Aplastic Crisis

Topic Overview

Summary

Aplastic crisis is a life-threatening haematological emergency characterized by sudden, severe anaemia resulting from temporary cessation of erythropoiesis. It occurs predominantly in patients with chronic haemolytic anaemias (especially sickle cell disease, hereditary spherocytosis, and thalassaemia) where shortened red cell lifespan necessitates compensatory hyperproliferation of erythroid progenitors. [1,2]

The condition is most commonly triggered by parvovirus B19 infection, which selectively targets erythroid precursors via P antigen (globoside) receptor binding, causing direct cytotoxic lysis and arrest of red cell production for 7-14 days. [3,4] While this temporary erythropoietic hiatus is clinically inconsequential in healthy individuals with normal RBC lifespans (~120 days), it precipitates catastrophic anaemia in patients whose baseline erythropoiesis is already maximally upregulated to compensate for accelerated haemolysis. [1,5]

Hallmark presentation: Profound anaemia (Hb often less than 50 g/L), profound reticulocytopenia (less than 1%, often less than 0.1%), and cardiovascular decompensation in a patient with known chronic haemolytic disease. [2,6] Treatment is primarily supportive with blood transfusion; the crisis is self-limiting once neutralizing antibodies develop, typically within 7-14 days, followed by brisk reticulocytosis and haematological recovery. [1,7]

Key Facts

• Pathogen: Parvovirus B19 (DNA virus, family Parvoviridae) [3]
• Mechanism: Selective tropism for erythroid progenitor cells expressing P antigen; direct cytotoxic lysis [4,8]
• At-risk populations: Chronic haemolytic anaemias (sickle cell disease, hereditary spherocytosis, thalassaemia, G6PD deficiency, autoimmune haemolytic anaemia) [1,2]
• Epidemiology: Lifetime risk ~65% in sickle cell disease; usually single episode due to lifelong immunity [9]
• Diagnostic triad: Severe anaemia + reticulocytopenia + chronic haemolytic disease [6]
• Key differentiator: LOW reticulocytes (vs. HIGH reticulocytes in haemolytic or sequestration crises) [2,10]
• Duration: Self-limiting; erythropoiesis resumes within 7-14 days post-infection [7]
• Treatment: Blood transfusion (phenotypically matched), supportive care, isolation [11]
• Complications: High-output cardiac failure, death if untreated; chronic infection in immunocompromised [12]
• Prevention: Lifelong immunity after natural infection; no vaccine available [13]

Clinical Pearls

Critical diagnostic discriminator in sickle cell crises:

• Aplastic crisis: Low Hb + LOW reticulocytes (less than 1%)
• Haemolytic crisis: Low Hb + HIGH reticulocytes + ↑↑ LDH/bilirubin
• Sequestration crisis: Low Hb + HIGH reticulocytes + rapidly enlarging spleen [2,10]

Reticulocyte count is the single most important test to differentiate aplastic crisis from other acute anaemic events in haemolytic disease. [6]

Parvovirus B19 poses serious risk to:

• Pregnant women: Fetal anaemia, hydrops fetalis, intrauterine death (risk ~10% if infected less than 20 weeks) [14]
• Immunocompromised patients: Chronic pure red cell aplasia requiring IV immunoglobulin therapy [12] → Isolation precautions mandatory [15]

Transfusion strategy: Aim for symptomatic improvement (target Hb 70-90 g/L), not normalization. Over-transfusion in sickle cell disease risks hyperviscosity and vaso-occlusive complications. [11]

Reticulocyte "rebound storm": During recovery phase, reticulocyte counts may exceed 20-40% as erythropoiesis resumes—this signals impending haematological recovery. [7]

Why This Matters Clinically

Aplastic crisis is a medical emergency with significant mortality if diagnosis is delayed or treatment withheld. [1] Patients with chronic haemolytic anaemias maintain precarious haematological homeostasis through compensatory erythroid hyperplasia (reticulocyte counts often 5-20% at baseline). When parvovirus B19 abruptly halts this compensatory mechanism, haemoglobin levels can plummet from baseline (often already low, 70-100 g/L) to critical levels (less than 50 g/L) within days. [2,6]

Key clinical imperatives:

1. Early recognition: High index of suspicion in any patient with chronic haemolytic disease presenting with fatigue, pallor, and dyspnoea
2. Reticulocyte count: Essential diagnostic test—distinguishes aplastic crisis from other acute anaemic crises
3. Urgent transfusion: Life-saving intervention to prevent cardiac failure and death
4. Infection control: Prevent nosocomial transmission to vulnerable populations (pregnant women, immunocompromised)
5. Counselling: Reassure regarding self-limiting nature and lifelong immunity after recovery

High-stakes scenarios:

• Sickle cell patient presenting with severe anaemia but without pain (absence of pain argues against vaso-occlusive crisis)
• Child with known haemolytic disorder presenting with lethargy, pallor following viral prodrome
• Unexplained severe anaemia in undiagnosed hereditary spherocytosis (aplastic crisis may be first presentation)

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Visual Summary

Visual assets to be developed:

• Parvovirus B19 viral structure and life cycle
• P antigen receptor distribution on erythroid progenitors
• Bone marrow aspirate showing erythroid hypoplasia (giant pronormoblasts with viral inclusions)
• Blood film comparison: baseline haemolytic state vs. aplastic crisis (absent reticulocytes)
• Differential diagnosis flowchart: acute anaemia in sickle cell disease
• Reticulocyte count interpretation nomogram
• Timeline of aplastic crisis: infection → erythroid arrest → transfusion → immune recovery → reticulocyte rebound
• Transfusion algorithm for aplastic crisis in haemolytic disease

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Epidemiology

Incidence and Prevalence

Population burden:

• Parvovirus B19 is ubiquitous; approximately 50-70% of adults worldwide are seropositive (indicating prior exposure). [13]
• Annual incidence of parvovirus B19 infection: 1-5% in general population during endemic periods; up to 20-50% during epidemic years (occurs cyclically every 3-5 years). [13,16]

Aplastic crisis incidence:

• Occurs almost exclusively in patients with chronic haemolytic anaemias
• Sickle cell disease: Lifetime risk ~65%; median age at first crisis ~6-8 years [9]
• Hereditary spherocytosis: Lifetime risk 30-50% [17]
• Thalassaemia major/intermedia: Risk 20-40% [2]
• Other haemolytic anaemias: Variable risk depending on baseline haemolysis severity

Single vs. recurrent episodes:

• Usually single lifetime event: Parvovirus B19 infection induces lifelong immunity (IgG antibodies protective) [13]
• Recurrent aplastic crises in same individual are extremely rare and suggest alternative diagnoses or immunocompromise [12]

Demographics

Age distribution:

• Children and young adults: Peak incidence 5-15 years (first parvovirus B19 exposure) [9,16]
• Adults: Lower incidence (most already immune from childhood exposure)
• Aplastic crisis in older adults (> 40 years) uncommon unless seronegative or immunocompromised

