Parvoviruses are the smallest DNA animal viruses. Because of the limited coding capacity of their genome, viral replication is dependent on functions supplied by replicating host cells or by coinfecting helper viruses. Parvovirus B19 is pathogenic for humans and has a tropism for erythroid progenitor cells. It is the cause of erythema infectiosum (“fifth disease”), a common childhood exanthem; of a polyarthralgia-arthritis syndrome in normal adults; of aplastic crisis in patients with hemolytic disorders; of chronic anemia in immunocompromised individuals; and of fetal death. The human bocaviruses have been detected in respiratory specimens from children with acute respiratory disease and in stool samples, but a role in disease is unproven.
PROPERTIES OF PARVOVIRUSES
Important properties of parvoviruses are listed in Table 31-1. It is noteworthy that there are both autonomously replicating and defective parvoviruses.
TABLE 31-1Important Properties of Parvoviruses ||Download (.pdf) TABLE 31-1 Important Properties of Parvoviruses
|Virion: Icosahedral, 18–26 nm in diameter, 32 capsomeres |
|Composition: DNA (20%), protein (80%) |
|Genome: Single-stranded DNA, linear, 5.6 kb, MW 1.5–2.0 million |
|Proteins: One major (VP2) and one minor (VP1) |
|Envelope: None |
|Replication: Nucleus, dependent on functions of dividing host cells |
|Outstanding characteristics: |
| Very simple viruses |
| Human pathogen, B19, has tropism for red blood cell progenitors |
| One genus contains viruses that are replication-defective and require a helper virus |
Structure and Composition
The icosahedral, nonenveloped particles are 18–26 nm in diameter (Figure 31-1). The particles have a molecular weight of 5.5–6.2 × 106 and a heavy buoyant density of 1.39–1.42 g/cm3. Virions are extremely resistant to inactivation. They are stable between a pH of 3 and 9 and withstand heating at 56°C for 60 minutes, but they can be inactivated by formalin, β-propiolactone, and oxidizing agents.
Electron micrograph of parvovirus particles. (Courtesy of FA Murphy and EL Palmer.)
Virions contain two coat proteins that are encoded by an overlapping, in-frame DNA sequence, so that VP2 is identical in sequence to the carboxy portion of VP1. The major capsid protein, VP2, represents about 90% of virion protein. The genome is about 5 kb, linear, single-stranded DNA. Whereas an autonomous virus, B19, contains 5596 nucleotides, a defective parvovirus, AAV-2, contains 4680 bases. Autonomous parvoviruses usually encapsidate primarily DNA strands complementary to viral mRNA; defective viruses tend to encapsidate DNA strands of both polarities with equal frequency into separate virions.
There are two subfamilies of Parvoviridae: the Parvovirinae, which infect vertebrates, and the Densovirinae, which infect insects. The Parvovirinae comprise five genera. Human parvovirus B19 is the most common member of the Erythrovirus genus. There are three human genotypes in this genus. The ...