Medicine chapters
Dengue fever
Dengue is a mosquito-borne flavivirus infection. Learn its three phases, the warning signs of plasma leakage, the right test for each day of illness and safe fluid management.
- Dengue is a flavivirus (four serotypes) spread by the day-biting Aedes aegypti mosquito, causing high fever, retro-orbital pain, myalgia, rash and low platelets.
- The danger is the critical phase around defervescence (days 3–7), when plasma leaks out of capillaries: haematocrit rises, platelets fall and shock can follow.
- Diagnose with NS1 antigen or PCR in the first 5 days, IgM after day 5; treat with paracetamol and careful fluids, never NSAIDs or aspirin.
On this page
Overview
Dengue is the commonest arthropod-borne viral infection of humans. It is endemic across the tropics and has caused large outbreaks in Pakistan, especially after the monsoon (roughly August to November).
Most infections are mild or silent. A small number develop severe dengue, with plasma leakage, shock, bleeding or organ failure. Deaths are mostly preventable with early recognition and careful fluid therapy.
Aetiology and transmission
- Agent: dengue virus, a flavivirus (same family as yellow fever, Zika, West Nile, Japanese encephalitis). There are four serotypes (DENV 1–4).
- Vector: Aedes aegypti (main) and Aedes albopictus. They bite in the daytime, especially early morning and late afternoon, and breed in clean stored water (tyres, pots, coolers, flower vases).
- Humans are infectious to mosquitoes during the viraemic period (first few days of fever).
- Incubation: about 3–10 days, commonly 5–6 days.
Immunity and severe disease
- Infection gives lifelong immunity to the same serotype, but only brief cross-protection against the others.
- A second infection with a different serotype carries a higher risk of severe dengue. Non-neutralising antibodies from the first infection help the new virus enter cells (antibody-dependent enhancement).
Pathophysiology
- The key event in severe dengue is increased capillary permeability: plasma leaks into the pleural and peritoneal spaces.
- Loss of plasma volume causes haemoconcentration (rising haematocrit) and, if severe, hypovolaemic shock.
- Thrombocytopenia is due to marrow suppression plus immune-mediated destruction of platelets.
- Bleeding comes from low platelets, vascular fragility and, in prolonged shock, DIC.
Clinical features: the three phases
| Phase | Timing | What happens | Danger |
|---|---|---|---|
| Febrile | Days 1–3 (to 7) | High fever, headache, retro-orbital pain, severe myalgia and back pain ("breakbone fever"), flushing, rash | Dehydration; febrile fits in infants |
| Critical | Around defervescence, days 3–7, lasts 24–48 hours | Plasma leakage: rising haematocrit, falling platelets, pleural effusion, ascites | Shock, bleeding, organ damage |
| Recovery | Next 48–72 hours | Leaked fluid is reabsorbed; appetite and diuresis return; itchy rash ("isles of white in a sea of red"); bradycardia | Fluid overload if IV fluids are not reduced |
The patient is most at risk just as the fever settles. Falling temperature does not mean recovery in dengue.
Classic dengue fever
- Sudden high fever, retro-orbital pain worse on eye movement, headache, severe back and muscle pain.
- Flushed face, conjunctival suffusion, maculopapular rash.
- Fever may be biphasic ("saddleback"): it falls, then returns briefly.
- Positive tourniquet test: cuff at mid-point between systolic and diastolic for 5 minutes; positive with 10 or more petechiae per square inch.
Warning signs (need admission and close watch)
- Abdominal pain or tenderness
- Persistent vomiting
- Clinical fluid accumulation (pleural effusion, ascites)
- Mucosal bleeding
- Lethargy or restlessness
- Liver enlargement more than 2 cm
- Rising haematocrit with a rapid fall in platelets
Severe dengue (any one of)
- Severe plasma leakage causing shock or fluid accumulation with breathing difficulty
- Severe bleeding
- Severe organ involvement (liver, brain, heart)
Dengue shock
- Early shock is compensated: the patient is alert, systolic BP may look normal, but there is tachycardia, cold peripheries, delayed capillary refill and a narrow pulse pressure (20 mmHg or less), for example 100/80.
- Late shock: blood pressure becomes unrecordable; acidosis, DIC and major bleeding follow.
In dengue shock the diastolic pressure rises towards the systolic. A "normal" systolic reading can hide shock.
Older WHO grading (still asked)
| Grade | Features |
|---|---|
| DF | Fever with two or more typical symptoms; no plasma leakage |
| DHF I | Fever + positive tourniquet test; platelets 100,000/mm3 or less; haematocrit rise 20% or more |
| DHF II | Grade I + spontaneous bleeding |
| DHF III | Circulatory failure (weak fast pulse, narrow pulse pressure, cold skin) |
| DHF IV | Profound shock, undetectable BP or pulse |
Grades III and IV are dengue shock syndrome.
Investigations
Blood counts
- Leucopenia early, then thrombocytopenia.
- Haematocrit: record a baseline early; a rise of 20% or more shows significant plasma leakage.
- Raised AST and ALT (AST often higher).
Baseline haematocrit on day 2: 40%
Haematocrit on day 5: 48%
Rise = (48 − 40) ÷ 40 × 100 = 20% → significant plasma leakage
Confirming dengue: test by day of illness
| Day of illness | Best test | Why |
|---|---|---|
| Days 1–5 | NS1 antigen (rapid or ELISA) or RT-PCR | Virus and its proteins are in the blood during viraemia |
| After day 5 | IgM ELISA (IgM capture) | Antibodies appear as viraemia ends |
| Paired sera | Rising IgG | Retrospective confirmation; high early IgG suggests secondary infection |
NS1 antigen is the test of choice in the first few days; IgM is useful only after about day 5.
