B19V is the type member of the genus Erythrovirus. On the basis of viral sequence, B19V is divided into three genotypes (designated 1, 2, and 3), but only a single B19V antigenic type has been described. Genotype 1 is predominant in most parts of the world; genotype 2 is rarely associated with active infection; and genotype 3 appears to predominate in parts of western Africa.
B19V exclusively infects humans, and infection is endemic in virtually all parts of the world. Transmission occurs predominantly via the respiratory route and is followed by the onset of rash and arthralgia. By the age of 15 years, ∼ 50% of children have detectable IgG; this figure rises to >90% among the elderly. In pregnant women, the estimated annual seroconversion rate is ∼1%. Within households, secondary infection rates approach 50%.
Detection of high-titer B19V in blood is not unusual (see “Pathogenesis,” below). Transmission can occur as a result of transfusion, most commonly of pooled components. To reduce the risk of transmission, plasma pools are screened by nucleic acid amplification technology, and high-titer pools are discarded. B19V is resistant to both heat and solvent-detergent inactivation.
B19V replicates primarily in erythroid progenitors. This specificity is due in part to the limited tissue distribution of the primary B19V receptor, blood group P antigen (globoside). Infection leads to high-titer viremia, with >1012 virus particles (or IU)/mL detectable in the blood at the apex (Fig. 184-1), and virus-induced cytotoxicity results in cessation of red cell production. In immunocompetent individuals, viremia and arrest of erythropoiesis are transient and resolve as the IgM and IgG antibody response is mounted. In individuals with normal erythropoiesis, there is only a minimal drop in hemoglobin levels; however, in those with increased erythropoiesis (especially with hemolytic anemia), this cessation of red cell production can induce a transient crisis with severe anemia (Fig. 184-1). Similarly, if an individual (or, after maternal infection, a fetus) does not mount a neutralizing antibody response and halt the lytic infection, erythroid production is compromised and chronic anemia develops (Fig. 184-1).
Schematic of the time course of parvovirus B19V infection in (A) normals (erythema infectiosum), (B) transient aplastic crisis (TAC), and (C) chronic anemia/pure red-cell aplasia (PRCA). (Reprinted with permission from Young and Brown, 2004. © 2004 Massachusetts Medical Society. All rights reserved.)
The immune-mediated phase of illness, which begins 2–3 weeks after infection as the IgM response peaks, manifests as the rash of fifth disease together with arthralgia and/or frank arthritis. Low-level B19V DNA can be detected by polymerase chain reaction (PCR) in blood and tissues for months to years after acute infection. The B19V receptor is found in a variety of other cells and tissues, including megakaryocytes, endothelial cells, placenta, myocardium, and liver. Infection of these tissues by B19V may be responsible for some of the more unusual presentations of the infection. Rare individuals who lack P antigen are naturally resistant to B19V infection.
Most B19V infections are asymptomatic or are associated with only a mild nonspecific illness. The main manifestation of symptomatic B19V infection is erythema infectiosum, also known as fifth disease or slapped-cheek disease (Fig. 184-2). Infection begins with a minor febrile prodrome ∼7–10 days after exposure, and the classic facial rash develops several days later; after 2–3 days, the erythematous macular rash may spread to the extremities in a lacy reticular pattern. However, its intensity and distribution vary, and B19V-induced rash is difficult to distinguish from other viral exanthems. Adults typically do not exhibit the “slapped-cheek” phenomenon but present with arthralgia, with or without the macular rash.
Young child with erythema infectiosum, or fifth disease, showing typical "slapped-cheek" appearance.
Although uncommon among children, arthropathy occurs in ∼50% of adults and is more common among women than among men. The distribution of the affected joints is often symmetrical, with arthralgia affecting the small joints of the hands and occasionally the ankles, knees, and wrists. Resolution usually occurs within a few weeks, but recurring symptoms can continue for months. The illness may mimic rheumatoid arthritis, and rheumatoid factor can often be detected in serum. B19V infection may trigger rheumatoid disease in some patients and has been associated with juvenile idiopathic arthritis.
Transient Aplastic Crisis
Asymptomatic transient reticulocytopenia occurs in most individuals with B19V infection. However, in patients who depend on continual rapid production of red cells, infection can cause transient aplastic crisis (TAC). Affected individuals include those with hemolytic disorders, hemoglobinopathies, red cell enzymopathies, and autoimmune hemolytic anemias. Patients present with symptoms of severe anemia (sometimes life-threatening) and a low reticulocyte count, and bone marrow examination reveals an absence of erythroid precursors and characteristic giant pronormoblasts. As its name indicates, the illness is transient, and anemia resolves with the cessation of cytopathic infection in the erythroid progenitors.
