Erythema Infectiosum ("fifth disease")

Summary / Overview

Parvovirus B19 causes erythema infectiosum, a mild childhood exanthem characterized by the “slapped-cheek” rash.
It infects erythroid precursor cells → temporary suppression of red-cell production, significant in hemolytic disorders.
Usually self-limiting in children, but can cause aplastic crisis, fetal hydrops, or arthritis in adults.

Etiology

Parvovirus B19 is a single-stranded DNA virus belonging to the Parvoviridae family.
Transmission occurs via respiratory droplets, vertical (mother → fetus), and blood products.
Virus has a strong tropism for erythroid progenitor cells in the bone marrow → transient red cell aplasia.
Infectivity is highest *before* rash onset when viremia peaks.

Pathogenesis

Parvovirus B19 targets erythroid precursor cells in the bone marrow via the P-antigen receptor.
After respiratory entry → virus replicates in the nasopharynx → viremia develops.
Mass destruction of erythroid precursors → transient arrest of erythropoiesis (RBC production stops for 7–10 days).
Immune complex formation (virus + IgM) causes the characteristic “slapped-cheek” rash and arthralgia.
During pregnancy → infection can cause fetal anemia → hydrops fetalis due to high-output cardiac failure.

Symptoms

Bright red “slapped-cheek” rash
Lacy (reticular) rash on arms and trunk
Persistent anemia in immunocompromised

Signs

Lacy, reticular rash on limbs and trunk
No significant pharyngitis or conjunctivitis

Clinical Features

Biphasic illness pattern
Initial phase (non-specific):
Rash phase:
Arthropathy (common in adults):
Hematologic features:
Pregnancy-specific features:
Immunocompromised hosts:

Investigations

Diagnosis usually clinical during classic rash phase
Serology:
PCR for Parvovirus B19 DNA:
CBC:
Reticulocyte count:
Ultrasound (in pregnancy):
Bone marrow (rare):

Differential Diagnosis

Rubella (German measles)
Measles
Roseola (HHV-6/7)
Scarlet fever
Dengue fever rash
Drug-induced exanthem
Systemic juvenile idiopathic arthritis (Still's disease)
Lupus flare or viral-type malar rash
Aplastic anemia of other causes

Complications

Transient aplastic crisis (TAC)
Chronic red-cell aplasia
Hydrops fetalis (in pregnancy)
Fetal loss or miscarriage
Arthropathy
Myocarditis
Neurologic complications (rare)
Hepatitis or hepatic dysfunction
Vasculitic or purpuric eruptions

Treatment

No specific antiviral therapy required in most cases
Supportive care
Arthropathy management
IVIG for immunocompromised with chronic anemia
Blood transfusion for severe anemia
Fetal management during pregnancy
Isolation not required once rash appears
Avoid exposure during pregnancy (preventive advice)

Prevention

No vaccine available
Good hand hygiene reduces transmission
Avoid close contact during the viremic phase
Pregnancy precautions
Infection control in schools & healthcare
Protection of high-risk groups
Environmental cleaning

Serotypes / Subtypes

Only one major human pathogenic serotype (B19)
Genotypes show minor molecular variation
No clinically relevant subtype differences
Stable virus with low antigenic drift

Pathology

Selective tropism for erythroid precursor cells
Transient red cell aplasia
Characteristic “giant pronormoblasts”
Immune complex–mediated rash & arthralgia

Radiology / Imaging

Usually normal chest radiograph in erythema infectiosum
Key modality for detecting fetal complications

Notes / Teaching points

Why does Parvovirus B19 cause sudden severe anemia?
Why is transient aplastic crisis seen only in patients with chronic hemolytic anemias?
Why does the classic “slapped-cheek rash” occur?
Why do adults with B19 get arthralgia instead of rash?
Why can Parvovirus B19 infect the fetus but not cause structural malformations?
Why is fetal infection dangerous in the 2nd trimester?
Why does Parvovirus not cause chronic hemolysis?
Why are immunocompromised patients at risk for chronic infection?
Why do some patients show only mild flu-like symptoms?
Why doesn’t Parvovirus B19 commonly cause respiratory symptoms?

