Meet the Parvoviridae

In this step we are going to focus on the parvovirus, which are one of the most common causes of disease amongst pets, but also in other species. We will learn which are the most important members of the family, their strategy of replication, the morphology of the viral particle and its genome.

The family Parvoviridae is divided into two subfamilies: Densovirinae and Parvovirinae, which infect insects and mammals, respectively. The subfamily Parvovirinae is classified into the five genera that you see. Genus Parvovirus contains the members that we will be studying in ensuing steps, which affect dogs, swine and cats. Bocavirus is an acronym for bovine parvovirus and canine minute virus. Human bocavirus has been involved in respiratory and diarrhoeal syndromes. Dependovirus are the smallest parvovirus, and require or depend on the presence of other viruses for replication.

Amdovirus is also an acronym: for Aleutian Mink Disease. Finally, Erythroparvovirus contains an important pathogen of humans, parvovirus B19, which produces the so-called “fifth disease” in children, a type of erythema. Parvovirus are non-enveloped viruses, which means that they are going to be very resistant. They are small, around 18 to 26 nm in diameter, with an icosahedral capsid formed by 60 copies of VP2.

The genome is a single strand of DNA of about 4 to 6 kb in size, which encodes only four proteins: NS1, NS2, VP1 and VP2. We have already mentioned in a previous activity that this genomic simplicity implies that parvovirus needs the cell to be actively dividing, even more so than other viruses. The virus enters the cell by endocytosis and escapes the endosome when its membrane permeabilizes. The virion is transported towards the nucleus by cell microtubules and the DNA genome penetrates into the nucleus. The single-stranded DNA genome is converted into double-stranded DNA by a DNA polymerase which is present only in dividing cells. It is transcribed to viral messenger RNAs and translated to produce viral proteins.

The genome replicates, originating many molecules of single-stranded DNA, which can either be converted to double-stranded DNA and serve as a template for transcription and replication, repeating the cycle, or be introduced into the capsids assembled with newly formed proteins. Finally, when there are many new virions, they are released by cell lysis. From this step it is important to remember that parvovirus are very small and that they require actively dividing cells in order to replicate. As they are non-enveloped, they are released by cell lysis, destroying the lining of the intestine. This is the mechanism by which they produce diarrhoea.