Parvovirus B19

The only single-stranded DNA (ssDNA) virus!

Pathophysiology

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• Parvovirus B19 binds to the P antigen (globoside) on erythroid progenitor cells → cellular invasion → viral DNA enters the nucleus of erythroid cells → viral DNA replication → cytotoxicity → clinical manifestations + transient cessation of erythropoiesis

Clinical features

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• Erythema Infectiosum (Fifth Disease - Children):
  • Prodrome: Low-grade fever, malaise, upper respiratory symptoms.
  • Rash: “Slapped-cheek” rash (spares nasolabial folds) -> reticular/lace-like maculopapular rash on trunk and extremities (evanescent, fluctuates w/ heat/sunlight).
• Acute Polyarthropathy (Adults):
  • Symmetric, small-joint polyarthralgia/arthritis (hands, wrists, knees, ankles).
  • Mimics RA (seronegative, self-limiting within 2-4 weeks, non-destructive).
• Transient Aplastic Crisis:
  • Occurs in underlying hemolytic anemia (SCA, thalassemia, spherocytosis).
  • Severe acute anemia w/ profound reticulocytopenia (<1%).
• Pure Red Cell Aplasia (PRCA):
  • Chronic, severe anemia in immunocompromised pts due to persistent viral infection.
• Congenital Infection:
  • Fetal infection in 1st/2nd trimester -> destruction of fetal erythroid precursors -> severe fetal anemia -> high-output HF -> Hydrops fetalis (ascites, pleural/pericardial effusions, skin edema) -> fetal demise. c

Childhood exanthems

Disease	Pathogen	Classic Presentation & Buzzwords
Measles	Measles virus	4 C’s (Cough, Coryza, Conjunctivitis, Koplik spots). Rash: Face $\to$ Body (Confluent). Assoc: Vitamin A deficiency.
Rubella	Rubella virus	Post-auricular LAD. Rash: Face $\to$ Body (spares 3 days, non-confluent). Assoc: Congenital (PDA, cataracts, deaf), Adult arthritis.
Roseola	HHV-6	High fever breaks $\to$ Rash appears. Rash: Trunk $\to$ Face. Assoc: Febrile seizures.
Fifth Disease	Parvovirus B19	”Slapped Cheek” $\to$ Lacy/Reticular body rash. Assoc: Aplastic crisis (Sickle Cell), Hydrops fetalis.
Varicella	VZV	”Dewdrop on a rose petal”. Lesions in different stages of healing. t Rash: Trunk $\to$ Extremities.
Scarlet Fever	Strep pyogenes	Sandpaper rash, Strawberry tongue, Circumoral pallor. Assoc: Strep throat, Desquamation (palms/soles).

Fever-rash relationship

• Measles (Rubeola): Fever first (with cough, coryza, conjunctivitis) → Rash appears 3-5 days later, spreading from head to toe.
• Rubella (German Measles): Low-grade fever → Rash appears 1-2 days later, spreading quickly from head to toe.
• Scarlet Fever: Fever and sore throat begin together → “Sandpaper” rash appears 1-2 days later.
• Erythema Infectiosum (Fifth): Low-grade fever/prodrome resolves → “Slapped cheek” rash appears days later.
• Roseola Infantum (Sixth): High fever for 3-5 days → Fever breaks → Rash appears after the fever is gone.
• Varicella (Chickenpox): Fever and vesicular rash appear at the same time (lesions in various stages).

Link to original

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Diagnosis

• Initial/Screening:
  • Normal host w/ classic presentation: Clinical diagnosis or anti-B19 IgM antibodies.
• Confirmatory / Special Populations:
  • Viral PCR for B19 DNA: Diagnostic test of choice in immunocompromised pts, transient aplastic crisis, or pregnant women (serology is often negative or unreliable).
• Key Labs:
  • Precipitous drop in Hb w/ very low reticulocyte count (< 1%) in aplastic crisis.
• Fetal Assessment (Pregnant Pts):
  • Obstetric US + Middle Cerebral Artery (MCA) Doppler (high peak systolic velocity indicates fetal anemia).