Pure Red-Cell Aplasia/Chronic Anemia
Chronic B19V infection has been reported in a wide range of immunosuppressed patients, including those with congenital immunodeficiency, AIDS (Chap. 189), lymphoproliferative disorders (especially acute lymphocytic leukemia), and transplantation (Chap. 132). Patients have persistent anemia with reticulocytopenia, absent or low levels of B19V IgG, high titers of B19V DNA in serum, and—in many cases—scattered giant pronormoblasts in bone marrow. Rarely, nonerythroid hematologic lineages are also affected. Transient neutropenia, lymphopenia, and thrombocytopenia (including idiopathic thrombocytopenic purpura) have been observed. B19V occasionally causes a hemophagocytic syndrome.
A recent study in Papua New Guinea, where malaria is endemic, suggested that B19V infection plays a major role in the development of severe anemia. Further studies must determine whether B19V infection contributes to severe anemia in other malarial regions.
B19V infection during pregnancy can lead to hydrops fetalis and/or fetal loss. The risk of transplacental fetal infection is ∼30%, and the risk of fetal loss (predominantly early in the second trimester) is ∼9%. The risk of congenital infection is <1%. Although B19V does not appear to be teratogenic, anecdotal cases of eye damage and central nervous system (CNS) abnormalities have been reported. Cases of congenital anemia have also been described. B19V probably causes 10–20% of all cases of nonimmune hydrops.
B19V infection may rarely cause hepatitis, vasculitis, myocarditis, glomerulosclerosis, or meningitis. A variety of other cardiac manifestations, CNS diseases, and autoimmune infections have also been reported. However, B19V DNA can be detected by PCR for years in many tissues; this finding is of no known clinical significance, but its interpretation may cause confusion regarding B19V disease association.
Diagnosis of B19V infection in immunocompetent individuals is generally based on detection of B19V IgM antibodies (Table 184-1). IgM can be detected at the time of rash in erythema infectiosum and by the third day of TAC in patients with hematologic disorders; these antibodies remain detectable for ∼3 months. B19V IgG is detectable by the seventh day of illness and persists throughout life. Detection of B19V DNA should be used for the diagnosis of early TAC or chronic anemia. Although B19V levels fall rapidly with the development of the immune response, DNA can be detectable by PCR for months or even years after infection, even in healthy individuals; therefore, quantitative PCR should be used. In acute infection at the height of viremia, >1012 B19V DNA IU/mL of serum can be detected; however, titers fall rapidly within 2 days. Patients with aplastic crisis or B19V-induced chronic anemia generally have >105 B19V DNA IU/mL.
Table 184-1 Diseases Associated With Human Parvovirus B19 Infection and Methods of Diagnosis |Favorite Table|Download (.pdf)
Table 184-1 Diseases Associated With Human Parvovirus B19 Infection and Methods of Diagnosis
|Fifth disease||Healthy children||Positive||Positive||Positive||>103 IU/mL|
|Polyarthropathy syndrome||Healthy adults (more often women)||Positive within 3 months of onset||Positive||Positive||>103 IU/mL|
|Transient aplastic crisis||Patients with increased erythropoiesis||Negative/positive||Negative/positive||Positive||Often >1012 IU/mL, but rapidly decreases|
|Persistent anemia/pure red-cell aplasia||Immunodeficient or immunocompetent patients||Negative/weakly positive||Negative/weakly positive||Positive||Often >1012 IU/mL, but should be >106 in the absence of treatment|
|Hydrops fetalis/congenital anemia||Fetus (<20 weeks)||Negative/positive||Positive||Positive amniotic fluid or tissue||n/a|
Treatment: Parvovirus B19 Infection
No antiviral drug effective against B19V is available, and treatment of B19V infection often targets symptoms only. TAC precipitated by B19V infection frequently necessitates symptom-based treatment with blood transfusions. In patients receiving chemotherapy, temporary cessation of treatment may result in an immune response and resolution. If this approach is unsuccessful or not applicable, commercial immune globulin (IVIg; Gammagard, Sandoglobulin) from healthy blood donors can cure or ameliorate persistent B19V infection in immunosuppressed patients. Generally, the dose used is 400 mg/kg daily for 5–10 days. Like patients with TAC, immunosuppressed patients with persistent B19V infection should be considered infectious. Administration of IVIg is not beneficial for erythema infectiosum or B19V-associated polyarthropathy. Intrauterine blood transfusion can prevent fetal loss in some cases of fetal hydrops.
No vaccine has been approved for the prevention of B19V infection. A vaccine based on virus-like particles expressed in insect cells is under development; the results of phase 1 trials were promising.