🦟 Dengue Fever - Complete Conceptual Notes

(MI 3.9 | As per Apoorba Sastry / Standard Microbiology Curriculum)

PART 1 - THE VIRUS

Classification (Mnemonic: "FARSS4")

Mnemonic: "DENSE FEVER"

DEN viruses 1-4
Enveloped flavivirus
Non-structural protein 1 (NS1) is key diagnostic antigen
Single stranded +ve sense RNA
Enzyme: RNA-dependent RNA polymerase
Flaviviridae family
Egypti mosquito (Aedes) = vector
Viremia lasts 7-10 days
Endemic in >100 countries
Reinfection with different serotype = severe disease

Structural Proteins (Important for NS1 Understanding)

DENGUE VIRUS GENOME
┌─────────────────────────────────────────────────────────┐
│  5'cap ─── Structural Proteins ──── Non-structural proteins ─── 3'UTR │
│              C   prM   E              NS1  NS2A  NS2B  NS3  NS4A  NS4B  NS5 │
└─────────────────────────────────────────────────────────┘

C  = Capsid protein (protects RNA)
prM = Precursor Membrane protein (prevents premature fusion)
E  = Envelope glycoprotein (binds host receptor → key for entry)
NS1 = Non-Structural Protein 1 (secreted into blood → DIAGNOSTIC MARKER)
NS3 = Protease + Helicase
NS5 = RNA-dependent RNA polymerase

PART 2 - EPIDEMIOLOGY & TRANSMISSION

Vector - "4As"

• Aedes aegypti = primary vector (urban)
• Aedes albopictus = secondary vector (less common)
• Aedes polynesiensis = tertiary
• Aedes are day-biters (peak morning + late afternoon)

Transmission Cycle

URBAN CYCLE (Dengue = Urban cycle, same as Yellow Fever)

    🦟 Infected Aedes aegypti
         ↓ bites
    🧑 Viremic Human (virus circulates in blood for 7-10 days)
         ↓ Aedes takes blood meal during viremia
    🦟 Mosquito acquires virus (extrinsic incubation: 8-12 days)
         ↓ virus replicates in mosquito salivary glands
    🧑 New susceptible human bitten → infection

No animal reservoir in urban cycle → Human IS the amplifying host
(Sylvatic cycle: monkeys in forests of Africa/SE Asia - rarely crosses to humans)

PART 3 - PATHOGENESIS

Concept 1: Primary vs Secondary Infection

PRIMARY INFECTION (1st serotype encountered)
───────────────────────────────────────────
Virus → enters via mosquito bite
       ↓
Infects dendritic cells at skin (Langerhans cells)
       ↓
Travels to regional lymph nodes → replication
       ↓
Viremia → infects monocytes/macrophages
       ↓
Mild-moderate dengue fever (DF)
       ↓
Serotype-specific lifelong immunity develops
Cross-reactive immunity to OTHER serotypes → temporary (1-3 years)

SECONDARY INFECTION (different serotype)
────────────────────────────────────────
New serotype enters bloodstream
       ↓
Cross-reactive (NON-NEUTRALIZING) antibodies from previous infection are present
       ↓
These antibodies bind virus BUT CANNOT neutralize it
       ↓
Virus-antibody complex enters MACROPHAGES via Fc receptors
(antibodies "escort" virus INTO macrophages = Trojan horse!)
       ↓
Massive viral replication inside macrophages (new tropism)
       ↓
Macrophages secrete: IFN-γ, TNF-α, IL-6, other cytokines
       ↓
CYTOKINE STORM
       ↓
↑ Vascular permeability + coagulation defects + platelet destruction
       ↓
DENGUE HEMORRHAGIC FEVER (DHF) / DENGUE SHOCK SYNDROME (DSS)

This is called: Antibody-Dependent Enhancement (ADE)

Fc receptor mediated entry
Enhancing (non-neutralizing) antibodies
New serotype (second infection)
Cytokine storm ensues
Exacerbated disease (DHF/DSS)

Concept 2: Why Platelets Fall?

1. Immune complex deposition on platelet surface → complement activation → platelet destruction
2. Cross-reactive antibodies against platelets (molecular mimicry with platelet surface antigens)
3. Endothelial damage → platelet consumption + splenic sequestration