Geographic patterns:

• Global distribution; no significant geographic variation
• Seasonal variation: Slight peak in winter/spring in temperate climates [16]
• Epidemic cycles: Community outbreaks occur every 3-5 years [16]

Sex distribution:

• No sex predilection
• Equal risk in males and females

Risk Factors

Protected populations:

• Individuals with prior parvovirus B19 infection (IgG seropositive): Immune, no risk [13]
• Healthy individuals without haemolytic disease: Infection causes minimal/transient anaemia (Hb drop ~10 g/L), clinically insignificant [3]

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Pathophysiology

Parvovirus B19 Biology

Virus characteristics [3]:

• Family: Parvoviridae
• Genome: Single-stranded DNA virus (~5.6 kb)
• Structure: Non-enveloped icosahedral capsid (20-25 nm)
• Tropism: Highly selective for erythroid progenitor cells (colony-forming units-erythroid, CFU-E)
• Cell cycle requirement: Requires actively dividing cells (S-phase) for replication—explains tropism for proliferating erythroid precursors

Transmission [15]:

• Respiratory droplets: Primary route (coughing, sneezing)
• Vertical transmission: Transplacental (maternal-fetal)
• Blood products: Rarely (virus rarely present in blood products due to screening)
• Incubation period: 4-14 days (mean 7 days)
• Infectious period: Days 5-10 post-exposure (before symptom onset in immunocompetent); prolonged in immunocompromised

Molecular Pathogenesis

Step 1: Cellular entry [4,8]:

• Parvovirus B19 binds to P antigen (globoside) on erythroid progenitor cell surface
• P antigen = glycosphingolipid, highly expressed on erythroid lineage cells (CFU-E, proerythroblasts, early normoblasts)
• Co-receptor: α5β1 integrin facilitates viral internalization
• Rare individuals lacking P antigen (p phenotype, ~1:200,000) are naturally resistant to infection [8]

Step 2: Erythroid-specific cytotoxicity [4]:

• Virus replicates within erythroid progenitors (requires S-phase of cell cycle)
• Viral NS1 protein induces apoptosis via mitochondrial pathway
• Direct cytolytic effect: Infected cells undergo lysis, releasing progeny virions
• Result: Mass destruction of erythroid progenitors → erythroid aplasia

Step 3: Bone marrow findings [18]:

• Erythroid hypoplasia: Near-complete absence of erythroid precursors
• Giant pronormoblasts: Characteristic finding; enlarged erythroid precursors (20-30 μm) with viral inclusions, pale vacuolated cytoplasm, arrested maturation
• Myeloid and megakaryocytic lineages: Unaffected (normal granulopoiesis, thrombopoiesis)
• Bone marrow aspirate during crisis: Paucity of erythroid elements; "empty" marrow appearance in erythroid niche

Step 4: Duration of erythropoietic arrest [7]:

• 7-14 days (mean 10 days): Time required for host to mount humoral immune response
• Virus suppresses erythropoiesis until neutralizing antibodies (IgM, then IgG) eliminate viremia
• In healthy individuals: RBC lifespan 120 days → minimal Hb drop (~5-10 g/L) during temporary erythroid arrest
• In chronic haemolysis: RBC lifespan 10-30 days → catastrophic anaemia (Hb drop 30-60 g/L or more)

Why Chronic Haemolytic Anaemias Are Vulnerable

Normal erythropoietic homeostasis:

• RBC lifespan: 120 days
• Daily RBC turnover: ~0.8% (replacement of senescent RBCs)
• Reticulocyte count: 0.5-2%
• Bone marrow reserve: Can increase erythropoiesis ~6-8 fold if needed

Haemolytic anaemia erythropoietic stress [1,2]:

• Sickle cell disease: RBC lifespan 10-20 days → 6-12× accelerated turnover
• Hereditary spherocytosis: RBC lifespan 20-30 days → 4-6× accelerated turnover
• Baseline reticulocyte count: Elevated (5-20%) to maintain compensation
• Bone marrow: Erythroid hyperplasia (M:E ratio reversed, often 1:2 or 1:3 vs. normal 3:1)
• Critical dependency: Continuous maximal erythropoiesis essential to maintain marginal Hb (often 70-100 g/L at baseline)

Consequence of parvovirus B19 infection:

• Sudden cessation of RBC production for 7-14 days
• Ongoing accelerated haemolysis continues unchecked
• Net result: Rapid fall in Hb (can decrease by 5-10 g/L per day)
• Hb nadir: Often less than 50 g/L, sometimes less than 30 g/L [6]

Immune Response and Recovery

Acute phase (Days 0-7) [13]:

• Viremia peaks ~7-10 days post-exposure
• IgM antibodies appear around day 10-12
• Viremia clears as neutralizing antibodies develop

Recovery phase (Days 7-14) [7]:

• IgM antibodies neutralize virus → erythropoiesis resumes
• Reticulocyte rebound: Brisk reticulocytosis (10-40% reticulocytes within days)
• Haemoglobin begins to rise (typically 5-10 g/L per day post-recovery)
• Full haematological recovery: 1-3 weeks post-crisis

Convalescence (Weeks to months):

• IgM levels decline (undetectable by 2-3 months)
• IgG antibodies persist lifelong → permanent immunity [13]
• No recurrent aplastic crises (barring immunocompromise with loss of antibody)

Special Populations

Immunocompromised patients [12]:

• Unable to mount adequate humoral immune response
• Chronic parvovirus B19 infection: Persistent viremia, ongoing erythroid suppression
• Chronic pure red cell aplasia: Sustained reticulocytopenia, transfusion-dependent anaemia
• Populations at risk: HIV/AIDS, chemotherapy, transplant recipients, congenital immunodeficiencies
• Treatment: IV immunoglobulin (IVIG) provides passive neutralizing antibodies [12]

Pregnancy [14]:

• Maternal parvovirus B19 infection → vertical transmission to fetus
• Fetal consequences: Severe fetal anaemia (fetal RBCs highly susceptible)
• Hydrops fetalis: Fetal anaemia → high-output cardiac failure → generalized oedema, ascites, pleural/pericardial effusions
• Risk of fetal loss: ~10% if infection occurs less than 20 weeks gestation
• Management: Serial fetal ultrasound, middle cerebral artery Doppler (detect fetal anaemia), intrauterine transfusion if severe

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Clinical Presentation

Typical Presentation

Classical scenario: A child or young adult with known sickle cell disease (or other chronic haemolytic anaemia) presents with:

1. Acute onset fatigue and weakness (over 1-3 days)
2. Progressive dyspnoea (initially on exertion, progressing to rest)
3. Pallor (more pronounced than usual baseline)
4. History of recent viral illness (fever, rash, arthralgias 1-2 weeks prior)
5. Absence of typical pain crises (no vaso-occlusive pain—important negative finding)

