What is a Parvovirus?

Parvoviridae is a name for a viral family that encompasses small, non-enveloped, isometric DNA viruses with linear, single-stranded genomes. Viruses from this family are commonly known as parvoviruses, with two subfamilies and a dozen of distinct genera. In the known biosphere, only parvoviruses have DNA genomes that are both single-stranded and linear.

The parvoviruses infect both humans and animals. They can be defined by their size, hence the name which comes from the Latin word for small (i.e. “parvum”). Their capsids are small, about 25 nm in diameter, and a single genomic molecule is between 4 and 6 kilobases in length, terminating in short palindromic sequences that create duplex hairpin telomeres.

Classification of parvoviruses

The classification of parvoviruses primarily relies on morphology and functional characteristics, although genetic factors come into play as well. As of 2015, the family Parvoviridae includes 134 viruses that have a possibility to infect a broad range of hosts. This family is further divided in two subfamilies – Parvovirinae and Densovirinae – members of which infect vertebrates and arthropods, respectively.

Using DNA sequence-based phylogenetic analysis, members of the subfamily Parvovirinaecan be further divided into eight genera: Amdoparvovirus, Aveparvovirus, Bocaparvovirus, Copiparvovirus, Dependoparvovirus, Erythroparvovirus, Protoparvovirus and Tetraparvovirus.

Three distinct genotypes can be found within the humanotropic Erythroparvovirus genus. Parvovirus B19 represents the predominant parvovirus pathogen in humans and the prototype genotype 1 strain. Two related viral genotypes (so-called LaLi/A6 and V9) are less commonly found and appear to be immunologically indistinguishable from Parvovirus B19.

Subfamily Densovirinae which infects invertebrates comprises of five genera: Ambidensovirus, Brevidensovirus, Hepandensovirus, Iteradensovirus and Penstyldensovirus. Interest in this group is based on the medical and economic importance, as these viruses are widely distributed among one million insect species (blood-sucking mosquitoes among others).

Phylogenetic analysis and identification

Although phylogenetic analysis is taking a pivotal role in taxonomic classification, its reliability is dubious when applied at the species level for viruses in general, and parvoviruses in particular. The reason is that phylogenetic approach principally depends upon the assumption that genetic information is only transmitted vertically over generation, which is not always true for viruses due to frequent reassortment of genetic information by horizontal mechanisms.

Therefore parvovirus serotype has been used as a major criterion for taxonomic species demarcation in the Parvoviridae family due to its exceptional stability. Despite the difference in capsid serology, the non-structural proteins of most species within a single genus reveal substantial antigenic cross-reactivity, so that genus-specific infections can be diagnosed serologically.

Diagnosis of infections with parvoviruses relies on serologic and DNA tests, as propagating the virus in standard tissue culture is often cumbersome. With the continual improvement of techniques used to identify these viruses and proper epidemiologic intelligence, it is possible to remain just far enough ahead of them to prevent a widespread outbreak – both in humans and animal species.