Parvovirus Infections¶
Chapter 202 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- B19V is a small, nonenveloped, single-stranded DNA virus causing erythema infectiosum (fifth disease) and severe complications in immunocompromised hosts.
- Genotype 1 is globally predominant; genotypes 2 and 3 are rare and more common in Africa.
- B19V infects erythroid progenitors via receptors (globoside and VP1u), causing transient aplastic crisis (TAC) or chronic anemia (PRCA).
- Diagnosis relies on serology (IgM/IgG) and PCR, with IVIG as the mainstay for immunocompromised patients.
- Pregnancy-associated B19V infection can lead to hydrops fetalis, requiring intrauterine transfusions or IVIG.
1. DEFINITION & OVERVIEW¶
Parvoviruses are small, nonenveloped, single-stranded DNA viruses. B19V (parvovirus B19) is the most clinically significant human parvovirus, belonging to the genus Erythroparvovirus. It is divided into three genotypes (1, 2, 3), with genotype 1 predominant globally.
Table 202-1: Diseases Associated with Human Parvovirus B19 Infection and Methods of Diagnosis¶
| DISEASE | HOSTS | IgM | IgG | PCR | QUANTITATIVE PCR |
|---|---|---|---|---|---|
| Fifth disease | Healthy children | Positive | Positive | Positive | >10^4 IU/mL |
| Polyarthropathy syndrome | Healthy adults (more often women) | Positive within 3 months of onset | Positive | Positive | >10^4 IU/mL |
| Transient aplastic crisis | Patients with increased erythropoiesis | Negative/positiv e | Negative/positiv e | Positive | Often >10^12 IU/mL, but rapidly decreases |
| Persistent anemia/PRCA | Immunodeficient or immunosuppr essed patients | Negative/weakly positive | Negative/weakly positive | Positive | >10^6 IU/mL in absence of treatment |
| Hydrops fetalis/congenital anemia | Fetuses (of mothers infected <20 weeks) | Negative/positiv e | Positive | Positive (amniotic fluid/tissue) | n/a |
1.1 Classification¶
Parvoviruses infect diverse hosts, including humans. Human parvoviruses include B19V, adeno-associated viruses (AAVs), parvovirus 4 (parv4), human bocaviruses (HBoVs), and human protoparvoviruses (bufavirus and cutavirus).
1.2 Pathogenesis¶
B19V replicates in erythroid progenitors via receptors (globoside and VP1u). In immunocompetent hosts, infection is self-limiting, but in immunocompromised patients, it can cause TAC or PRCA. Viral load peaks at 10^4–10^12 particles/mL, with DNAemia persisting for months.
2. EPIDEMIOLOGY¶
B19V exclusively infects humans, with genotype 1 predominant globally. Genotype 2 is rare, and genotype 3 is more common in Africa. Outbreaks occur in temperate climates, with 50% infection rates in households/schools. Seroprevalence reaches 50% by age 15 and 80% in the elderly.
2.1 Risk Factors¶
Young children, pregnant women, immunocompromised patients (e.g., sickle cell disease, HIV, transplant recipients), and healthcare workers at risk. Transmission occurs via respiratory route, blood transfusions, and vertical transmission.
2.2 Demographics¶
Children are most commonly affected; adults may present with arthralgia. Fetal infection risk is ~30% during pregnancy, with 9% fetal loss if infected before 20 weeks of gestation.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
B19V replicates in erythroid progenitors, causing suppression of red cell production. In immunocompetent hosts, this is transient; in immunocompromised patients, it leads to TAC or PRCA. High viral load (up to 10^14 particles/mL) increases transmission risk.
3.1 Receptors¶
Primary receptor: globoside (blood group P antigen). Secondary receptor: VP1u. Individuals lacking globoside are naturally resistant.
3.2 Immune Response¶
Neutralizing antibodies (IgG) develop within 7 days. Avidity testing and epitope-specific assays differentiate acute vs. past infection.
4. CLINICAL FEATURES¶
Most infections are asymptomatic. Symptomatic cases present with erythema infectiosum (slapped-cheek rash), polyarthropathy, or aplastic crises. Complications include fetal hydrops, anemia, and hemophagocytic syndrome.
4.1 Erythema Infectiosum¶
Classic rash on face and extremities, with recurrent episodes. Adults may present with arthralgia without rash.
4.2 Polyarthropathy Syndrome¶
Symmetrical joint pain in adults, mimicking rheumatoid arthritis. Rheumatoid factor may be present.
4.3 Aplastic Crisis¶
Severe anemia with low reticulocyte count. Bone marrow shows absent erythroid precursors. Common in hemolytic disorders.
5. DIFFERENTIAL DIAGNOSIS¶
Other viral exanthems (e.g., rubella, measles), autoimmune disorders, and hemolytic anemias. Laboratory confirmation is needed for definitive diagnosis, especially in pregnant women.
6. INVESTIGATIONS & DIAGNOSIS¶
Serology (IgM/IgG), PCR for DNAemia, and bone marrow biopsy for aplastic crisis. Quantitative PCR (qPCR) detects viral load (IU/mL).
6.1 Diagnostic Criteria¶
IgM positivity for acute infection; IgG positivity for immunity. qPCR confirms active infection (threshold >10^4 IU/mL).
6.2 Imaging¶
Ultrasound for fetal hydrops. MRI/CT for CNS complications.
7. MANAGEMENT & TREATMENT¶
No antivirals approved. Treatment includes IVIG (400 mg/kg daily for 5–10 days) for immunocompromised patients. Blood transfusions for TAC. Fetal hydrops may require intrauterine transfusions.
7.1 Pharmacologic¶
IVIG is the mainstay for PRCA/TAC. Cidofovir/brincidofovir may inhibit replication in vitro.
7.2 Non-Pharmacologic¶
Supportive care, monitoring, and avoiding blood transfusions in asymptomatic patients.
8. PROGNOSIS & COMPLICATIONS¶
Most cases resolve spontaneously. Complications include severe anemia, fetal loss, and chronic anemia in immunocompromised patients. Long-term DNAemia may persist for years.
8.1 Fetal Complications¶
Hydrops fetalis (30% risk) and congenital anemia. Intrauterine transfusions may prevent fetal loss.
8.2 Chronic Anemia¶
PRCA in immunocompromised patients requires prolonged IVIG therapy.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Monitor for hydrops fetalis. Pediatrics: Common in children with hemolytic disorders. Elderly: Higher seroprevalence but milder disease. Immunocompromised: Risk of chronic anemia.
9.1 Pregnancy¶
B19V infection may cause fetal loss or hydrops. IVIG reduces transmission risk to fetus.
9.2 Immunocompromised¶
High-risk for TAC/PRCA. IVIG is the primary treatment. Avoid blood transfusions unless necessary.
10. KEY POINTS & CLINICAL PEARLS¶
B19V is a leading cause of aplastic crisis in hemolytic disorders. IVIG is the mainstay for immunocompromised patients. Fetal infection risk is highest in early pregnancy. PCR confirms active infection, while serology identifies immunity. Avidity testing differentiates acute vs. past infection.