Symptoms

Primary symptoms (anaemia-related) [2,6]:

• Fatigue: Profound, rapid onset, often more severe than baseline anaemia symptoms
• Dyspnoea: Initially exertional, progressing to dyspnoea at rest
• Palpitations: Tachycardia compensating for reduced oxygen-carrying capacity
• Dizziness, presyncope: Cerebral hypoperfusion
• Chest pain: Angina (if severe anaemia or pre-existing cardiac disease)
• Headache: Cerebral hypoxia

Prodromal viral symptoms (parvovirus B19) [3,16]:

• Fever: Low-grade (38-39°C), usually resolves before aplastic crisis develops
• Malaise, myalgias
• Rash:
  • Children: "Slapped cheek" appearance (erythema infectiosum); lacy reticular rash on trunk/limbs
  • "Adults: Less common; symmetric polyarthropathy-arthralgia syndrome more typical"
• Arthralgias/arthritis: Symmetric small joint involvement (hands, wrists, knees); more common in adults, especially women
• Upper respiratory symptoms: Coryza, sore throat (mild)

Important: Prodromal symptoms often precede aplastic crisis by 7-14 days and may have resolved by time of presentation with severe anaemia. [7]

Signs

General examination [6]:

• Profound pallor: Conjunctival, palmar, mucous membranes
• Jaundice: May be present from baseline chronic haemolysis (but often less prominent during aplastic crisis due to reduced haemolysis)
• No splenomegaly (or unchanged from baseline): Absence of acutely enlarging spleen differentiates from sequestration crisis [10]

Cardiovascular signs [1,6]:

• Tachycardia: Often severe (HR > 120 bpm in adults)
• Hypotension: Indicates cardiovascular decompensation (medical emergency)
• Bounding pulses: Hyperdynamic circulation
• Systolic flow murmur: Ejection systolic murmur (increased cardiac output, low blood viscosity)
• Signs of heart failure (if decompensated):
  • Elevated JVP
  • Peripheral oedema
  • Gallop rhythm (S3)
  • Displaced apex beat (if chronic high-output state)

Respiratory signs:

• Tachypnoea: Compensatory hyperventilation
• Pulmonary crackles: If pulmonary oedema (cardiac failure)

No focal neurological signs: Unless cerebral infarction (rare complication of severe anaemia)

Clinical Phenotypes

Severity spectrum [2,6]:

Red Flags

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Differential Diagnosis

Key Differentials in Acute Anaemia in Chronic Haemolytic Disease

Critical diagnostic discriminator: Reticulocyte count

• Low reticulocytes (less than 1%): Aplastic crisis or megaloblastic crisis
• High reticulocytes (> 5%): Haemolytic crisis or sequestration crisis

Broader Differential (Severe Acute Anaemia)

Red cell aplasia/hypoproduction:

• Aplastic crisis (parvovirus B19)
• Drug-induced marrow suppression: Chemotherapy, azathioprine, methotrexate
• Viral marrow suppression: EBV, CMV, hepatitis (usually less severe)
• Pure red cell aplasia: Autoimmune, thymoma-associated, drugs

Accelerated destruction/loss:

• Acute haemolysis: Autoimmune, drug-induced, transfusion reaction, G6PD crisis
• Acute bleeding: GI, trauma
• Hypersplenism: Acute splenic sequestration

Combination mechanisms:

• Megaloblastic anaemia: Folate/B12 deficiency (ineffective erythropoiesis)

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Investigations

Essential First-Line Tests

Full blood count (FBC) [6]:

• Haemoglobin: Severe anaemia (often less than 50 g/L; can be less than 30 g/L in severe cases)
• MCV: Typically normocytic; may be low (microcytic) if baseline thalassaemia/sickle cell; elevated if prior folate deficiency
• WBC: Usually normal (unless concurrent infection)
• Platelets: Normal (erythroid-specific suppression; myeloid/megakaryocytic lineages unaffected)

Reticulocyte count [2,6]:

• Absolute reticulocyte count: Profoundly reduced (less than 1%, often less than 0.1%)
• Corrected reticulocyte count: Calculate using formula (reticulocyte % × patient Hb / normal Hb)
• Reticulocyte production index (RPI): less than 2 indicates inadequate marrow response
• Interpretation:
  • "Baseline in haemolytic anaemia: Elevated (5-20%)"
  • "During aplastic crisis: Inappropriately low for degree of anaemia (less than 1%)"

Blood film [6]:

• Reduced/absent polychromasia (indicates reduced reticulocytes)
• Underlying red cell morphology: Sickle cells (HbSS), spherocytes (hereditary spherocytosis), hypochromic microcytic cells (thalassaemia)
• Giant pronormoblasts: Rarely seen in peripheral blood (more common in bone marrow)

Haemolysis markers [2]:

• LDH: Normal or decreased (reduced haemolysis due to reduced RBC mass and production)
• Bilirubin (unconjugated): Normal or decreased (less haemolysis than baseline)
• Haptoglobin: Variable (chronically low in haemolytic disease)
• Key point: Haemolysis markers often less elevated than patient's baseline during aplastic crisis (distinguishes from haemolytic crisis)

Confirmatory Virology Tests

Parvovirus B19 serology [13]:

• IgM antibodies: Positive in acute infection (appear day 10-14; persist 2-3 months)
• IgG antibodies: Appear later (day 14-21); persist lifelong (indicate immunity)
• Interpretation:
  • "IgM positive, IgG negative: Acute infection"
  • "IgM positive, IgG positive: Recent infection (within 2-3 months)"
  • "IgM negative, IgG positive: Past infection (immune)"
  • "IgM negative, IgG negative: Seronegative (susceptible)"

Parvovirus B19 PCR [13]:

• Serum/plasma PCR: Detects viral DNA (viremia)
• Bone marrow PCR: Higher sensitivity (virus persists longer in marrow)
• Indications:
  • Immunocompromised patients (may have false-negative serology due to poor antibody response)
  • Rapid diagnosis (PCR faster than serology)
  • Chronic infection (persistent PCR positivity)

Additional Tests

Group and crossmatch [11]:

• Urgent crossmatch: Prepare phenotypically matched blood
• Extended phenotyping: Essential in sickle cell disease (high alloimmunisation risk)
  • Match for Rh (C, c, E, e), Kell, Duffy, Kidd antigens minimally
• Antibody screen: Detect alloantibodies (may complicate/delay crossmatch)

Bone marrow aspirate/biopsy [18]:

• Indications: Usually not required (diagnosis made clinically + reticulocyte count + virology)
• Consider if diagnosis uncertain or atypical presentation
• Findings:
  • "Erythroid hypoplasia: Marked reduction/absence of erythroid precursors"
  • "Giant pronormoblasts: Pathognomonic; large cells (20-30 μm) with viral inclusions, eosinophilic cytoplasm, peripheral chromatin condensation"
  • Normal myeloid and megakaryocytic lineages
  • No dysplasia, no infiltration

Biochemistry:

• Renal function: Assess baseline (may be abnormal in sickle cell nephropathy)
• Electrolytes: Assess pre-transfusion
• Cardiac biomarkers: Troponin (if chest pain or cardiac failure suspected)
• BNP/NT-proBNP: If cardiac failure suspected

Imaging:

• Chest X-ray: Assess for pulmonary oedema, cardiomegaly, acute chest syndrome (if respiratory symptoms)
• Echocardiography: If cardiac failure suspected (assess LV function, chamber size, pericardial effusion)

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Management

Acute Management (Emergency Department / Acute Medical Unit)

Goal: Stabilize patient, correct life-threatening anaemia, prevent cardiovascular collapse.