Biphasic illness pattern

1) Initial viremic phase

2) Immune-mediated rash/arthropathy phase

Initial phase (non-specific):

Mild fever

Headache

Myalgia

Malaise

Sore throat

Sometimes mild cough or rhinorrhea

Rash phase:

Slapped-cheek facial erythema

Lacy, reticular maculopapular rash on trunk and limbs

Rash may recur with heat, sunlight, exercise, or stress

Arthropathy (common in adults):

Symmetric pain and stiffness in small joints (hands, wrists)

Knees and ankles may be involved

Usually resolves in weeks

Hematologic features:

Transient aplastic crisis in patients with hemolytic disorders

Worsening anemia in pregnancy or immunocompromised patients

Pregnancy-specific features:

Risk of fetal hydrops due to severe fetal anemia

Possible fetal loss in early pregnancy

Immunocompromised hosts:

Prolonged anemia

Chronic infection with persistent viremia

Transient aplastic crisis (TAC)

Occurs in patients with increased red-cell turnover (sickle cell disease, thalassemia, hereditary spherocytosis). Sudden drop in Hb + reticulocytopenia.

Chronic red-cell aplasia

Seen in immunocompromised patients (HIV, transplant, leukemia). Persistent anemia due to inability to clear virus.

Hydrops fetalis (in pregnancy)

Maternal infection → fetal anemia → high-output cardiac failure → hydrops. Risk highest in 1st and 2nd trimester.

Fetal loss or miscarriage

If severe fetal anemia develops; usually between 17–24 weeks gestation.

Arthropathy

Immune-mediated symmetric small-joint pain/swelling; common in adult women; usually self-limited.

Myocarditis

Rare but reported; can cause arrhythmia or heart failure.

Neurologic complications (rare)

Encephalitis, meningitis, peripheral neuropathy—immune-mediated.

Hepatitis or hepatic dysfunction

Mild transient elevation of transaminases in some cases.

Vasculitic or purpuric eruptions

Including papular-purpuric glove-and-sock syndrome (PPGSS).

Rubella (German measles)

Postauricular lymphadenopathy, mild fever, faster-spreading rash; IgM/IgG serology differentiates.

Measles

High fever, cough–coryza–conjunctivitis, Koplik spots, confluent rash spreading cephalocaudally.

Roseola (HHV-6/7)

High fever for 3–4 days → fever suddenly resolves → rash appears afterward.

Scarlet fever

Sandpaper rash, strawberry tongue, Pastia lines; throat swab positive for Group A Streptococcus.

Dengue fever rash

Generalized erythematous rash or “islands of white in a sea of red”; thrombocytopenia and severe myalgia.

Drug-induced exanthem

History of new medication, pruritic morbilliform rash; no reticulocytopenia.

Systemic juvenile idiopathic arthritis (Still's disease)

Evanescent salmon-pink rash + fever spikes; arthritis prominent.

Lupus flare or viral-type malar rash

Photosensitive malar rash sparing nasolabial folds; ANA/anti-dsDNA assist diagnosis.

Aplastic anemia of other causes

Medication-induced, autoimmune, or bone-marrow failure syndromes; marrow study differentiates.

Diagnosis usually clinical during classic rash phase

Serology:

IgM antibodies → indicates recent infection

IgG antibodies → indicates past infection or immunity

Useful in pregnancy, atypical cases, or immunocompromised patients

PCR for Parvovirus B19 DNA:

Highly sensitive

Detects viremia

Indicated in: pregnancy, aplastic crisis, immunocompromised patients, ambiguous serology

CBC:

Mild anemia

Reticulocytopenia (important clue)

Possible neutropenia or thrombocytopenia in some cases

Reticulocyte count:

Low retic count confirms temporary marrow suppression

Ultrasound (in pregnancy):

Evaluate fetus for hydrops fetalis

Look for ascites, skin edema, pleural/pericardial effusion, placentomegaly

Bone marrow (rare):

“Giant pronormoblasts” with viral inclusions

Considered only in unclear severe anemia cases

Why does Parvovirus B19 cause sudden severe anemia?