Concept 3: Three Phases of Dengue Disease

FEBRILE PHASE (Day 1-3)
┌─────────────────────────────────────────────┐
│ • High fever (39-40°C), sudden onset        │
│ • Severe myalgia/"breakbone fever"          │
│ • Retro-orbital pain, headache              │
│ • Maculopapular rash, flushed face          │
│ • Leukopenia, thrombocytopenia begins       │
│ • VIRUS IS DETECTABLE (RT-PCR, NS1+, culture)│
└─────────────────────────────────────────────┘
              ↓
CRITICAL PHASE (Day 4-6, around defervescence)
┌─────────────────────────────────────────────┐
│ • Plasma leakage = HALLMARK               │
│ • ↑ Hematocrit (hemoconcentration ≥20%)   │
│ • Pleural effusion, ascites               │
│ • Platelet count ≤100,000/mm³             │
│ • WARNING SIGNS: abdominal pain,          │
│   persistent vomiting, mucosal bleeding,  │
│   restlessness, liver >2 cm              │
│ → DHF/DSS if severe                      │
└─────────────────────────────────────────────┘
              ↓
RECOVERY PHASE (Day 7-10)
┌─────────────────────────────────────────────┐
│ • Plasma reabsorption                     │
│ • "Isles of white in sea of red" rash     │
│ • Bradycardia (convalescent sign)         │
│ • Platelet rises (later than WBC rise)   │
│ • Risk: fluid overload if over-transfused │
└─────────────────────────────────────────────┘

PART 4 - LABORATORY DIAGNOSIS

The Big Picture: WHAT to test WHEN

                TIMELINE OF DENGUE LAB MARKERS

    Fever onset
         │
Day 1 ───┼──── NS1 Antigen APPEARS ✅
         │     RT-PCR positive ✅
         │     Virus isolation possible ✅ (up to Day 5-6)
         │
Day 3 ───┼──── IgM begins to appear (primary infection)
         │     (3-5 days after onset)
         │
Day 5 ───┼──── IgM detectable in >90% by Day 10
         │     (NS1 still positive up to Day 9-10)
         │
Day 7 ───┼──── NS1 DISAPPEARS around Day 9-10
         │     RT-PCR becomes less sensitive
         │
Day 10 ──┼──── IgM peaks at ~2 weeks
         │     IgG rises (in secondary infections, IgG rises EARLY - by Day 4)
         │
Day 15+ ─┼──── Convalescent serology (4-fold rise in IgG)
         │     IgG persists for LIFE

KEY DIAGNOSTIC TESTS

1. Virus Isolation

• Sample: Acute serum/plasma (Days 1-5), buffy coat, autopsy tissue
• Method: Mosquito cell culture (C6/36 cells) or mosquito inoculation
• Time: 1 week or more
• Use: Research/reference labs only

2. RT-PCR (Gold Standard for early phase)

• Detects viral RNA
• Day 1 - Day 7 of illness
• Offers serotyping
• Time: 1-2 days
• Advantage: High sensitivity + specificity + serotyping possible
• Real-time RT-PCR is superior (faster, no post-PCR contamination risk)

3. NS1 Antigen Detection ⭐ (High-yield)

• Non-Structural Protein 1 - secreted by all 4 DENV serotypes into bloodstream
• Appears in serum from Day 1 of fever
• Present up to Day 9-10 of illness
• Detectable in BOTH primary AND secondary infections

• NS1 ELISA (takes 1 day)
• NS1 Rapid Test (card test, results in minutes)
• Immunohistochemistry (tissue)

• Early diagnosis (before antibodies appear)
• Works for all 4 serotypes
• Does NOT require paired sera
• Commercial kits widely available

4. Serology - IgM & IgG (MAC-ELISA) ⭐⭐

PRIMARY INFECTION:
• Low IgG early (rises slowly after 2 weeks)
• Prominent IgM (high IgM/IgG ratio > 1.2)
• Diagnosis: IgM detection confirms recent primary infection

SECONDARY INFECTION (ADE prone):
• IgG rises VERY EARLY (by Day 4) and rapidly - "anamnestic response"
• IgM may be low or appear late
• Low IgM/IgG ratio (< 1.2)
• Diagnosis: High IgG with low IgM = secondary infection
           Fourfold rise in IgG titer between acute and convalescent sera

More IgM = first (primary) infection
Greater IgG early = secondary (Grave danger - ADE risk!)
Measure both together for best accuracy

5. Haemagglutination Inhibition Assay (HIA)

• Detects antibodies that inhibit dengue virus from agglutinating RBCs
• For diagnosis: Single titer ≥1:1280 = presumptive
• For confirmation: 4-fold rise between acute and convalescent
• Time: Minimum 7 days (requires paired sera)
• Less used now (replaced by ELISA)

6. Complement Fixation (CF) & Neutralization Test (NT)

• CF: Less sensitive than HIA, rarely used
• Neutralization (Plaque Reduction Neutralization Test - PRNT): Gold standard for serotyping and vaccine evaluation, but complex and slow

7. Rapid Diagnostic Tests (RDTs)

• Lateral flow immunochromatography
• Detects IgM + IgG ± NS1 in 15 minutes
• Results in 15 minutes, no equipment needed
• Limitation: Variable accuracy, not all validated; NS1 + IgM combo RDTs are best

