Dengue
Published on September 20, 2026
Risk Factors
Residents or travelers in tropical/subtropical endemic areas (Southeast Asia, Latin America, Caribbean, Pacific Islands); prior dengue infection with a different serotype (risk for severe disease); rainy season; urban/peri-urban settings with standing water; lack of vector control
Etiology
Dengue virus (DENV), a single-stranded RNA Flavivirus with 4 serotypes (DENV-1 through DENV-4), transmitted by the Aedes aegypti mosquito (daytime biter)
Presentation
Acute high-grade fever (40 C / 104 F), severe headache, retro-orbital pain, myalgia, arthralgia ("breakbone fever"), nausea/vomiting. In severe cases: abdominal pain, persistent vomiting, mucosal bleeding, lethargy
Classic Exam
Maculopapular or petechial rash (classically described as "islands of white in a sea of red"), facial flushing, positive tourniquet test (>20 petechiae per 1-inch square area), hepatomegaly, signs of plasma leakage (ascites, pleural effusion) in severe disease
Diagnostics
Thrombocytopenia (platelet count < 100,000/mm³), hemoconcentration (hematocrit rise ≥ 20% from baseline), leukopenia, elevated AST/ALT, positive NS1 antigen (within first 5 days), positive IgM anti-dengue (after day 5)
Management
Supportive care only: oral rehydration for mild disease, IV isotonic crystalloids (normal saline or Ringer's lactate) for warning signs or severe dengue, serial hematocrit and platelet monitoring, avoid aspirin and NSAIDs (bleeding risk), use acetaminophen for fever
Keywords
"Breakbone fever," "saddle-back fever," "islands of white in a sea of red," "retro-orbital pain," "tourniquet test positive," "hemoconcentration," "critical phase around defervescence," "secondary infection," "antibody-dependent enhancement"
01Pathophysiology
Dengue virus enters the bloodstream through the bite of an infected Aedes aegypti mosquito. The virus initially replicates in dendritic cells, macrophages, and monocytes at the site of inoculation, then disseminates through the lymphatic system and bloodstream, producing a systemic viremia. This viremia is directly responsible for the abrupt onset of high fever, and the intense activation of the immune response with cytokine release explains the severe myalgias and arthralgias that give dengue its classic nickname, "breakbone fever."
The most important concept to understand for exam purposes is antibody-dependent enhancement (ADE). When a patient is infected with one serotype of dengue (for example, DENV-1), they develop lifelong immunity to that serotype. However, upon reinfection with a different serotype (for example, DENV-2), pre-existing non-neutralizing antibodies from the first infection actually facilitate viral entry into Fc-receptor-bearing cells (monocytes and macrophages). This paradoxically increases viral replication and triggers an exaggerated immune response. The result is a cytokine storm with release of TNF-alpha, IL-6, IL-8, and other vasoactive mediators.
This cytokine storm causes increased vascular permeability, which is the central pathological event in severe dengue. Plasma leaks from the intravascular compartment into the interstitial space and serous cavities (pleural effusion, ascites). This explains the hemoconcentration (rising hematocrit) seen on labs, as plasma volume decreases while red cell mass remains the same. If plasma leakage is severe enough, it leads to hypovolemic shock (Dengue Shock Syndrome).
The thrombocytopenia in dengue results from two mechanisms: direct bone marrow suppression by the virus (reduced platelet production) and increased peripheral destruction through immune-mediated clearance and consumption. Combined with endothelial dysfunction, this creates a hemorrhagic tendency, manifesting as petechiae, mucosal bleeding, and in severe cases, gastrointestinal hemorrhage.
A critical timing concept is the "critical phase," which occurs around defervescence (when the fever breaks, typically days 3 to 7 of illness). Counterintuitively, the patient appears to improve as the fever resolves, but this is precisely when plasma leakage peaks and the risk of shock is highest. This is a frequently tested point.
02Classification and Clinical Manifestation
WHO 2009 Revised Classification
Classification | Clinical Features |
|---|---|
Dengue Without Warning Signs | Fever plus two or more of: nausea/vomiting, rash, aches and pains (myalgia, arthralgia, retro-orbital pain), leukopenia, positive tourniquet test |
Dengue With Warning Signs | All of the above plus any of: abdominal pain or tenderness, persistent vomiting, clinical fluid accumulation (ascites, pleural effusion), mucosal bleeding, lethargy or restlessness, hepatomegaly > 2 cm, rising hematocrit concurrent with rapid platelet drop |
Severe Dengue | One or more of: Severe plasma leakage leading to shock (Dengue Shock Syndrome) or respiratory distress from fluid accumulation; Severe bleeding (as evaluated by clinician); Severe organ involvement (AST or ALT ≥ 1000, impaired consciousness, myocarditis, or other organ failure) |
Classic Dengue Fever Phases
Febrile Phase
Timing
Days 1 to 3
Key Features
Abrupt high fever (39 to 40 C), facial flushing, generalized erythema, headache, retro-orbital pain, myalgia, arthralgia. Viremia is highest during this phase.