B19 selectively infects and destroys erythroid precursors in the bone marrow, stopping red-cell production for 7–10 days.

Why is transient aplastic crisis seen only in patients with chronic hemolytic anemias?

These patients already have rapid RBC turnover; when marrow production stops even briefly, hemoglobin falls dramatically.

Why does the classic “slapped-cheek rash” occur?

Immune-complex deposition in the skin → erythematous malar rash → followed by lacy, reticular rash on limbs.

Why do adults with B19 get arthralgia instead of rash?

Adult immune response favors immune-complex deposition in joints rather than skin, causing symmetric small-joint pain.

Why can Parvovirus B19 infect the fetus but not cause structural malformations?

The virus targets erythroid precursor cells, not organ-forming tissues. Damage occurs by anemia → hydrops, not by teratogenesis.

Why is fetal infection dangerous in the 2nd trimester?

RBC production is maximal; severe anemia → high-output cardiac failure → hydrops fetalis.

Why does Parvovirus not cause chronic hemolysis?

It suppresses erythropoiesis but does not destroy mature RBCs directly; once marrow recovers, anemia resolves.

Why are immunocompromised patients at risk for chronic infection?

They cannot mount adequate neutralizing antibody response → persistent viral replication → chronic pure red-cell aplasia.

Why do some patients show only mild flu-like symptoms?

Host immunity and viral load determine severity; many infections are subclinical.

Why doesn’t Parvovirus B19 commonly cause respiratory symptoms?

Primary infection is viremia-driven; upper airway involvement is minimal compared to viruses like RSV or influenza.

Parvovirus B19 targets erythroid precursor cells in the bone marrow via the P-antigen receptor.

After respiratory entry → virus replicates in the nasopharynx → viremia develops.

Mass destruction of erythroid precursors → transient arrest of erythropoiesis (RBC production stops for 7–10 days).

Immune complex formation (virus + IgM) causes the characteristic “slapped-cheek” rash and arthralgia.

In patients with hemolytic disorders (e.g., thalassemia, sickle cell anemia) → severe anemia occurs due to inability to compensate.

During pregnancy → infection can cause fetal anemia → hydrops fetalis due to high-output cardiac failure.

Virus can persist in immunocompromised hosts → chronic pure red cell aplasia.

Selective tropism for erythroid precursor cells

B19 virus targets pronormoblasts in the bone marrow (via P antigen), causing arrest of erythropoiesis.

Transient red cell aplasia

Destruction of erythroid precursors leads to a drop in reticulocyte count and temporary anemia; severe in patients with hemolytic anemia.

Characteristic “giant pronormoblasts”

Bone marrow shows enlarged pronormoblasts with viral inclusions—pathognomonic.

Immune complex–mediated rash & arthralgia

As viremia declines, IgM and IgG antibodies form complexes → deposition in skin and joints → rash and arthropathy.

Hydrops fetalis pathology

• Fetal anemia → high-output cardiac failure → generalized edema.

• Placental edema and hepatosplenomegaly may be present.

Other tissue effects

• Myocarditis (rare): lymphocytic infiltration of myocardium.

• Vasculitis (rare): immune complex–related vascular involvement.

No vaccine available

There is currently no licensed vaccine for Parvovirus B19.

Good hand hygiene reduces transmission

Frequent handwashing and avoiding touching the face help reduce spread.

Avoid close contact during the viremic phase

Patients are infectious **before the rash appears**. Once rash starts, they are usually no longer contagious.

Pregnancy precautions

Non-immune pregnant women exposed to Parvovirus B19 should undergo IgG/IgM testing and fetal monitoring.

