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.

Parvovirus B19 infections during pregnancy: clinical pictures in women
About 50-65% of pregnant women have protective antibodies against Parvo B19 (lifelong immunity). In non-immune pregnant women the highest risk of infection is during epidemics and following exposure to infected children in the home. Most pregnant women are asymptomatic, some may experience exanthema and arthralgia. In 33% of cases Parvo B19 can be transmitted transplacentally to the fetus during active maternal infection (33%).

Parvovirus B19 infections during pregnancy: consequences for the fetus
Parvo B19 infects and destroys the fetal erythroid precursor cells of the bone marrow which results in anemia. Anemia is an underlying factor in the development of non immune hydrops fetalis (NIHF), fetal ascites and can lead to intrauterine fetal death/fetal loss. On average, there is a 10% risk of congenital hydrops. It is manifested at birth by severe anemia, high-output cardiac failure and extramedullary hematopoiesis. Up to 10% of Parvo B19 infections during pregnancy are associated with fetal death/fetal loss. Intrauterine fetal death usually occurs before the 20th week of gestation, fetal loss normally in the 2nd trimester.

Diagnosis
In cases of a suspected Parvo B19 infection in pregnant women, immediately the immune status should be determined by detection of antibodies in maternal blood samples. If the results point to an primary infection (IgM antibodies) a control should be performed instantaneously. With close monitoring by ultrasound (every 8-10 days), indications of the development of hydrops fetalis can be detected. These indications should be confirmed by antibody detection in cord blood as well as the direct virus detection with PCR in fetal ascites, in cord blood or in chorionic villi.

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