Diagnosis of Dengue Fever - Study Notes

1. The Dengue Virus - Quick Overview

• Causative agent: Dengue virus (DENV), a flavivirus - 4 serotypes (DENV 1-4)
• Vector: Aedes aegypti and Aedes albopictus mosquitoes (daytime biters)
• Incubation period: 3-10 days (commonly 5-6 days)
• Severe dengue is most commonly associated with DENV-2 and secondary infections (a second infection with a different serotype carries higher risk)

2. Clinical Classification of Dengue Infection

3. Clinical Phases of Dengue

Phase 1: Febrile Phase

• Sudden onset, high fever (39-40°C), chills
• Intense headache, retro-orbital pain (especially on eye movement or pressure)
• Myalgia, arthralgia ("breakbone fever")
• Photophobia, extreme weakness, anorexia
• Maculopapular rash, flushed face
• Tourniquet test (Rumpel-Leede test) may be positive

Phase 2: Critical Phase (Days 3-7)

• Temperature drops to 37.5-38°C (defervescence)
• Increased capillary permeability → plasma leakage begins
• Rising haematocrit + rapid fall in platelet count
• Warning signs: abdominal pain/tenderness, persistent vomiting, mucosal bleeding, restlessness, liver enlargement >2 cm, oliguria
• Pleural effusion (usually right-sided), ascites

Phase 3: Recovery Phase

• Plasma leakage resolves
• Reabsorption of fluids, risk of fluid overload
• Bradycardia may occur

4. Diagnostic Criteria

Probable Dengue Fever

• Headache
• Retro-orbital pain
• Myalgia / Arthralgia / Bone pain
• Rash
• Haemorrhagic manifestations
• Leucopenia (WBC ≤ 5,000 cells/mm³)
• Thrombocytopenia (Platelets < 150,000 cells/mm³)
• Rising haematocrit (5-10%)

• Supportive serology (HI titre ≥ 1:1280, comparable IgG on ELISA, or positive IgM)
• Occurrence at same location/time as confirmed dengue cases

Confirmed Dengue Fever

• Virus isolation from serum, CSF, or autopsy tissue
• Fourfold or greater rise in IgG (by HI test) or rise in IgM to dengue virus
• Antigen detection in tissue/serum/CSF (immunohistochemistry, immunofluorescence, ELISA)
• RT-PCR detection of dengue viral genome

Dengue Haemorrhagic Fever (DHF) - All 4 must be present:

1. Acute fever lasting 2-7 days
2. Haemorrhagic manifestations (positive tourniquet test, petechiae, ecchymoses, purpura, or bleeding from mucosa/GI tract)
3. Platelet count ≤ 100,000 cells/mm³
4. Objective evidence of plasma leakage:
   • Haematocrit rise ≥ 20% from baseline, OR
   • Pleural effusion / ascites / hypoproteinaemia

Dengue Shock Syndrome (DSS)

• Tachycardia, cool extremities, delayed capillary refill, weak pulse
• Pulse pressure ≤ 20 mmHg (e.g., BP 100/80 mmHg)
• Hypotension: systolic BP < 80 mmHg (age < 5 years) or 80-90 mmHg (older children/adults)

5. Laboratory Diagnosis - Detailed

A. Virus Isolation

• Best specimen: Acute-phase serum taken within the first 6 days of illness
• Other specimens: plasma, washed buffy coat, autopsy tissue (liver, spleen, lymph nodes, thymus)
• Gold standard but slow turnaround

B. Viral Nucleic Acid Detection (RT-PCR)

• Detects dengue RNA
• Most useful in early illness (days 1-5) - during viraemia
• Better specificity than simple PCR; faster than virus isolation
• Can serotype the virus

C. Serological Tests

Important: IgG remains positive for life and is NOT useful for diagnosing new acute infections. Cross-reactivity with other flaviviruses (Zika, West Nile, Japanese encephalitis, yellow fever) can cause false-positive serology. Plaque Reduction Neutralization Test (PRNT) is needed to distinguish dengue from Zika.

D. NS1 Antigen Detection

• Detects dengue nonstructural protein-1 (NS1)
• Can be detected in both primary and secondary infections
• Positive for up to 6 days after illness onset
• Commercial rapid kits available; does not differentiate serotypes
• A key early diagnostic marker

E. Rapid Diagnostic Tests (RDT)

• Detect anti-dengue IgM/IgG antibodies
• Results within 15 minutes
• Accuracy varies; many not fully validated yet
• Used in field/resource-limited settings

F. Haematological Parameters

• Platelet count - falls progressively; < 100,000/mm³ in DHF
• Haematocrit - rises ≥ 20% indicating plasma leakage
• Leucopenia - WBC ≤ 5,000/mm³ is a diagnostic feature
• Monitor daily during critical phase

6. Diagnostic Tests Summary by Timing

7. Tourniquet Test (Rumpel-Leede Sign)

• Inflate BP cuff to midpoint between systolic and diastolic for 5 minutes
• Positive: ≥ 10 petechiae per 2.5 × 2.5 cm (1 inch square)
• In DHF: usually ≥ 20 petechiae (strongly positive)
• Indicates capillary fragility / thrombocytopenia

8. Differential Diagnosis

• Other arboviral fevers (Chikungunya, Zika, West Nile)
• Malaria
• Leptospirosis
• Typhoid fever
• Viral hepatitis
• Meningococcemia

9. Key Points to Remember for Exam

1. NS1 antigen = best early marker (days 1-6)
2. RT-PCR = best for confirmation in first 5 days
3. IgM (MAC-ELISA) = appears from day 3-5, stays positive for weeks
4. IgG = not useful for acute diagnosis (stays positive for life)
5. Tourniquet test is the most common haemorrhagic sign in dengue
6. Platelet ≤ 100,000 + haematocrit rise ≥ 20% = hallmarks of DHF
7. Cross-reactivity with Zika and other flaviviruses is a major pitfall of serology
8. Reference testing for dengue is available through the CDC dengue branch