensive steroid treatment or second-line GvHD therapy to identify weakly pathogenic, commensal, and unexpected viruses.

RESULTS

Median duration of intensive immunosuppression was 5.1 months (IQR 5.5). GvHD-related mortality rate was 36%. mNGS analysis detected viral nucleotide sequences in 24/25 patients. Sequences of ≥ 3 distinct viruses were detected in 16/25 patients; Anelloviridae (24/25) and human pegivirus-1 (9/25) were the most prevalent. In 7 patients with fatal outcomes, viral sequences not assessed by routine investigations were identified with mNGS and confirmed by RT-PCR. These cases included Usutu virus (1), rubella virus (1 vaccine strain and 1 wild-type), novel human astrovirus (HAstV) MLB2 (1), classic HAstV (1), human polyomavirus 6 and 7 (2), cutavirus (1), and bufavirus (1).

CONCLUSIONS

Clinically unrecognized viral infections were identified in 28% of highly immunocompromised allo-HSCT recipients with steroid-refractory/dependent GvHD in consecutive samples. These identified viruses have all been previously described in humans, but have poorly understood clinical significance. Rubella virus identification raises the possibility of re-emergence from past infections or vaccinations, or re-infection. Video abstract.

Reorganizing the family Parvoviridae: a revised taxonomy independent of the canonical approach based on host association

Parvoviridae, a diverse family of small single-stranded DNA viruses was established in 1975. It was divided into two subfamilies, Parvovirinae and Densovirinae, in 1993 to accommodate parvoviruses that infect vertebrate and invertebrate animals, respectively. This relatively straightforward segregation, using host association as the prime criterion for subfamily-level classification, has recently been challenged by the discovery of divergent, vertebrate-infecting parvoviruses, dubbed "chapparvoviruses", which have proven to be more closely related to viruses in certain Densovirinae genera than to members of the Parvovirinae. Viruses belonging to these genera, namely Brevi-, Hepan- and Penstyldensovirus, are responsible for the unmatched heterogeneity of the subfamily Densovirinae when compared to the Parvovirinae in matters of genome organization, protein sequence homology, and phylogeny. Another genus of Densovirinae, Ambidensovirus, has challenged traditional parvovirus classification, as it includes all newly discovered densoviruses with an ambisense genome organization, which introduces genus-level paraphyly. Lastly, current taxon definition and virus inclusion criteria have significantly limited the classification of certain long-discovered parvoviruses and impedes the classification of some potential family members discovered using high-throughput sequencing methods. Here, we present a new and updated system for parvovirus classification, which includes the introduction of a third subfamily, Hamaparvovirinae, resolves the paraphyly within genus Ambidensovirus, and introduces new genera and species into the subfamily Parvovirinae. These proposals were accepted by the ICTV in 2020 March.

Genetic heterogeneity of canine bufaviruses

Canine bufavirus (CBuV) is a protoparvovirus, genetically related to human and non-human primate bufaviruses and distantly related to canine parvovirus type 2 (CPV-2). CBuV was initially identified from young dogs with respiratory signs but subsequent studies revealed that this virus is also a common component of the canine enteric virome. In this survey, by assessing archival and recent collections of dogs faecal samples, CBuV DNA was detected with a higher prevalence rate (8.8%) in animals with enteritis than in control animals (5.0%), although this difference was not statistically significant. The rate of co-infections with other enteric viruses in diarrhoeic dogs was high (84.6%), mostly in association with canine parvovirus CPV-2 (90.1%). The complete ORF2 gene was determined in five samples, and the nearly full-length genome was reconstructed for three strains, 62/2017/ITA, 9AS/2005/ITA and 35/2018/ITA. Upon sequence comparison, the viruses appeared highly conserved in the NS1 (97.2%-97.9% nt and 97.5%-98.1% aa identities). In the complete VP2 coding region, three strains were similar to the prototype viruses (99.7-99.8 nt and 99.6%-99.8% aa) whilst strains 9AS/2005/ITA and 35/2016/ITA were distantly related (87.6%-89.3% nt and 93.9%-95.1% aa identities). Interestingly, genetic diversification occurred downstream conserved regions such as the VP1/VP2 splicing signals and/or the G-rich motif in the N terminus of the VP2, suggesting a potential recombination nature. Upon phylogenetic analysis, the two divergent CBuV strains formed a distinct cluster/genotype.