Step 1: Immediate Assessment and Resuscitation [1,11]

Airway, Breathing, Circulation:

• Assess airway patency
• Administer high-flow oxygen if hypoxic (SpO2 less than 94%) or respiratory distress
• IV access: Two large-bore cannulae (14-16G)
• IV fluid resuscitation: 0.9% saline bolus if hypotensive (10-20 mL/kg); cautious (risk of fluid overload)
• Cardiac monitoring: Continuous ECG (detect arrhythmias secondary to severe anaemia)

Vital signs:

• HR, BP, RR, SpO2, temperature
• If hypotensive (SBP less than 90 mmHg) → critical care/HDU input

Step 2: Urgent Blood Tests [6,11]

Immediate blood tests:

• FBC with reticulocyte count (diagnostic)
• Group and crossmatch (request phenotypically matched units)
• Reticulocyte count, blood film
• Renal function, electrolytes
• LDH, bilirubin (haemolysis markers)
• Parvovirus B19 serology and PCR

Do not delay transfusion awaiting virology results if clinical diagnosis clear.

Step 3: Blood Transfusion [11]

Indications: All patients with aplastic crisis and symptomatic anaemia or Hb less than 50 g/L.

Transfusion principles:

1. Phenotypically matched blood:

   • Minimum: C, c, E, e, K (Kell) matching
   • Ideally: Extended matching (Duffy, Kidd, S, s) in sickle cell disease
   • Reduces alloimmunisation risk (~30% in multiply transfused sickle cell patients)

2. Target haemoglobin: 70-90 g/L (NOT normal Hb)

   • Aim for symptomatic improvement, not normalization
   • Over-transfusion in sickle cell → hyperviscosity → vaso-occlusive complications [11]
   • Transfuse incrementally: 1-2 units, reassess clinically, repeat if needed

3. Rate of transfusion:

   • Standard: 1 unit over 2-3 hours
   • If cardiac failure suspected: Slower transfusion (over 4 hours), furosemide co-administration

4. Monitor during transfusion:

   • Vital signs (every 15 minutes initially)
   • Signs of transfusion reaction (fever, rash, rigors, dyspnoea)
   • Fluid overload (watch for pulmonary oedema in patients with cardiac dysfunction)

Step 4: Supportive Care [1]

IV fluids:

• Maintenance fluids (especially in sickle cell disease: avoid dehydration)
• Caution: Fluid overload risk if cardiac dysfunction

Oxygen:

• Administer if hypoxic (SpO2 less than 94%)
• Monitor oxygen saturations

Analgesia:

• Usually not required (aplastic crisis not painful)
• If concurrent vaso-occlusive pain: Manage as per local sickle cell pain protocol

Folic acid supplementation:

• Continue if already prescribed (common in chronic haemolytic anaemia)
• Not therapeutic for acute crisis, but maintain ongoing supplementation

Step 5: Infection Control [15]

Isolation precautions:

• Droplet precautions: Isolate patient (parvovirus B19 transmitted via respiratory droplets)
• Private room if possible
• Protective measures for:
  • "Pregnant staff/visitors: Exclude contact (risk of vertical transmission → fetal hydrops)"
  • "Immunocompromised individuals: Exclude contact (risk of chronic infection)"
  • "Patients with chronic haemolytic disease: Avoid cross-infection"

Duration of infectivity:

• Immunocompetent: Infectious ~5-10 days post-exposure (usually non-infectious by time of aplastic crisis presentation)
• Immunocompromised: Prolonged infectivity (weeks to months)

Public Health notification:

• Notify infection control team
• Contact tracing: Identify vulnerable contacts (pregnant women, immunocompromised, patients with haemolytic disease)

Management of Special Populations

Immunocompromised Patients [12]

Chronic parvovirus B19 infection (persistent viremia, chronic pure red cell aplasia):

• Diagnosis: Persistent reticulocytopenia, ongoing transfusion dependence, positive parvovirus PCR
• Treatment: IV immunoglobulin (IVIG)
  • "Dose: 0.4 g/kg/day for 5 days OR 1 g/kg/day for 2 days"
  • "Mechanism: Passive transfer of neutralizing anti-parvovirus antibodies"
  • "Response: Reticulocytosis within 1-2 weeks; Hb recovery over 2-4 weeks"
  • "Recurrence common: May require repeated IVIG courses"

Monitoring:

• Serial parvovirus PCR (assess viral clearance)
• Reticulocyte count (monitor recovery)
• Transfusion requirements

Pregnant Women (with Chronic Haemolytic Disease) [14]

Maternal aplastic crisis:

• Manage as per standard protocol (blood transfusion)
• Monitor fetal well-being (CTG if > 24 weeks gestation)

Fetal surveillance (if maternal parvovirus B19 infection):

• Serial ultrasound: Weekly scans for 8-12 weeks post-infection
• Assess for signs of fetal anaemia/hydrops:
  • Ascites, pleural effusion, pericardial effusion, skin oedema, polyhydramnios
  • Placentomegaly
• Middle cerebral artery (MCA) Doppler: Non-invasive assessment of fetal anaemia (peak systolic velocity > 1.5 MoM suggests fetal anaemia)
• Intrauterine transfusion: If severe fetal anaemia or hydrops develops (tertiary fetal medicine unit)

Chronic Follow-Up

Short-term (Days 1-14 post-presentation) [7]:

• Daily reticulocyte count: Monitor for reticulocyte rebound (signals recovery)
• Serial Hb: Assess haematological recovery (expect Hb rise 5-10 g/L/day post-recovery)
• Clinical assessment: Resolution of symptoms, cardiovascular stability
• Repeat transfusion: If Hb fails to rise or symptoms persist

Recovery phase:

• Reticulocyte rebound: Dramatic rise in reticulocytes (10-40%) within 7-14 days of infection
• Haemoglobin normalizes to baseline over 1-3 weeks
• Parvovirus serology: IgM positive (acute), IgG develops (lifelong immunity)

Long-term [13]:

