Dengue - Comprehensive Overview

Etiology

• Family: Flaviviridae; genus Flavivirus
• Single-stranded, positive-sense RNA virus
• Infection with one serotype confers lifelong immunity to that serotype and transient cross-protection (1-3 years) against the other three
• A person may have up to four lifetime dengue infections

Epidemiology

• Infects an estimated 400 million people per year, 100 million symptomatic, ~40,000 deaths from severe dengue
• Endemic in South and Central America, Mexico, Africa, Asia, the Caribbean, and Pacific islands
• In 2025, >3.6 million reported infections across nearly 100 countries
• Vector mosquitoes breed in peridomestic standing water; strongly associated with poor sanitation, crowding, and tropical climate
• Incubation period: 3-14 days (intrinsic); mosquito extrinsic incubation 8-12 days
• Viremic persons (symptomatic or asymptomatic) can infect mosquitoes 1-2 days before symptoms and throughout ~7-day viremia

Pathophysiology

Primary Infection

Antibody-Dependent Enhancement (ADE)

• Cross-reactive antibodies from the first serotype cannot neutralize the second serotype
• Instead, these non-neutralizing IgG antibodies bind the new virus and facilitate entry into macrophages via Fc receptors, amplifying viral replication
• Higher viral load drives cytokine storm, endothelial dysfunction, and massive plasma leakage
• Severe dengue is most likely with secondary DENV-2 infection
• Same mechanism explains severe dengue in infants born to dengue-immune mothers (maternally acquired antibodies)

Plasma Leakage Mechanism

Clinical Phases

Phase 1: Febrile Phase (Days 1-3)

• Abrupt high fever (39-40°C), lasting 2-7 days ("saddleback" pattern possible)
• Severe headache, retro-orbital pain, myalgia, arthralgia ("breakbone fever")
• Facial flushing, pharyngeal injection, conjunctival suffusion
• Macular or maculopapular rash
• Nausea/vomiting
• Petechiae, mild hemorrhagic manifestations
• Labs: leukopenia, thrombocytopenia beginning

Phase 2: Critical Phase (Days 3-7, around defervescence)

• Temperature drops - this is the danger window
• Plasma leakage due to increased vascular permeability (lasts 24-48 hours)
• Rising hematocrit (hemoconcentration) is a key marker
• Rapid drop in platelet count
• Warning signs (requiring hospitalization):
  • Abdominal pain or tenderness
  • Persistent vomiting
  • Clinical fluid accumulation (ascites, pleural effusion)
  • Mucosal bleeding
  • Lethargy or restlessness
  • Liver enlargement >2 cm
  • Rising hematocrit with rapid platelet decline

Phase 3: Recovery/Convalescent Phase (Days 7-10+)

• Reabsorption of leaked fluids
• Risk of fluid overload if IV fluids not reduced
• Bradycardia, confluent rash with islets of normal skin ("white islands in a sea of red")
• Improving platelet count

WHO Classification

2009 WHO Classification (Current)

Classical DHF Grading (WHO 1997 - Still Used)

Diagnosis

Timing-Based Approach

• Testing a single specimen in the first 10 days for both NS1 antigen AND IgM detects ≥90% of cases
• RT-PCR or NS1: preferred in first 5 days
• IgM: preferred after Day 5
• FBC findings: leukopenia, thrombocytopenia, elevated hematocrit, raised liver enzymes

Tourniquet (Rumpel-Leede) Test

Management

No specific antiviral exists. Treatment is entirely supportive.

Dengue without Warning Signs (Outpatient)

• Oral rehydration (ORS, fruit juices, electrolyte solutions)
• Paracetamol for fever (≥6-hour intervals); keep temp <39°C
• Avoid NSAIDs (ibuprofen), aspirin, corticosteroids - increase bleeding risk
• Return immediately if warning signs develop
• Monitor: daily FBC, hematocrit, platelet count, fluid balance

Dengue with Warning Signs (Hospital Admission)

• IV crystalloids (isotonic saline or Ringer's lactate)
• Titrate fluid rate based on clinical status and hematocrit
• Monitor urine output (target ≥0.5 mL/kg/hr)
• Watch for plasma reabsorption in convalescence - reduce fluids to avoid overload

Severe Dengue / DSS (ICU-Level Care)

• Aggressive IV fluid resuscitation - crystalloids first; colloids or blood products if refractory
• Monitoring: serial hematocrit, blood pressure, pulse, urine output
• Platelet transfusion: only for severe bleeding or pre-procedural, not prophylactic
• Vasopressors for refractory shock
• With early recognition and aggressive supportive care, CFR drops from 5-10% to <1%

Complications

High-Risk Groups for Severe Disease

• Infants (maternal antibodies - ADE)
• Pregnant women
• Second (heterologous) dengue infection
• Patients with chronic diseases: asthma, sickle cell, diabetes mellitus

Vaccination

• Dengvaxia (CYD-TDV) - chimeric live-attenuated tetravalent vaccine; approved in several countries but only recommended for seropositive individuals - risk of severe dengue on first natural infection post-vaccination in seronegative recipients (paradoxically acts like a "first infection")
• TAK-003 (Qdenga) - live-attenuated tetravalent; approved in multiple countries; trial (TIDES) showed ~80% efficacy for symptomatic dengue and ~90% for hospitalization; can be given regardless of prior serostatus, though most benefit in seropositive
• WHO 2025 updated guidelines on arboviral disease management (including dengue) were published in July 2025, emphasizing against corticosteroid use in non-severe dengue (conditional recommendation, low certainty evidence)

Key Points Summary

2025 Note: WHO issued updated clinical management guidelines for dengue and other arboviral diseases in July 2025, recommending against corticosteroid use in non-severe dengue. In 2025, dengue surged globally with >3.6 million reported cases across ~100 countries - the highest burden in recent years.