Cutavirus DNA in Malignant and Nonmalignant Skin of Cutaneous T-Cell Lymphoma and Organ Transplant Patients but Not of Healthy Adults

BACKGROUND

Three new parvoviruses of Protoparvovirus genus, bufavirus (BuV), tusavirus (TuV), and cutavirus (CuV), have recently been discovered in diarrheal stools. CuV was further detected in a proportion of cutaneous T-cell lymphoma (CTCL)/mycosis fungoides skin samples and in one melanoma.

PATIENTS AND METHODS

With novel multiplex quantitative polymerase chain reaction and antibody assays, we studied 3 patient groups for BuV, TuV, and CuV DNA and immunoglobulin G (IgG): CTCL patients, immunosuppressed solid-organ transplant recipients, and immunocompetent healthy adults.

RESULTS

CuV DNA was detected in skin biopsies of 4/25 (16.0%) CTCL and 4/136 (2.9%) transplant patients but not in any of 159 skin samples of 98 healthy adults. The dermal CuV-DNA prevalence was significantly higher in CTCL patients than in the other subjects. CuV DNA was further detected in healthy skin of 4 organ transplant recipients, 2 of whom also had CuV-positive skin carcinomas. One CTCL patient harbored CuV DNA in both malignant (CTCL, melanoma) and nonmalignant skin and sentinel lymph nodes but not in his prostate. The CuV IgG seroprevalences were among CTCL patients 9.5% (4/42), transplant recipients 6.5% (8/124), and healthy adults 3.8% (3/78). BuV and TuV DNAs were absent and antibodies infrequent in all cohorts. Parvoviral antibodies were shown to persist for ≥20 years and dermal CuV DNA for 4 years. All 3 CuV-DNA-positive patients, with both biopsies and sera available, were CuV-IgG positive.

CONCLUSION

Our results suggest that dermal CuV DNA carriage is associated with CTCL. Any putative roles of CuV in the carcinogenesis must be determined in forthcoming studies.

Structural Characterization of Cuta- and Tusavirus: Insight into Protoparvoviruses Capsid Morphology

Several members of the Protoparvovirus genus, capable of infecting humans, have been recently discovered, including cutavirus (CuV) and tusavirus (TuV). To begin the characterization of these viruses, we have used cryo-electron microscopy and image reconstruction to determine their capsid structures to ~2.9 Å resolution, and glycan array and cell-based assays to identify glycans utilized for cellular entry. Structural comparisons show that the CuV and TuV capsids share common features with other parvoviruses, including an eight-stranded anti-parallel β-barrel, depressions at the icosahedral 2-fold and surrounding the 5-fold axes, and a channel at the 5-fold axes. However, the viruses exhibit significant topological differences in their viral protein surface loops. These result in three separated 3-fold protrusions, similar to the bufaviruses also infecting humans, suggesting a host-driven structure evolution. The surface loops contain residues involved in receptor binding, cellular trafficking, and antigenic reactivity in other parvoviruses. In addition, terminal sialic acid was identified as the glycan potentially utilized by both CuV and TuV for cellular entry, with TuV showing additional recognition of poly-sialic acid and sialylated Lewis X (sLeXLeXLeX) motifs reported to be upregulated in neurotropic and cancer cells, respectively. These structures provide a platform for annotating the cellular interactions of these human pathogens.

Detection and molecular characterization of novel porcine bufaviruses in Guangxi province

Bufavirus (BuV) can infect a variety of hosts, including human, bats, rats, dog, swine and shrew species and are suggested related to diarrhea disease. Porcine bufaviruses (PoBuV) were first detected in Hungarian pig farms in 2016. To determine the prevalence and genetic diversity of PoBuV in China, we developed SYBR Green-based real-time PCR assays to detect PoBuV in Guangxi pigs. Real-time PCR detected PoBuV in 30 (29.13%, 30/103) of the samples with diarrhoeal intestinal tissues and rectal swabs. PoBuV-positive intestinal tissues and rectal swabs samples, co-infection with PEDV (15/30, 50.0%), followed by PDCoV (8/30, 26.67%), PoRV (6/30, 20.0%), PRRSV (5/30, 16.67%), and PCV2 (3/30, 10.0%) were observed. Fourteen complete genomes were cloned and sequenced. The results showed that they were 4189 bp in length and combined three open reading frames (ORFs) in the order 5'-NS1-VP1/VP2-3'. Fourteen strains shared 96.5%-99.8% identity among themselves and 92.7%-97.9% with the PoBuV reference sequences. Phylogenetic analysis based on the deduced amino acid sequence of the VP2 gene showed fourteen strains belonging to PoBuV and were grouped into the three branches. These results help to provide new insight into the molecular epidemiology of PoBuV in the world.