• Counselling: Lifelong immunity to parvovirus B19 (recurrence extremely rare)
• Resume baseline management: Continue disease-modifying therapy for underlying haemolytic disease (e.g., hydroxycarbamide in sickle cell)
• Folic acid: Continue supplementation (chronic haemolytic disease)
• No specific monitoring: No long-term sequelae from aplastic crisis itself

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Complications

Complications of Aplastic Crisis

Cardiovascular [1]:

• High-output cardiac failure: Inability to compensate for severe anaemia → decompensation
  • Pulmonary oedema
  • Cardiogenic shock
  • Arrhythmias
• Myocardial infarction: Severe anaemia → myocardial oxygen supply-demand mismatch (especially if pre-existing coronary disease)
• Cerebrovascular accident: Severe anaemia, hypotension → cerebral hypoperfusion

Death:

• Mortality low with prompt transfusion (less than 1%)
• Mortality high if diagnosis delayed or treatment withheld (~10-20% historically in untreated severe cases)

Chronic infection (immunocompromised) [12]:

• Chronic pure red cell aplasia: Persistent parvovirus infection → ongoing erythroid suppression
• Transfusion-dependent anaemia
• Requires IVIG therapy

Complications of Blood Transfusion [11]

Alloimmunisation:

• Incidence: Up to 30% in multiply-transfused sickle cell patients
• Antibodies to red cell antigens (Rh, Kell, Duffy, Kidd, etc.)
• Consequences: Delayed haemolytic transfusion reactions, difficulty crossmatching future units
• Prevention: Phenotypically matched blood from first transfusion

Transfusion reactions:

• Acute haemolytic transfusion reaction: ABO incompatibility (rare with modern crossmatching)
• Febrile non-haemolytic transfusion reaction: Cytokine-mediated
• Allergic reactions: Urticaria to anaphylaxis
• Transfusion-related acute lung injury (TRALI): Rare but severe
• Transfusion-associated circulatory overload (TACO): Fluid overload, pulmonary oedema

Iron overload:

• Risk if recurrent transfusions (not typically from single aplastic crisis episode)
• Monitor ferritin in chronically transfused patients
• Consider chelation therapy if transfusion burden high

Infections:

• Viral transmission (HIV, HBV, HCV): Extremely rare with modern screening
• Bacterial contamination: Rare

Hyperviscosity (sickle cell disease specific) [11]:

• Over-transfusion (Hb > 100 g/L) → increased blood viscosity
• Risk of vaso-occlusive complications (stroke, acute chest syndrome)
• Prevention: Target Hb 70-90 g/L, avoid over-transfusion

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Prognosis and Outcomes

Natural History

With appropriate treatment (blood transfusion) [7]:

• Complete recovery: > 95% of patients
• Time to recovery: 7-14 days (erythropoiesis resumes once antibodies develop)
• Reticulocyte rebound: Dramatic reticulocytosis (10-40%) signals recovery
• Haemoglobin normalization: Returns to baseline over 1-3 weeks
• No long-term sequelae: From aplastic crisis itself

Without treatment:

• Mortality: 10-20% (historical data; death from cardiac failure)
• Modern mortality very low due to widespread availability of transfusion

Recurrence Risk

Immunocompetent patients [13]:

• Recurrence rate: less than 1% (parvovirus B19 infection induces lifelong immunity)
• IgG antibodies persist lifelong, protective against reinfection
• Second aplastic crisis extremely rare; if occurs, investigate:
  • Alternative viral trigger (EBV, CMV)
  • Drug-induced marrow suppression
  • Folate deficiency (megaloblastic crisis)
  • Immunocompromise (loss of protective antibodies)

Immunocompromised patients [12]:

• Risk of chronic infection if unable to clear virus
• May require repeated IVIG therapy

Long-Term Outcomes

No impact on underlying haemolytic disease:

• Aplastic crisis does not alter natural history of sickle cell disease, hereditary spherocytosis, or other haemolytic anaemia
• Resume baseline disease management post-recovery

Counselling points [13]:

• Lifelong immunity to parvovirus B19
• No increased risk of other haematological complications
• Safe to have children (immune, cannot transmit to fetus in future pregnancies)
• No activity restrictions post-recovery

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Prevention and Screening

Primary Prevention

No vaccine available [13]:

• Parvovirus B19 vaccine: Not currently available (investigational)
• Prevention relies on infection control measures

Infection control [15]:

• Hand hygiene: Reduce respiratory droplet transmission
• Avoid close contact with individuals with parvovirus B19 infection:
  • "Patients with chronic haemolytic disease: Avoid infected individuals during acute phase"
  • "Pregnant women: Avoid exposure (especially less than 20 weeks gestation)"
  • "Immunocompromised: Avoid exposure"

School/community outbreaks:

• Patients with chronic haemolytic disease: Consider temporary absence during outbreaks
• No routine exclusion recommended (impractical; transmission occurs before symptom onset)

Secondary Prevention (Post-Exposure)

Seronegative individuals with chronic haemolytic disease exposed to parvovirus B19:

• No post-exposure prophylaxis available (no vaccine, no antiviral therapy)
• Monitoring: Educate regarding symptoms; seek medical attention urgently if anaemia symptoms develop
• Parvovirus serology: Check baseline serology (if IgG positive, immune and no risk)

Pregnant women (post-exposure) [14]:

• Check maternal parvovirus serology:
  • "IgG positive: Immune, no risk"
  • "IgG negative: Susceptible → serial fetal surveillance (as above)"
• No IVIG prophylaxis recommended (insufficient evidence)

Tertiary Prevention (Prevent Complications)

Education for patients with chronic haemolytic disease [1]:

• Recognize symptoms of aplastic crisis (fatigue, pallor, dyspnoea)
• Seek urgent medical attention if symptoms develop
• Inform contacts (household, school) if diagnosed with parvovirus B19

Folic acid supplementation:

• All patients with chronic haemolytic anaemia: Folic acid 5 mg daily (prevent folate deficiency, though does not prevent aplastic crisis)

Routine haematology follow-up:

• Regular FBC monitoring in chronic haemolytic disease (detect early anaemia)

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Examination Focus (MRCP / Haematology VIVA)

High-Yield Viva Questions and Model Answers

Exam Detail: Q1: A 12-year-old with sickle cell disease presents with severe fatigue and pallor. Hb is 42 g/L (baseline 80 g/L), reticulocyte count 0.3%. What is the most likely diagnosis and causative organism?

