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The Parvovirus: The Smallest of Them All

Timeline of Historical Events and Discoveries:

            Through a mix of chance events and a deliberate search for the causes of certain animal diseases, the parvoviruses were discovered. The diseases caused by the parvoviruses such as erythema infectiosum have been known since the beginning of the century. In 1959, Kilham and Olivier published the account of a latent virus in rates that had been isolated using tissue culture. This was the Rat Virus (RV) which later became known as a type species of the genus Parvovirus. Once the size and morphology of RV had been characterized, other parvoviruses in animals were identified by chance and cell culture experiments.
            It was not until 1975 during a screening of healthy blood donors for hepatitis B surface antigens that human parvovirus B19 was discovered. B19 alludes to the sample from which the parvovirus was first discovered. Initially, B19 was identified as the causative agent of erythema infectiosum, a common childhood rash found in outbreaks among schoolchildren during the winter and spring months. Soon after, virologists discovered that parvovirus B19 was linked to fifth disease. Since then, B19 has been shown to be the causative agent of many diseases. Please link to Clinical Manifestations.
            Finally, outbreaks of B19 occurred during January 1993 and 1994. Please refer to figure 1 below (

Figure 1: Report of Parvovirus B19


Viral Replication:

                The replication of the parvovirus is closely linked to cellular replication. The virus replicates in the nucleus, initiates replication only after the host cell has completed the S phase of the cell cycle, and uses cellular DNA polymerases. Studies have shown that either the alpha or gamma DNA polymerase is utilized.

General Features:
1) Parvovirus DNA replication is accounted for by a single-strand displacement model, similar to the one used to describe adenovirus DNA replication. There are no lagging, discontinuous strands involved in the DNA synthesis.
2) There are two stages of replication. Both stages use terminal DNA sequences as primers. Thus, RNA or protein primers or primases are not involved.
3) This model predicts site-specific cleavage of intermediates during the replication.

Specific Steps of Replication:

                 The model for parvovirus replication is shown in Figure 2 below. The numbers in parentheses correspond to the numbers in the figure.

Figure 2: Parvovirus DNA replication


Key: ABa. 3'-terminal palindrome of virion strand; FGf. 5'-terminal palindrome of virion strand; e. 18-26-nucleotide sequence present in replicative intermediates but not in DNA of nuclease-treated virions; T, site-specific nuclease; V, virion strand; Vpar, parental virion strand; Vprog, progeny virion strand; C, complementary strand; X, site of possible topoisomerase action. (Fields, Bernard. Virology, 2180, 3rd edition, Philadelphia; Lippincott-Raven, 1996.)

                 The DNA replication of the parvovirus can be thought of as a modified rolling hairpin. The ends of the viral genome are palindromic. In order to initiate DNA synthesis, the 3' end of the viral genome folds back on itself to become a primer (1). After some DNA synthesis along the complementary strand (2-5), the 3' terminus of the complementary strand folds into a hairpin structure to serve as a primer for further DNA replication (6). Replication continues along the complementary strand (7). Next, an obligatory dimer duplex replicative intermediate forms is formed. Within the palindrome regions lie small, unpaired sequences which serve as cleavage sites for the NS1 nuclease. The nuclease subsequently becomes covalently attached to the 5' side of the nick (8). DNA replication continues along the viral progeny strand (9-10). In the final steps (11-13), capsid protein recognizes the 3' hairpin structure and packages the single strands. As can be seen in step 13, about 18-26 DNA nucleotides at the 5' end, along with NS1, are found outside the capsid. These extra sequences will be cleaved off in the extracellular environment or upon entry into the host cell.

The "Star" Researchers of the Parvovirus Family:
Researcher Name Contribution (s)
Cossart, Y.  Discovered human parvovirus. She reacted sera from blood bank  donors with samples from hepatitis patients and noticed a series of false positive reactions, which when investigated further, turned out  to be parvoviruses.
Filatov, N.  Formulated classification of red rashes of childhood. Parvovirus is on Filatov's list because it causes Fifth Disease of Childhood.
Kilham and Olivier  Discovered the Rat Virus (RV) which became the type species of the genus Parvovirus.
Ward, D. Studies of minute-virus-of-mice.