Feline Virome-A Review of Novel Enteric Viruses Detected in Cats

Recent advances in the diagnostic and metagenomic investigations of the feline enteric environment have allowed the identification of several novel viruses that have been associated with gastroenteritis in cats. In the last few years, noroviruses, kobuviruses, and novel parvoviruses have been repetitively detected in diarrheic cats as alone or in mixed infections with other pathogens, raising a number of questions, with particular regards to their pathogenic attitude and clinical impact. In the present article, the current available literature on novel potential feline enteric viruses is reviewed, providing a meaningful update on the etiology, epidemiologic, pathogenetic, clinical, and diagnostic aspects of the infections caused by these pathogens.

Emerging Human Parvoviruses: The Rocky Road to Fame

Parvoviruses are structurally simple viruses with linear single-stranded DNA genomes and nonenveloped icosahedral capsids. They infect a wide range of animals from insects to humans. Parvovirus B19 is a long-known human pathogen, whereas adeno-associated viruses are nonpathogenic. Since 2005, many parvoviruses have been discovered in human-derived samples: bocaviruses 1-4, parvovirus 4, bufavirus, tusavirus, and cutavirus. Some human parvoviruses have already been shown to cause disease during acute infection, some are associated with chronic diseases, and others still remain to be proven clinically relevant-or harmless commensals, a distinction not as apparent as it might seem. One initially human-labeled parvovirus might not even be a human virus, whereas another was originally overlooked due to inadequate diagnostics. The intention of this review is to follow the rocky road of emerging human parvoviruses from discovery of a DNA sequence to current and future clinical status, highlighting the perils along the way.

Novel Parvovirus Related to Primate Bufaviruses in Dogs

A novel protoparvovirus species, related genetically to human bufaviruses, was identified in dogs with respiratory signs. The canine bufavirus was distantly related to the well-known canine protoparvovirus, canine parvovirus type 2, sharing low amino acid identities in the nonstructural protein 1 (40.6%) and in the capsid protein 1 (33.4%). By screening collections of fecal, nasal, and oropharyngeal samples obtained from juvenile dogs (<1 year of age), canine bufavirus DNA appeared as a common component of canine virome. The virus was common in the stool samples of dogs with or without enteric disease and in the nasal and oropharyngeal swab samples of dogs with respiratory signs. However, the virus was not detected in nasal and oropharyngeal swab samples from animals without clinical signs.

Global Distribution of Human Protoparvoviruses

Development of next-generation sequencing and metagenomics has revolutionized detection of novel viruses. Among these viruses are 3 human protoparvoviruses: bufavirus, tusavirus, and cutavirus. These viruses have been detected in feces of children with diarrhea. In addition, cutavirus has been detected in skin biopsy specimens of cutaneous T-cell lymphoma patients in France and in 1 melanoma patient in Denmark. We studied seroprevalences of IgG against bufavirus, tusavirus, and cutavirus in various populations (n = 840), and found a striking geographic difference in prevalence of bufavirus IgG. Although prevalence was low in adult populations in Finland (1.9%) and the United States (3.6%), bufavirus IgG was highly prevalent in populations in Iraq (84.8%), Iran (56.1%), and Kenya (72.3%). Conversely, cutavirus IgG showed evenly low prevalences (0%–5.6%) in all cohorts, and tusavirus IgG was not detected. These results provide new insights on the global distribution and endemic areas of protoparvoviruses.