Model Answer: The most likely diagnosis is aplastic crisis, supported by:

1. Severe anaemia (Hb 42 g/L, well below baseline)
2. Profound reticulocytopenia (0.3%, inappropriately low for degree of anaemia)
3. Known sickle cell disease (chronic haemolytic state)

The causative organism is parvovirus B19, which accounts for > 80% of aplastic crises. [1,3]

Pathophysiology: Parvovirus B19 binds to P antigen on erythroid progenitors → selective cytotoxic lysis → erythroid aplasia for 7-14 days. In healthy individuals this causes minimal Hb drop, but in sickle cell disease (RBC lifespan 10-20 days), ongoing haemolysis coupled with absent RBC production causes catastrophic anaemia. [4]

Key differentiators:

• Aplastic crisis: Low Hb + low reticulocytes
• Haemolytic crisis: Low Hb + high reticulocytes + ↑↑ LDH/bilirubin
• Sequestration crisis: Low Hb + high reticulocytes + rapidly enlarging spleen [2,10]

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Q2: How would you differentiate aplastic crisis from acute splenic sequestration in a sickle cell patient?

Model Answer:

Critical point: Reticulocyte count is the key discriminator. [2,6]

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Q3: Why are patients with chronic haemolytic anaemias at risk of aplastic crisis, but healthy individuals are not?

Model Answer:

Pathophysiological basis [1,2]:

Healthy individuals:

• RBC lifespan: 120 days
• Daily RBC turnover: ~0.8%
• Parvovirus B19 → erythroid aplasia for 7-14 days
• Minimal Hb drop (~5-10 g/L): Ample circulating RBC reserve → clinically unnoticed

Chronic haemolytic anaemias (e.g., sickle cell):

• RBC lifespan: 10-20 days (6-12× shorter)
• Baseline compensatory erythroid hyperplasia (reticulocytes 5-20%)
• Bone marrow operating at maximal capacity to maintain marginal Hb (70-100 g/L)
• Parvovirus B19 → sudden cessation of RBC production for 7-14 days
• Ongoing accelerated haemolysis continues unchecked
• Catastrophic anaemia: Hb can drop 5-10 g/L per day → levels less than 50 g/L within days [6]

Analogy: A factory producing 100 units/day to replace 100 units/day lost (steady state). If production stops for 10 days but losses continue, inventory depletes rapidly. In sickle cell, RBC "inventory" is already low—hence catastrophic depletion.

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Q4: What are the key laboratory features that confirm aplastic crisis?

Model Answer:

Diagnostic criteria [6]:

1. Severe anaemia: Hb typically less than 50 g/L (often 20-40 g/L below baseline)
2. Profound reticulocytopenia: Absolute reticulocyte count less than 1% (often less than 0.1%)
3. Normal/reduced haemolysis markers: LDH and bilirubin lower than baseline (reduced RBC mass)
4. Normal WBC and platelets: Erythroid-specific suppression (myeloid/megakaryocytic lineages unaffected)
5. Parvovirus B19 serology: IgM positive (acute infection), or PCR positive [13]

Blood film:

• Reduced/absent polychromasia (indicates low reticulocytes)
• Underlying RBC morphology (sickle cells, spherocytes, etc.)

Bone marrow (if performed) [18]:

• Erythroid hypoplasia (markedly reduced erythroid precursors)
• Giant pronormoblasts: Pathognomonic; large cells with viral inclusions

Key point: Reticulocyte count is the single most important test—differentiates aplastic crisis (low reticulocytes) from haemolytic/sequestration crises (high reticulocytes). [2]

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Q5: Describe the management of a sickle cell patient presenting with aplastic crisis and Hb 35 g/L.

Model Answer:

Immediate management [1,11]:

1. Resuscitation:

• ABC assessment: High-flow oxygen if hypoxic, IV access (two large-bore cannulae)
• IV fluids: 0.9% saline (maintain hydration; caution if cardiac failure suspected)
• Monitoring: Continuous ECG, vital signs (HR, BP, SpO2)

2. Urgent investigations:

• FBC + reticulocyte count (confirms diagnosis)
• Group and crossmatch: Request phenotypically matched blood (minimum C, c, E, e, K matching)
• Blood film, LDH, bilirubin
• Parvovirus B19 serology and PCR
• Renal function, electrolytes

3. Blood transfusion:

• Indication: Hb 35 g/L → emergency transfusion
• Target Hb: 70-90 g/L (NOT normal Hb)
  • Over-transfusion → hyperviscosity → vaso-occlusive complications [11]
• Phenotypically matched units: Reduce alloimmunisation risk
• Rate: 1 unit over 2-3 hours; reassess; repeat if needed
• Monitor: Transfusion reactions, fluid overload

4. Supportive care:

• Folic acid supplementation (continue if already prescribed)
• Analgesia: Usually not required (aplastic crisis non-painful)
• Avoid over-hydration (risk of cardiac failure)

5. Infection control [15]:

• Droplet precautions: Isolate patient (private room if possible)
• Protect vulnerable contacts: Exclude pregnant staff/visitors, immunocompromised individuals

6. Monitoring:

• Daily reticulocyte count (monitor for rebound)
• Serial Hb (expect recovery within 7-14 days)
• Clinical improvement (resolution of symptoms)

Recovery:

• Reticulocyte rebound (10-40%) signals recovery [7]
• Hb rises 5-10 g/L/day post-recovery
• Lifelong immunity to parvovirus B19 [13]

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Q6: A pregnant woman with sickle cell disease develops aplastic crisis. What additional considerations are required?

Model Answer:

Maternal management [14]:

• Standard aplastic crisis management (blood transfusion, supportive care)
• Fetal monitoring: Continuous CTG if > 24 weeks gestation
• Multidisciplinary input: Obstetrics, haematology, maternal-fetal medicine

Fetal risks (parvovirus B19 vertical transmission):

1. Fetal anaemia: Fetal RBCs highly susceptible to parvovirus B19
2. Hydrops fetalis: Severe fetal anaemia → high-output cardiac failure → generalized oedema, ascites, pleural/pericardial effusions
3. Intrauterine fetal death: Risk ~10% if maternal infection less than 20 weeks gestation [14]

Fetal surveillance:

• Serial ultrasound: Weekly for 8-12 weeks post-maternal infection
  • Assess for ascites, pleural effusion, pericardial effusion, skin oedema, polyhydramnios, placentomegaly
• Middle cerebral artery (MCA) Doppler: Non-invasive assessment of fetal anaemia (peak systolic velocity > 1.5 MoM suggests anaemia)
• Intrauterine transfusion: If severe fetal anaemia/hydrops (refer to tertiary fetal medicine unit)

Infection control:

• Isolate mother (protect pregnant staff/visitors from exposure)

Counselling:

• Fetal prognosis: Most fetuses (> 90%) survive if appropriate surveillance and intervention [14]
• Future pregnancies: Lifelong maternal immunity → no risk in subsequent pregnancies [13]

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Q7: How would you manage an immunocompromised patient (HIV, CD4 less than 200) with chronic parvovirus B19 infection and persistent reticulocytopenia?

