Protoparvovirus

Taxonomy

Group: ssDNA

Five species are currently recognized, most containing several named viruses, virus strains, genotypes or serotypes. When applied to viruses, the definition of species is a little unusual. It is simply an abstract taxonomic concept that clusters a selected range of genetic variants, helping to distinguish branches in a phylogenetic lineage, but it is not a physical entity like a virus that can infect an animal or be isolated. If the diversity level used to define a species is set very low, many will effectively contain a single virus, and the virus and species may even be given the same name, resulting in confusion between the two concepts in the literature, and marginalizing the phylogenetic role of the species taxon. To counter this problem, the diversity level now recognized for species in the Parvoviridae is relatively broad: species are defined as a cluster of similar viruses that encode a particular replication protein, typically called NS1, that is at least 85% identical to the protein encoded by other members of the species, as discussed in and.

Recognized species in genus Protoparvovirus include:

A new virus in this genus was recently discovered in the feces of children from Burkina Faso, and named using the siglum Bufavirus. Three genotypes of bufaviruses have so far been detected, circulating in Tunisia, Finland and Bhutan

Another candidate virus in this group—Tusavirus 1—has been reported in the feces of a single human, but whether or not it is able to infect humans or was simply ingested remains to be clarified.

Another set of viruses—provisionally termed cutaviruses—has been isolated from the feces of children with diarrhea. These appear to be related to the bufoviruses.

Structure

Viruses in genus Protoparvovirus have non-enveloped protein capsids around 18–26 nm in diameter, which show T=1 icosahedral symmetry. Genomes are single-stranded linear DNA between 4–6kb in length, with small (100–500b) imperfect palindromic sequences at each terminus that fold to form distinctive duplex hairpin telomeres.

Life Cycle

Viral replication is nuclear. Entry into the host cell is achieved by attachment to host receptors, which mediates clathrin-mediated endocytosis. Replication follows the rolling-hairpin model. In some virus/host cell combinations, progeny virions can be trafficked through the cytoplasm in vesicles and released from the parental host cell prior to cell death, while the remaining virions are released following cell lysis. Vertebrates from 4 orders are currently known to serve as natural hosts. Transmission routes are fecal-oral and/or respiratory.

History

Kilham rat virus, isolated in 1959, was the first member of this family of small, linear, single-stranded DNA viruses to be identified.

In following years a series of physically similar viruses, including H1, LuIII, minute virus of mice and tumor virus X, were extracted from cells or tissues in routine use in research laboratories and porcine parvovirus, PPV, one of the major causes of reproductive failure in swine, was isolated from infected pigs. In 1971 these viruses were all recognized as part of a taxonomic genus called Parvovirus in the First Report of the newly created International Committee on Taxonomy of Viruses (ICTV)

The Second ICTV Report, published in 1976, established the family Parvoviridae, which at that time included three genera, one of which retained the name Parvovirus and contained all of the aforementioned viruses plus feline panleukopenia virus (now called feline parvovirus, abbreviated to FPV), which had been shown to cause epidemics of enteritis, panleukopenia and congenital cerebellar ataxia in domestic cats. In 1978 a virus from the same species as FPV emerged that was able to infect dogs (called canine parvovirus or CPV), which rapidly spread globally, causing pandemics of severe intestinal and coronary disease.

Genus Parvovirus continued to accrue new viruses until 2014, when its name was changed to Protoparvovirus.