Infection control in schools & healthcare

Routine exclusion after rash is unnecessary, but symptomatic individuals (fever, malaise) should stay home.

Protection of high-risk groups

Immunocompromised patients and those with chronic hemolytic anemia should avoid exposure where possible.

Environmental cleaning

Virus spreads via respiratory secretions; regular cleaning of shared surfaces can help reduce transmission.Dr

Usually normal chest radiograph in erythema infectiosum

Most children have no respiratory involvement, so CXRs are normal.

Bone marrow imaging (rarely required)

• MRI may show diffuse marrow edema during transient aplastic crisis.

• Findings are nonspecific; diagnosis is clinical + laboratory.

Fetal ultrasound (in maternal infection)

Key modality for detecting fetal complications

• Signs of fetal anemia: increased middle cerebral artery (MCA) peak systolic velocity.

• Hydrops fetalis: skin edema, ascites, pleural or pericardial effusion.

• Placental thickening.

Fetal echocardiography

• May reveal high-output cardiac failure due to severe anemia.

Other imaging notes

• No specific CT / MRI findings tied to parvovirus infection in children or adults.

• Imaging is mainly used to evaluate complications such as myocarditis (MRI), or to rule out alternate causes of anemia.

Only one major human pathogenic serotype (B19)

Human parvovirus B19 has a single dominant serotype worldwide, unlike many other viral exanthems.

Genotypes show minor molecular variation

Three main genetic groups are identified: Genotype 1 (most common), Genotype 2, and Genotype 3—but they behave similarly clinically.

No clinically relevant subtype differences

All genotypes produce the same disease spectrum (erythema infectiosum, aplastic crisis, hydrops fetalis), with no difference in severity.

Stable virus with low antigenic drift

Parvovirus B19 undergoes minimal mutation; immunity after infection is typically long-lasting.

*Slapped-cheek erythema* (bright red cheeks with perioral pallor)

Lacy, reticular rash on limbs and trunk

Rash may fluctuate with heat, sunlight, stress

Low-grade fever

Mild cervical lymphadenopathy

Joint swelling/tenderness (adults)

Reduced range of motion due to arthralgia

Pallor (if anemia present)

Tachycardia (in severe anemia)

No significant pharyngitis or conjunctivitis

Helps distinguish from other viral exanthems

Bright red “slapped-cheek” rash

Lacy (reticular) rash on arms and trunk

Low-grade fever

Malaise

Headache

Sore throat

Mild arthralgia (children)

*Prominent arthralgia / arthritis in adults* (hands, wrists, knees, ankles)

Fatigue

Low-grade fever

Mild or absent rash (adults)

*Often asymptomatic in pregnancy* (maternal infection)

— but can affect fetus (hydrops fetalis)

Persistent anemia in immunocompromised

Fatigue

Pallor

Minimal or no rash

No specific antiviral therapy required in most cases

Illness is mild and self-limited in immunocompetent children and adults.

Supportive care

Adequate hydration, rest, antipyretics (paracetamol) for fever and discomfort. Avoid NSAIDs in pregnancy unless medically advised.

Arthropathy management

Mild joint pain → paracetamol or short course of NSAIDs (if not contraindicated). Usually resolves spontaneously.

IVIG for immunocompromised with chronic anemia

Intravenous immunoglobulin helps clear persistent infection and corrects red-cell aplasia.

Blood transfusion for severe anemia

Indicated in transient aplastic crisis (e.g., sickle cell disease, thalassemia) with symptomatic or profound anemia.

Fetal management during pregnancy

Weekly ultrasound Doppler monitoring (MCA-PSV) for fetal anemia.

Severe fetal anemia → *intrauterine transfusion* improves survival.

Isolation not required once rash appears

The patient is no longer contagious once the rash starts, so school exclusion is generally unnecessary.

Avoid exposure during pregnancy (preventive advice)

Pregnant contacts should be informed and tested for immunity (IgG).

References