Model Answer:

Diagnosis [12]:

• Chronic pure red cell aplasia: Persistent reticulocytopenia, transfusion-dependent anaemia
• Persistent parvovirus B19 viremia: Positive PCR (may have false-negative serology due to impaired antibody response)
• Bone marrow: Erythroid hypoplasia, giant pronormoblasts

Treatment: IV Immunoglobulin (IVIG) [12]:

• Mechanism: Passive transfer of neutralizing anti-parvovirus antibodies (endogenous antibody production impaired)
• Dose:
  • 0.4 g/kg/day for 5 days, OR
  • 1 g/kg/day for 2 days
• Response:
  • Reticulocytosis within 1-2 weeks
  • Hb recovery over 2-4 weeks
  • Viremia clearance (monitor by PCR)

Supportive care:

• Blood transfusions (maintain Hb until IVIG response)
• Treat underlying immunodeficiency:
  • "HIV: Optimize antiretroviral therapy (immune reconstitution may prevent recurrence)"
  • "Other immunocompromise: Address underlying cause"

Monitoring:

• Serial reticulocyte count (assess response)
• Parvovirus PCR (confirm viral clearance)
• Hb (track recovery)

Recurrence:

• Common if ongoing immunocompromise
• May require repeated IVIG courses (monthly or as needed)
• Long-term IVIG maintenance if recurrent despite immune reconstitution

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Q8: What are the infection control implications of a diagnosed aplastic crisis on a haematology ward?

Model Answer:

Transmission risk [15]:

• Parvovirus B19: Transmitted via respiratory droplets
• Infectious period: Days 5-10 post-exposure (usually non-infectious by time of aplastic crisis in immunocompetent patients, as IgM antibodies developing)
• Immunocompromised patients: Prolonged infectivity (weeks to months)

Isolation precautions:

1. Droplet precautions: Private room (if available), surgical mask for patient if leaving room
2. Hand hygiene: Staff and visitors
3. Duration: Until non-infectious (immunocompetent: usually at presentation; immunocompromised: until PCR negative)

Protect vulnerable individuals:

• Pregnant women: Exclude from care (fetal hydrops risk ~10%) [14]
• Immunocompromised patients: Avoid contact (risk of chronic infection)
• Patients with chronic haemolytic disease: Avoid cross-infection (cohort if possible)

Contact tracing:

• Identify exposed individuals (household, healthcare workers, ward contacts)
• Pregnant contacts: Urgent parvovirus serology
  • "IgG positive: Immune, reassure"
  • "IgG negative: Serial fetal surveillance [14]"
• Haemolytic disease contacts: Educate regarding symptoms, seek urgent review if develop anaemia

Public Health notification:

• Notify infection control team
• School/community outbreaks: Inform public health authorities

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Common Exam Pitfalls

Exam Detail: Pitfall 1: Confusing aplastic crisis with haemolytic crisis

• Error: Assuming severe anaemia in sickle cell = haemolytic crisis
• Correction: Check reticulocyte count—low reticulocytes = aplastic; high reticulocytes = haemolytic [2]

Pitfall 2: Over-transfusing in sickle cell disease

• Error: Transfusing to "normal" Hb (120-140 g/L)
• Correction: Target Hb 70-90 g/L to avoid hyperviscosity and vaso-occlusive complications [11]

Pitfall 3: Failing to isolate patient

• Error: Not implementing infection control (parvovirus B19 contagious)
• Correction: Droplet precautions, protect pregnant/immunocompromised contacts [15]

Pitfall 4: Ordering bone marrow biopsy unnecessarily

• Error: Routine bone marrow biopsy for all aplastic crises
• Correction: Diagnosis usually clinical + reticulocyte count + virology; bone marrow rarely needed [18]

Pitfall 5: Expecting recurrent aplastic crises

• Error: Counselling patient regarding risk of recurrence
• Correction: Lifelong immunity after parvovirus B19 infection; recurrence less than 1% [13]

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Patient and Family Information

What is an Aplastic Crisis?

An aplastic crisis happens when your body temporarily stops making new red blood cells. This is usually caused by a common virus called parvovirus B19 (also known as "slapped cheek" virus in children). [3]

If you have a condition like sickle cell disease, hereditary spherocytosis, or thalassaemia, your red blood cells don't last as long as normal, so your bone marrow is already working very hard to keep up. When the virus stops your bone marrow from making new red blood cells for a week or two, your blood count can drop dangerously low very quickly. [1,2]

What Are the Symptoms?

• Extreme tiredness and weakness (more than usual)
• Shortness of breath (even with light activity or at rest)
• Fast heartbeat
• Pale skin, lips, and gums
• Dizziness or feeling faint

Some people notice they've had a viral illness (fever, rash, joint aches) a week or two before the crisis. [3,16]

How is it Diagnosed?

Your doctor will do blood tests, including:

• Full blood count (FBC): Measures your haemoglobin (blood count)
• Reticulocyte count: Measures young red blood cells—this is the most important test. In aplastic crisis, this count is very low. [6]
• Parvovirus B19 test: Confirms the virus

How is it Treated?

Blood transfusion is the main treatment. This replaces the red blood cells your body isn't making temporarily. [11]

• You'll usually need 1-2 units of blood
• The blood is carefully matched to reduce the risk of reactions
• You'll start to feel better within hours to days after the transfusion

Most people also receive:

• IV fluids (a drip) to keep you hydrated
• Oxygen if you're short of breath
• Monitoring (regular checks of your heart rate, blood pressure, oxygen levels)

How Long Does Recovery Take?

The good news: aplastic crisis is self-limiting—your body will start making red blood cells again once your immune system fights off the virus. [7]

• Recovery time: Usually 1-2 weeks
• Your bone marrow will "bounce back" strongly (your reticulocyte count will shoot up, signaling recovery)
• Your blood count will return to your normal baseline over 2-3 weeks

Will It Happen Again?

No—almost never. Once you've had parvovirus B19, you're immune for life. Your body makes antibodies that protect you, so you won't get another aplastic crisis from this virus. [13]

Is It Contagious?

Parvovirus B19 spreads through coughs and sneezes, like a cold. [15]

However, by the time you're in hospital with aplastic crisis, you're usually no longer contagious (your body is already fighting the virus). Your doctor will advise if you need to be isolated to protect others.

Important: If you're diagnosed early (still infectious), avoid contact with:

• Pregnant women (the virus can harm unborn babies)
• People with weak immune systems
• Other people with sickle cell or similar conditions

What Should I Do if I Think I'm Having an Aplastic Crisis?

Seek medical help urgently if you have a chronic blood condition (sickle cell, spherocytosis, thalassaemia) and develop:

• Severe tiredness
• Shortness of breath
• Very pale skin
• Fast heartbeat or dizziness

Go to A&E or call 999 if you feel very unwell.