8. Haematological Parameters

• Platelet count ≤100,000/mm³ = thrombocytopenia (key for DHF diagnosis)
• Hematocrit ≥20% rise from baseline = hemoconcentration/plasma leakage (key for DHF)
• Leukopenia (in febrile phase)
• WBC begins to rise after defervescence (earlier than platelet recovery)

DIAGNOSTIC SUMMARY TABLE (Park's PSM Table 2 Format)

PART 5 - WHO CASE DEFINITIONS

Probable Case

• Acute febrile illness PLUS supportive serology (HIA titer ≥1:1280, or positive IgM, or comparable IgG ELISA)
• AND at same location/time as confirmed cases

Confirmed Case

1. Virus isolation from serum/CSF/autopsy
2. 4-fold rise in IgG (HIA) or rise in IgM antibody
3. Detection of dengue antigen in tissue/serum/CSF by IHC, IF, or ELISA
4. Detection of viral genome by RT-PCR

DHF Diagnosis (All 4 criteria needed):

1. Fever 2-7 days
2. Hemorrhagic manifestations (tourniquet test +, petechiae, purpura, bleeding)
3. Platelet ≤100,000/mm³
4. Evidence of plasma leakage (hematocrit rise ≥20%, pleural effusion/ascites/hypoalbuminemia)

Dengue Shock Syndrome = DHF + shock signs

MASTER FLOWCHART

PATIENT PRESENTS WITH ACUTE FEVER (suspected dengue)
                   │
          Is it Day 1-5 of fever?
         /                      \
       YES                       NO (Day 5+)
        │                         │
  Run BOTH:                  Serology:
  1. NS1 Ag (rapid/ELISA)    IgM ELISA (MAC-ELISA)
  2. RT-PCR                  IgG ELISA
        │                         │
  +ve NS1 or PCR?          IgM positive?
    → Early confirmation      → Recent infection confirmed
        │                    IgM−, IgG high early?
  Also send:                  → Secondary infection (ADE risk!)
  IgM (may be -ve yet)             │
        │                    Paired sera (Day 15-21)?
        └──────────┬──────────┘    → 4-fold IgG rise = confirmed
                   │
          MONITOR: Platelet count + Hematocrit daily
                   │
        Platelet ≤100k + Hematocrit ≥20% rise?
                   │
                  DHF
                   │
           Shock signs present?
                   │
                  DSS

QUICK REVISION MNEMONICS

"NS1 STAT" - for NS1 features:

Non-structural protein 1
Secreted into blood (unique)
1 day 1 of fever it appears
Serotype-agnostic (all 4 serotypes)
Ten days = disappears by day 9-10
Acute infection marker
Tests: rapid card test or ELISA

"ADE = Always Dangerous, Especially (in 2nd infection)":

Antibody Dependent Enhancement occurs in secondary infection
Non-neutralizing IgG escorts virus into macrophage via Fc receptor
Cytokine storm → DHF/DSS

"4S of Dengue Serotypes":

4 Serotypes (DENV 1-4)
Specific lifelong immunity to ONE
Short-term cross-protection to others
Second infection with different serotype → Severe disease

Clinical Phases: "FCR" (Febrile → Critical → Recovery)

• Febrile: Fever, flu-like, rash (Day 1-3)
• Critical: plasma leak, Shock risk (Day 4-6)
• Recovery: Reabsorption, Rash "isles of white" (Day 7-10)

SPECIAL POINTS (Frequently Asked in Exams)

1. NS1 vs IgM timing: NS1 appears EARLIER (Day 1) vs IgM (Day 3-5) → NS1 is the better early marker
2. Best test in 1st week: NS1 Antigen + RT-PCR (combo)
3. Best test after Day 5: IgM MAC-ELISA
4. Gold standard for confirmation: RT-PCR (early) or Virus isolation
5. Gold standard for serotyping: Plaque Reduction Neutralization Test (PRNT)
6. Combining NS1 + IgM in single serum collected in first 10 days identifies ≥90% of both primary and secondary dengue cases (Red Book 2021)
7. IgG rises early in secondary infection (by Day 4) due to anamnestic response - this is the key differentiator
8. Dengue is NOT spread person-to-person (no airborne, contact, or sexual route in routine clinical practice)
9. Wolbachia mosquitoes: New vector control - Aedes mosquitoes infected with Wolbachia bacteria reduce dengue transmission by 95-97% (Robbins 2025)
10. Vaccine (Dengvaxia/CYD-TDV): Tetravalent recombinant live attenuated; approved 9-45 years; ONLY recommended in seropositive individuals (if given to seronegative = risk of ADE on first natural infection!)