First identification of a novel parvovirus distantly related to human bufavirus from diarrheal dogs in China

Bufaviruses are small, nonenveloped, single-stranded DNA viruses belonging to the subfamily Parvovirinae. Human bufaviruses were first identified in 2012 in fecal samples from children with diarrhea. A new parvovirus of canines that was first detected in various samples from dogs with enteric and respiratory symptoms in Italy between 2014 and 2018 is closely related to the newly described human bufavirus. To explore the prevalence and genetic diversity of CBuV in Chinese dogs, 540 canine parvovirus (CPV)-positive serum and diarrhea samples were collected in Guangxi Province between 2016 and 2018. Among the samples, 6.25% (5/80) of rectal swabs and 2.5% (5/200) of CPV PCR-positive samples were positive for CBuV. However, the virus was not detected in CPV PCR-negative samples or nasal swabs. Two CBuV isolates were identified from CPV-positive fecal and serum samples by complete sequence analysis, with 99.8%-99.9% NS1 and VP2 protein identity to each another. Sequence analysis indicated that the CBuV GXNN01-2018 isolate VP2 protein shares 99.6% identity with the Italian CBuV ITA/2015/297 isolate and 62.3%-65.5% identity with human bufavirus. Phylogenetic analysis showed that CBuV was significantly distinct from other known bufaviruses and was most closely related to CBuV ITA/2015/297. This is the first report of the existence of CBuV in China, and our findings will strengthen the understanding of the epidemiology of bufaviruses in different animals.

Identification of a novel parvovirus in domestic cats

A novel protoparvovirus species was identified in domestic cats. The virus was distantly related to the well-known feline (feline panleukopenia virus) and canine (canine parvovirus type 2) parvoviruses, sharing low nucleotide identities in the capsid protein 2 (less than 43%). The virus was genetically similar (100% at the nucleotide level) to a newly identified canine protoparvovirus, genetically related to human bufaviruses. The feline bufavirus appeared as a common element of the feline virome, especially in juvenile cats, with an overall prevalence of 9.2%. The virus was more common in respiratory samples (9.5%-12.2%) than in enteric samples of cats (2.2%). The role of bufaviruses in the etiology of feline respiratory disease complex, either as a primary or a secondary agents, should be defined.

Atomic Resolution Structures of Human Bufaviruses Determined by Cryo-Electron Microscopy

Bufavirus strain 1 (BuV1), a member of the Protoparvovirus genus of the Parvoviridae, was first isolated from fecal samples of children with acute diarrhea in Burkina Faso. Since this initial discovery, BuVs have been isolated in several countries, including Finland, the Netherlands, and Bhutan, in pediatric patients exhibiting similar symptoms. Towards their characterization, the structures of virus-like particles of BuV1, BuV2, and BuV3, the current known genotypes, have been determined by cryo-electron microscopy and image reconstruction to 2.84, 3.79, and 3.25 Å, respectively. The BuVs, 65-73% identical in amino acid sequence, conserve the major viral protein, VP2, structure and general capsid surface features of parvoviruses. These include a core β-barrel (βB-βI), α-helix A, and large surface loops inserted between these elements in VP2. The capsid contains depressions at the icosahedral 2-fold and around the 5-fold axes, and has three separated protrusions surrounding the 3-fold axes. Structure comparison among the BuVs and to available parvovirus structures revealed capsid surface variations and capsid 3-fold protrusions that depart from the single pinwheel arrangement of the animal protoparvoviruses. These structures provide a platform to begin the molecular characterization of these potentially pathogenic viruses.

Human Protoparvoviruses

Next-generation sequencing and metagenomics have revolutionized the discovery of novel viruses. In recent years, three novel protoparvoviruses have been discovered in fecal samples of humans: bufavirus (BuV) in 2012, tusavirus (TuV) in 2014, and cutavirus (CuV) in 2016. BuV has since been studied the most, disclosing three genotypes that also represent serotypes. Besides one nasal sample, BuV DNA has been found exclusively in diarrheal feces, but not in non-diarrheal feces, suggesting a causal relationship. According to both geno- and seroprevalences, BuV appears to be the most common of the three novel protoparvoviruses, whereas TuV DNA has been found in only a single fecal sample, with antibody detection being equally rare. Moreover, the TuV sequence is closer to those of non-human protoparvoviruses, and so the evidence of TuV being a human virus is thus far insufficient. Interestingly, besides in feces, CuV has also been detected in skin biopsies of patients with cutaneous T-cell lymphoma and a patient with melanoma, while all other skin samples have tested PCR negative. Even if preliminary disease associations exist, the full etiological roles of these viruses in human disease are yet to be resolved.