Imaging
- Ultrasound: gall bladder wall oedema, ascites, pleural effusion (may precede clinical leakage).
- Chest X-ray: pleural effusion, mostly right-sided.
Differential diagnosis
| Condition | Clues that help separate it |
|---|---|
| Malaria | Rigors, splenomegaly, positive film or RDT |
| Typhoid | Gradual onset, relative bradycardia, abdominal symptoms, blood culture |
| Chikungunya | Same mosquito; severe, lasting joint pain; platelets less low |
| Leptospirosis | Flood or sewage exposure, calf pain, conjunctival suffusion, jaundice with kidney injury |
| CCHF | Tick bite or animal blood contact (often around Eid ul-Adha), early severe bleeding, very high AST |
| COVID-19, influenza | Respiratory symptoms; platelets usually less affected |
Management
Outpatient (no warning signs, drinking well, passing urine)
- Paracetamol for fever (no more often than every 6 hours), tepid sponging.
- Plenty of oral fluids: ORS, juices, soups.
- Avoid aspirin, ibuprofen and other NSAIDs (bleeding, gastritis; aspirin also risks Reye syndrome in children).
- Avoid IM injections.
In dengue, give paracetamol only. Aspirin and NSAIDs worsen bleeding and gastritis.
- Daily review of fluid intake, urine output, warning signs, haematocrit and platelets.
- Return at once if warning signs appear.
Inpatient: dengue with warning signs
- Check haematocrit before fluids.
- Start isotonic crystalloid (normal saline or Ringer's lactate), for example 6 ml/kg/hour for 1–2 hours, then step down (3 ml/kg/hour) as haematocrit and vital signs improve.
- If haematocrit keeps rising or vitals worsen, increase the rate (for example 10 ml/kg/hour) and reassess.
- Stop IV fluids within 24–48 hours once stable; the leak stops on its own.
Adult with warning signs, weight 60 kg
Initial crystalloid 6 ml/kg/h = 6 × 60 = 360 ml per hour for 1–2 hours
Step down to 3 ml/kg/h = 180 ml per hour when improving
Dengue shock
- Rapid crystalloid bolus (10–20 ml/kg over 1 hour), oxygen, then reassess.
- If no improvement and haematocrit still high: give colloid.
- If no improvement and haematocrit falling: suspect hidden bleeding; give blood.
- Monitor urine output, pulse pressure and haematocrit closely.
Platelets and blood
- No prophylactic platelet transfusion, even with very low counts, if there is no significant bleeding. It does not prevent bleeding and can cause fluid overload.
- Platelets recover by themselves in the recovery phase.
- Give blood for significant bleeding or shock with a falling haematocrit.
- Steroids have no role.
A falling haematocrit in a shocked patient is not a sign of improvement. It suggests internal bleeding.
In the recovery phase, keep giving IV fluids and you cause pulmonary oedema. Most deaths come from profound shock, often worsened by fluid overload.
Prevention
- Source reduction: empty, cover or clean water containers weekly; remove tyres and junk.
- Repellents and long sleeves during the day; nets help mainly for daytime sleepers and patients.
- Space spraying (fogging) during outbreaks.
- A dengue vaccine exists, but it can increase the risk of severe dengue in people never infected before, so it is restricted to those with proof of previous infection.
Pakistan note: provincial and national (NIH Pakistan) dengue guidelines set local admission and fluid protocols. These are revised between seasons, so check the current guideline.
"When the fever goes, the danger shows." Watch the haematocrit and pulse pressure as the temperature falls.
Expect: Aedes aegypti as a day-biter; four serotypes and severe disease with a second serotype; the critical phase at defervescence; rising haematocrit with falling platelets; warning signs; narrow pulse pressure in compensated shock; NS1 or PCR early versus IgM after day 5; paracetamol and avoidance of NSAIDs; no prophylactic platelet transfusion; fluid overload in the recovery phase; and differentiating dengue from malaria, typhoid, chikungunya and CCHF.
Quick revision
- Flavivirus, 4 serotypes, Aedes aegypti (day-biting, clean water).
- Febrile → critical (days 3–7, leak for 24–48 h) → recovery (reabsorption).
- Leak = haematocrit up 20% or more, platelets down, effusions, gall bladder wall oedema.
- Shock: pulse pressure 20 mmHg or less, cold peripheries, patient often alert.
- NS1/PCR in days 1–5; IgM after day 5.
- Paracetamol, oral fluids; no NSAIDs, aspirin or steroids.
- No prophylactic platelets; blood if bleeding.
- Reduce fluids in recovery to avoid overload.
Written by The House Job from standard references (Kumar and Clark's Clinical Medicine; Park's Textbook of Preventive and Social Medicine).
FAQ
Why is dengue most dangerous when the fever settles?
Defervescence often coincides with the critical phase, when capillaries leak plasma. Haematocrit rises and shock can develop over 24–48 hours.
Which test should be sent on day 2 of fever?
NS1 antigen or RT-PCR, because the virus is circulating early. IgM is often negative until about day 5.
Should platelets be transfused at a count of 15,000 without bleeding?
No. Prophylactic platelet transfusion does not prevent bleeding in dengue and can cause harm; counts recover on their own.
What does a falling haematocrit in a shocked dengue patient mean?
Suspect internal bleeding. Give blood rather than more crystalloid.