Parvovirus

Parvovirus is the common name applied to all the viruses in the Parvoviridae taxonomic family, although it can also be used specifically for members of one of the two Parvoviridae subfamilies, the Parvovirinae, which infect vertebrate hosts. Members of the second subfamily, the Densovirinae, which infect invertebrate hosts, are more commonly referred to as densoviruses. In subfamily Parvovirinae there are eight genera, containing a total of 58 recognized species, while in subfamily Densovirinae there are 5 genera and a total of 21 species. These viruses have small genomes, encoding just two genes, and must rely on the synthetic machinery of their host cell for their own preferential replication. This means that many parvoviruses require host cells to enter S-phase before viral DNA replication can initiate, but they do not encode any gene products that can drive this transition. Parvoviruses overcome this problem in various ways: viruses in many genera simply wait within the cell for it to enter S-phase under its own cell cycle control, which means that they can only infect actively-dividing cell populations. In contrast, the so-called adeno-associated viruses (AAVs) from genus Dependoparvovirus must wait until the cell is co-infected by a helper DNA virus, commonly an adenovirus or herpes virus, which does encode gene products that can drive the cell into S-phase, allowing AAV infection to initiate and out-compete the helper virus. A third strategy is used by human bocavirus 1 (HBoV1) from genus Bocaparvovirus, which appears to invoke a specific DNA-damage response in its host cell that ultimately supports viral DNA amplification and progeny virus production. Parvoviruses are linear, nonsegmented, single-stranded DNA viruses, with an average genome size of 5-6 kilo base pairs (kbp). They are classified as group II viruses in the Baltimore classification of viruses. Parvoviruses are among the smallest viruses (hence the name, from Latin parvus meaning small) and are 23–28 nm in diameter. Parvoviruses can infect and may cause disease in many animals, from arthropods such as insects and shrimp, to echinoderms such as starfish, and to mammals including humans. Because most of these viruses require actively dividing cells to replicate, the type of tissue infected varies with the age of the animal. The gastrointestinal tract and lymphatic system can be affected at any age, leading to vomiting, diarrhea, and immunosuppression, but cerebellar hypoplasia is only seen in cats that were infected with feline parvovirus (FPV) in the womb or at less than two weeks of age, and disease of the myocardium is seen in puppies infected with canine parvovirus 2 (CPV2) between the ages of three and eight weeks. Perhaps due to their extremely small size, the first parvoviruses were not discovered until the late 1950s. Parvovirus B19, the first known parvovirus to cause disease in humans, was discovered in London by Australian virologist Yvonne Cossart in 1974. Cossart and her group were focused on hepatitis B and were processing blood samples when they discovered a number of 'false positives' later identified as parvovirus B19. The virus is named for the patient code of one of the blood-bank samples involved in the discovery. The viral capsid of a parvovirus is made up of 60 copies of two or more size variants of a single protein sequence, called VP1, VP2 etc., which form a resilient structure with T=1 icosahedral symmetry. These virions are typically resistant to dilute acids, bases, solvents, and temperatures up to 50°C (122°F). Parvoviruses do not have envelopes, thus are considered 'naked' viruses. In addition, the shape of the virion is roughly spherical, with surface protrusions and canyons. Inside the capsid is a linear, single-stranded DNA genome in the size range 4–6 kbp, so the small genome of parvovirus can encode only a few proteins. At the 5’ and 3’ ends of this genome are short complementary sequences of roughly 120 to 550 nucleotides that form secondary structures as hairpins for example inverted terminal repeats (ITRs, which are two identical secondary structures at the termini) or unique sequences at the termini (two unique and different secondary structures are at each end of the DNA) and are essential for viral genome replication mechanism called rolling-hairpin replication. Parvoviruses that infect vertebrate hosts make up the subfamily Parvovirinae, while those that infect arthropods (currently only known to infect insects, crustacea and echinoderms) make up the subfamily Densovirinae. Prior to 2014, the name parvovirus was also applied to a genus within subfamily Parvovirinae, but this genus name has been amended to Protoparvovirus to avoid confusion between taxonomic levels. Many mammalian species sustain infection by multiple parvoviruses. Parvoviruses tend to be specific about the species of animal they will infect, but this is a somewhat flexible characteristic. Thus, all isolates of canine parvovirus affect dogs, wolves, and foxes, but only some of them will infect cats.