Dengue Virus - Microbiology

1. Classification & Taxonomy

2. Viral Structure

• Lipid envelope - derived from host cell membrane during budding; contains two surface glycoproteins:
  • Envelope (E) protein - the major surface glycoprotein; mediates receptor binding and membrane fusion; primary target for neutralizing antibodies; varies by 30-40% among the four serotypes - this divergence underlies non-neutralizing cross-reactive antibody formation
  • Membrane (M) protein (mature form of prM) - present in mature virions; prM is the precursor form in immature virions
• Nucleocapsid - the E and M proteins surround an icosahedral nucleocapsid
• Capsid (C) protein - encloses the +ssRNA genome

Genome Organization

3. Replication Cycle

1. Attachment - E protein binds host cell receptors (heparan sulfate proteoglycans, DC-SIGN/CD209, TIM-1, AXL, etc.) on monocytes, macrophages, dendritic cells, and endothelial cells
2. Entry - Receptor-mediated endocytosis; the E protein undergoes acid-triggered conformational change in the endosome → membrane fusion → release of nucleocapsid into cytoplasm
3. Translation - Positive-sense RNA acts directly as mRNA → ribosomes on the rough ER translate the single polyprotein
4. Polyprotein processing - NS2B/NS3 protease (viral) + host signal peptidase cleave the polyprotein into the 10 individual proteins
5. RNA replication - NS proteins (particularly NS3 helicase + NS5 RdRp) assemble a replication complex on ER-derived membranes; the +ssRNA is copied to a -ssRNA intermediate, then to new +ssRNA genomes
6. Assembly - New +ssRNA genomes packaged with C protein to form nucleocapsids → bud into the lumen of the ER acquiring prM and E glycoproteins; produces immature virions
7. Maturation - In the trans-Golgi, low pH causes conformational change → host furin protease cleaves prM to M → produces mature infectious virions
8. Release - Exocytosis via secretory vesicles; mature virions released into the extracellular space

4. Antigenic Properties & Serotypes

• Four serotypes exist: DENV-1, DENV-2, DENV-3, DENV-4
• Distinguished by neutralization tests and molecular assays (RT-PCR serotyping)
• The envelope (E) protein diverges by 30-40% between serotypes - sufficient to prevent cross-neutralization
• All four serotypes are antigenically related (cross-reactive IgG) but require distinct serotype-specific antibodies for neutralization
• DENV-2 is most commonly associated with severe dengue disease

5. Immune Response & Antibody-Dependent Enhancement (ADE)

Primary Infection

• Stimulates serotype-specific neutralizing antibodies (lifelong protection against that serotype)
• Also stimulates cross-reactive, non-neutralizing IgG against other serotypes
• Cross-protection against other serotypes lasts 1-3 years only

Secondary Infection (Different Serotype)

1. Pre-existing non-neutralizing cross-reactive antibodies from the first infection bind the new serotype
2. These sub-neutralizing antibody-virus complexes enter Fc receptor-bearing cells (monocytes, macrophages, dendritic cells) at dramatically higher rates than viral entry without antibody
3. This ADE results in:
   • Higher viral load in mononuclear phagocytes
   • Massive release of cytokines and vasoactive mediators (TNF-α, IL-6, IL-10)
   • Activation of procoagulants → DIC component
   • Endothelial dysfunction → plasma leakage
4. Cross-reactive CD4+ and CD8+ T cells (original antigenic sin) may also amplify immunopathology in secondary infection

The same mechanism explains severe dengue in infants of dengue-immune mothers - maternally acquired IgG mediates ADE upon their first natural infection.

6. Vector Biology

7. Laboratory Diagnosis (Microbiology Perspective)

Specimen: Acute-phase serum

Key Diagnostic Principle

• Days 1-7: RT-PCR + NS1 antigen (combined gives ≥90% sensitivity for primary + secondary cases)
• After Day 5: IgM ELISA becomes the preferred test
• Serologic cross-reactivity with other flaviviruses (Zika, WNV, JEV, YFV, and vaccines) complicates IgG/IgM interpretation - always correlate with travel and vaccination history

Dengue diagnostic test sensitivity over time - Goldman-Cecil Medicine

8. Manifestations of Dengue Virus Infection (Classification Tree)

Park's Textbook of Preventive and Social Medicine

9. Immunity & Infection Ratios

• Ratio of inapparent to apparent infections: approximately 15:1 in primary infections; lower in secondary infections (more symptomatic)
• Approximately 75% of dengue infections are asymptomatic
• Risk of DHF: ~0.2% in primary infection; at least 10-fold higher in secondary infection with a different serotype
• CFR of DHF/DSS: up to 15% untreated; <1% with proper fluid management

10. Inactivation & Physical Properties

• Acid pH
• Heat
• Lipid solvents (ether, chloroform)
• Detergents
• Bleach
• Phenol
• 70% alcohol
• Formaldehyde

11. Quick Microbiology Summary Card