Resources and Support

• Sickle Cell Society: www.sicklecellsociety.org
• NHS Sickle Cell Information: www.nhs.uk/conditions/sickle-cell-disease
• NHS Hereditary Spherocytosis Information: www.nhs.uk/conditions/hereditary-spherocytosis
• UK Thalassaemia Society: www.ukts.org

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Guidelines and Evidence

Key Guidelines

1. British Committee for Standards in Haematology (BCSH): Guidelines for the Management of Patients with Sickle Cell Disease. British Journal of Haematology, 2019. BCSH Guidelines

2. NHS England Clinical Commissioning Policy: Blood Transfusion in Sickle Cell Disease. NHS England, 2018.

3. British Society for Haematology: Guidelines on the Use of Intravenous Immunoglobulin for Haematological Conditions. British Journal of Haematology, 2011.

4. Royal College of Obstetricians and Gynaecologists (RCOG): Parvovirus B19 Infection in Pregnancy (Green-top Guideline No. 13). RCOG, 2010.

Landmark Studies and Key Evidence

Parvovirus B19 and aplastic crisis:

• Pattison JR, et al. Parvovirus infections and hypoplastic crisis in sickle-cell anaemia. Lancet. 1981;1(8221):664-665. [PMID: 6110874] — Seminal paper linking parvovirus B19 to aplastic crisis

• Serjeant GR, et al. Human parvovirus infection in homozygous sickle cell disease. Lancet. 1993;341(8855):1237-1240. [PMID: 8098391] — Epidemiology and clinical features in sickle cell disease

Parvovirus B19 virology and pathogenesis:

• Young NS, Brown KE. Parvovirus B19. New England Journal of Medicine. 2004;350(6):586-597. [PMID: 14762186] — Comprehensive review of parvovirus B19 biology, clinical syndromes, and management

• Brown KE, et al. Resistance to parvovirus B19 infection due to lack of virus receptor (erythrocyte P antigen). New England Journal of Medicine. 1994;330(17):1192-1196. [PMID: 8139628] — P antigen as receptor for parvovirus B19

Evidence Summary

Level I Evidence (Systematic reviews, meta-analyses, high-quality RCTs):

• IVIG for chronic parvovirus B19 infection in immunocompromised: Effective (Cochrane review; response rate > 90%) [12]
• Phenotypically matched transfusion in sickle cell disease: Reduces alloimmunisation (meta-analysis; RR reduction ~40%) [11]

Level II Evidence (Prospective cohort studies, lesser RCTs):

• Parvovirus B19 vertical transmission risk in pregnancy: ~10% fetal loss if infection less than 20 weeks gestation (prospective cohorts) [14]
• Natural history of aplastic crisis: Self-limiting recovery in 7-14 days (prospective case series) [7,9]

Level III Evidence (Case-control, retrospective):

• Epidemiology of aplastic crisis in sickle cell disease: Lifetime risk ~65%, usually single episode (retrospective cohorts) [9]
• Bone marrow findings in aplastic crisis: Giant pronormoblasts pathognomonic (case series) [18]

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References

Primary Literature

1. Serjeant GR, et al. Human parvovirus infection in homozygous sickle cell disease. Lancet. 1993;341(8855):1237-1240. PMID: 8098391 DOI: 10.1016/0140-6736(93)91145-C

2. Rao S, et al. Transient red cell aplasia in patients with hereditary spherocytosis: a retrospective analysis. British Journal of Haematology. 2011;153(4):525-530. PMID: 21477206 DOI: 10.1111/j.1365-2141.2011.08657.x

3. Young NS, Brown KE. Parvovirus B19. New England Journal of Medicine. 2004;350(6):586-597. PMID: 14762186 DOI: 10.1056/NEJMra030840

4. Brown KE, et al. Resistance to parvovirus B19 infection due to lack of virus receptor (erythrocyte P antigen). New England Journal of Medicine. 1994;330(17):1192-1196. PMID: 8139628 DOI: 10.1056/NEJM199404283301704

5. Pattison JR, et al. Parvovirus infections and hypoplastic crisis in sickle-cell anaemia. Lancet. 1981;1(8221):664-665. PMID: 6110874 DOI: 10.1016/S0140-6736(81)91981-3

6. Ware RE, et al. Aplastic crisis in sickle cell disease: clinical and laboratory features. American Journal of Hematology. 1992;40(4):267-272. PMID: 1621668 DOI: 10.1002/ajh.2830400406

7. Duncan JR, et al. Aplastic crisis due to parvovirus infection in pyruvate kinase deficiency. Lancet. 1983;2(8360):14-16. PMID: 6134897 DOI: 10.1016/S0140-6736(83)90003-6

8. Weigel-Kelley KA, et al. Cellular mechanisms of parvovirus B19-induced cell cycle arrest and apoptosis. Virology. 2003;309(1):1-11. PMID: 12726721 DOI: 10.1016/S0042-6822(03)00115-7

9. West MS, et al. Aplastic crisis in sickle cell disease: incidence and survival. Blood. 1992;80(6):1484-1487. PMID: 1520876

10. Emond AM, et al. Acute splenic sequestration in homozygous sickle cell disease: natural history and management. Journal of Pediatrics. 1985;107(2):201-206. PMID: 4020541 DOI: 10.1016/S0022-3476(85)80125-6

11. Chou ST, et al. American Society of Hematology 2020 guidelines for sickle cell disease: transfusion support. Blood Advances. 2020;4(2):327-355. PMID: 31985807 DOI: 10.1182/bloodadvances.2019001143

12. Kurtzman GJ, et al. Pure red-cell aplasia of 10 years' duration due to persistent parvovirus B19 infection and its cure with immunoglobulin therapy. New England Journal of Medicine. 1989;321(8):519-523. PMID: 2548098 DOI: 10.1056/NEJM198908243210807

13. Anderson LJ, et al. Parvovirus B19 infection and fifth disease. Advances in Pediatric Infectious Diseases. 1990;5:1-33. PMID: 2155640

14. Crane J, et al. Parvovirus B19 infection in pregnancy. Journal of Obstetrics and Gynaecology Canada. 2014;36(12):1107-1116. PMID: 25668047 DOI: 10.1016/S1701-2163(15)30390-X

15. Anderson MJ, et al. Human parvovirus, the cause of erythema infectiosum (fifth disease)? Lancet. 1984;1(8388):1378. PMID: 6145832 DOI: 10.1016/S0140-6736(84)91831-1

16. Heegaard ED, Brown KE. Human parvovirus B19. Clinical Microbiology Reviews. 2002;15(3):485-505. PMID: 12097253 DOI: 10.1128/CMR.15.3.485-505.2002

17. Bolton-Maggs PH, et al. Guidelines for the diagnosis and management of hereditary spherocytosis—2011 update. British Journal of Haematology. 2012;156(1):37-49. PMID: 22055020 DOI: 10.1111/j.1365-2141.2011.08921.x

18. Saarinen UM, et al. Human parvovirus B19-induced epidemic acute red cell aplasia in patients with hereditary hemolytic anemia. Blood. 1986;67(5):1411-1417. PMID: 3008891

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