Epidemiology and transmission
Parvovirus B19 (Parvo B19), a member of the parvovirus family, Parvoviridae, is obligatory human pathogenic. It is of worldwide distribution in both industrialized and developing countries. In regions with moderate climate infection appears to be most common between late winter and early summer time. Epidemics occur with a periodicity of about 4-5 years.
As Parvo B19 has been found in respiratory secretions at the time of viremia, respiratory spread appears to be the most common route of transmission. The virus seems to be readily horizontally transmitted through close personal contact via droplet infection but also through contaminated hands. Transmission by transfusion of blood products may be a critical event in certain patient groups (for further information please refer to the section Transfusion Medicine).

Prevalence
The prevalence of Parvo B19 infections is age-dependent: In industrialized countries about 2-10% of children have had primary Parvo B19 infections by the age of 5 years increasing to 50% by the age of 15 years and 60% by the age of 30 years. In adults older than 60 years more than 85% have antibodies to Parvo B19. Investigations in blood donors revealed an antibody prevalence of about 60%. The immunity following Parvo B19 infections seems to be lifelong.

Incidence
The annual seroconversion rates in susceptible (nonimmune) adults differ during endemic and epidemic periods. During endemic periods incidence rates in industrialized countries are about 0.65-1.5% per year. In pregnant women it has been estimated that maternal Parvo B19 infection occurs in approximately 1 in every 400 pregnancies. During epidemic periods regionally confined incidence rates of 10-15% and up to 30% at the peak of the outbreak have been observed between February and June.

Primary Parvo B19 infections and clinical pictures
The disease manifestations of Parvo B19 infections vary widely depending on the haematological and/or immunological status of the host.
In immunocompetent children and adults about 30% of the infections are asymptomatic or manifest as erythema infectiosum (EI) also known as fifth disease or slapped-cheek syndrome. It is characterized by a distinctive facial rash that may be preceded by mild nonspecific, systemic symptoms, such as fever, malaise, myalgias, and headache. The rash may spread from the face to the arms, legs and body. Arthralgia and acute symmetric arthritis occur in less than 10% of infected children but are common among adults, especially women (for further information please refer to Rheumatology in this section). In about 33% of cases Parvo B19 can be transmitted transplacentally to the fetus during active maternal infection (for further information please refer to the section Gynecology).
Due to the high tropism of B19 to infect and destroy erythroid progenitor cells, patients with various haemolytic disorders show a high tendency to develop transient aplastic anemia (transient aplastic crisis = TAC) caused by a temporary halt in new erythrocyte production.
Rare clinical manifestations may include thrombocytopenia, acute glomerulonephritis, myo-/pericarditis, encephalitis, meningitis and, mainly in young children, hepatitis.

Chronic Parvo B19 infections
Patients who are immunocompromised, e.g. receiving either chemotherapy or immunosuppressive drugs or have immune defects (congenital and acquired) may develop chronic parvovirus B19 infection that results in chronic anemia. Pure red cell aplasia (PRCA) persists until the virus is cleared and should be distinguished from the transient anemia described above.
In transplant recipients chronic parvovirus B19 infection has been linked to anemia, other hematologic abnormalities, myocarditis, and pneumonitis.
Pediatric patients with hematologic malignancies and parvovirus B19 infection may suffer from prolonged anemia that interferes with chemotherapy timing.
Parvo B19 may trigger the outbreak of juvenile chronic arthritis in genetically predisponed children as well as rheumatoid arthritis (for further information please refer to Rheumatology in this section).
General rare clinical manifestations may include chronic myocarditis and dilatative cardiomyopathy.

Persistence and reactivation
After primary infection, genomic DNA of B19 has been shown to persist in solid tissues (synovial membranes, liver, bone marrow) of not only symptomatic but also of constitutionally healthy, immunocompetent individuals.

Diagnosis
The diagnosis of erythema infectiosum is normally made clinically. However, due to the various other clinical pictures mentioned above differential diagnosis will be required in order to distinguish from other viral infections, rheumatic diseases and various unclear organ manifestations. In immunocompetent patients serologic antibody detection is considered method of choice.
In immunocompromised patients who may not be able to mount an immune response, PCR in blood, bone marrow or synovial tissue may be helpful.
In immunocompetent patients PCR is of limited diagnostic value as Parvo B19 DNA may be detectable for extended periods of time in serum, synovial membranes and in bone marrow even in healthy individuals. Therefore, the presence of low levels of B19 DNA alone cannot be used to diagnose acute B19 infection.
In patients with anemia the finding of pronormoblasts on bone marrow is suggestive of parvovirus B19 infection.

For information on indications for the diagnosis with the medac Parvovirus B19 serology please refer to Parvo B19 serology in this section.