Clinical Manifestations:

Table 1: Symptoms due to B19 Infection

                    Location or Time Period                               &nb sp;                                                     Clinical Symptoms Clinical Manifestation
Nerves neuralgic amyotrophy, plexus brachialis neuropathy
Pregnancy fetal anemia, hydrops fetalis non-immune, abortion, stillbirth, neonatal death, and B19 infections in utero
Skin erythema infectiosum (EI), fifth disease (for a description, link to

Connective tissue and joints 
acute arthropathy
Blood and vessels vaculitis, thromobocytopenia, hemophagocytic syndrome, transient aplastic crisis (TAC), petchial glove and sock syndrome, chronic infection in immunosuppressed individuals
Heart congestive heart failure, myocarditis, AV-block and Adams Stokes attacks, pericarditis


Further Notes on Clinical Manifestations:

Pregnancy: B19 infections in pregnant mothers are often overlooked because the disease is mainly asymptomatic. In some cases, infection may present with rash and joint complaints. For most pregnant women infected with B19, their child will be delivered normal. However, there is a significant chance of transplacental virus transmissions and fetal infection. After the mother experiences the onset of symptoms caused by B19 infection, she has the risk of losing her fetus up to 12 weeks after this initial onset. There is some evidence that B19 may produce teratogenic effects.

Skin: Please link to

Blood and Vessels: About 68-100% of TAC cases are attributable to B19 infection. In 20% of these cases, infection is subclinical. Few patients report a rash.

Heart: Heart complications due to B19 infection are rare. On a side note, B19 should be used as a differential diagnosis to borreolis in young and middle aged patients with acute AV-block. A picture of an AV-block is shown below in Figure 3. For further insight on differential diagnosis with B19, please link to

Figure 3. Image of AV-block due to B19 infection (



                B19 virus can be detected in both blood and respiratory secretions five to seven days before a rash is developed and before antibodies can be detected in serum. The virus targets erythroid progenitor cells (lantern cells) in the bone marrow and spleen, which, when infected, undergo lysis. There is a resulting decline in the erythrocyte count as well as in lymphocyte, granulocyte, and platelet counts.

                After the virus's incubation period of 10 days, fever, catarrhalia, and adenitis may start. Four days later, a rash may develop. The rash is believed to be an immune response because, simultaneously, B19-specific IgM antibodies can be detected in the host. B19-specific IgG antibodies can be detected a few days later. Also, the patient is no longer infectious via the normal transmission route a few days after the rash develops. Virus transmission via blood products is still possible. These persons without the blood group P antigen are naturally resistant to infection with B19 because it is its natural receptor. However, this occurrence is quite rare.


                Although parvovirus infection can be quite apparent clinically, the virus's inability to grow in standard cell culture systems has made widespread laboratory testing of B19 quite difficult. However, detection of B19 IgG and IgM allows for specific diagnosis of B19 infection. B19 infection can be detected by the presence of IgM antibodies 14 days after the virus first infects the individual. Right now, the most sensitive test to detect recent infection is the IgM-antibody assay. Through either enzyme immunoassay (EIA) or radio immunoassay (RIA), antibodies can be detected in about 90% of cases by the third day after the onset of symptoms. In addition, tests can also be done to test for the presence of viral DNA. Currently, the most sensitive test for detecting the virus is nucleic acid hybridization. These tests have suggested that B19 DNA was more likely to be present in leukocytes than in serum. Moreover, detection of B19 IgG antibodies in a patient's blood indicates protective immunity, which can last many years in a majority of cases.

For more information on parvoviruses, you can also check out the following web sites:

Chorba et al "The Role of Parvovirus B19 in Aplastic Crisis and Erythema Infectiosum (Fifth Disease)" Journal of Infectious Disease, 154, 383-93, 1986.
Gillespie et al "Occupational Risk of Human Parvovirus B19 Infection for School and Day-Care Personnel during an Outbreak of Erythema Infectiosum" JAMA, 263, 2061-5, 1990.
Siegl, Bates, Berns, Carter, Kelly, Kurstak, and Tattersall "Characteristics and Taxonomy of Parvoviridiae" Intervirology, 23: 61-73, 1985.
Thurn, Joseph "'Human Parvovirus B19: Historical and Clinical Review" Reviews of Infectious Diseases. Vol. 10, No. 5, 1005-1011, Sept-Oct 1998.
Webster and Granoff, Encyclopedia of Virology, Volume 3. Academic Press.
Weiland et al "Parvovirus B19 Associated with Fetal Abnormality." Lacnet, 1, 682-283, 1987.
For additional references, please refer to