Critical Phase
Timing
Days 3 to 7 (around defervescence)
Key Features
Temperature drops, but plasma leakage peaks. Rising hematocrit, progressive thrombocytopenia, pleural effusion, ascites. Risk of shock is highest here. Lasts 24 to 48 hours.
Recovery Phase
Timing
Days 7 to 10
Key Features
Reabsorption of leaked plasma. Hematocrit stabilizes or drops (hemodilution), platelet count recovers. Bradycardia and a confluent rash with petechiae ("islands of white in a sea of red") may appear. Risk of fluid overload if IV fluids are not tapered.
Phase | Timing | Key Features |
|---|---|---|
Febrile Phase | Days 1 to 3 | Abrupt high fever (39 to 40 C), facial flushing, generalized erythema, headache, retro-orbital pain, myalgia, arthralgia. Viremia is highest during this phase. |
Critical Phase | Days 3 to 7 (around defervescence) | Temperature drops, but plasma leakage peaks. Rising hematocrit, progressive thrombocytopenia, pleural effusion, ascites. Risk of shock is highest here. Lasts 24 to 48 hours. |
Recovery Phase | Days 7 to 10 | Reabsorption of leaked plasma. Hematocrit stabilizes or drops (hemodilution), platelet count recovers. Bradycardia and a confluent rash with petechiae ("islands of white in a sea of red") may appear. Risk of fluid overload if IV fluids are not tapered. |
Dengue Hemorrhagic Fever Grading (WHO 1997, still commonly tested)
Grade | Clinical Criteria |
|---|---|
Grade I | Fever with nonspecific constitutional symptoms; only hemorrhagic manifestation is a positive tourniquet test |
Grade II | Grade I plus spontaneous bleeding (petechiae, ecchymoses, epistaxis, gingival bleeding, GI bleeding) |
Grade III | Grade II plus circulatory failure (rapid, weak pulse; narrow pulse pressure ≤ 20 mmHg; hypotension; cold clammy skin; restlessness) |
Grade IV | Profound shock with undetectable pulse and blood pressure |
Grades III and IV constitute Dengue Shock Syndrome (DSS).
03Diagnostic Workup
NS1 Antigen
Timing
Days 1 to 5 of fever
Purpose
Best initial test during febrile phase
Notes
High sensitivity early in illness; can confirm dengue before antibodies develop
IgM Anti-Dengue (MAC-ELISA)
Timing
Day 5 onward
Purpose
Primary diagnostic test after febrile phase
Notes
Appears from day 3 to 5 and peaks at 2 weeks; can cross-react with other flaviviruses (Zika, Chikungunya)
IgG Anti-Dengue
Timing
Primary infection: day 14 onward; Secondary infection: day 1 to 2
Purpose
Distinguish primary vs. secondary infection
Notes
Rapid early IgG rise suggests secondary infection (and therefore higher risk for severe dengue)
RT-PCR
Timing
Days 1 to 5
Purpose
Most accurate test (gold standard for serotype identification)
Notes
Confirms diagnosis and identifies the serotype; often used in reference labs, not bedside
CBC
Timing
Throughout illness
Purpose
Monitor disease severity
Notes
Leukopenia (WBC < 5,000), thrombocytopenia (platelets < 100,000), hemoconcentration (hematocrit rise ≥ 20%)
Liver Function Tests
Timing
Throughout illness
Purpose
Detect hepatic involvement
Notes
Elevated AST typically more than ALT; AST/ALT ≥ 1000 indicates severe organ involvement
Ultrasound (abdomen/chest)
Timing
Critical phase
Purpose
Detect plasma leakage
Notes
Gallbladder wall thickening (> 3 mm), ascites, right-sided pleural effusion (earliest sign of leakage on imaging)
Test | Timing | Purpose | Notes |
|---|---|---|---|
NS1 Antigen | Days 1 to 5 of fever | Best initial test during febrile phase | High sensitivity early in illness; can confirm dengue before antibodies develop |
IgM Anti-Dengue (MAC-ELISA) | Day 5 onward | Primary diagnostic test after febrile phase | Appears from day 3 to 5 and peaks at 2 weeks; can cross-react with other flaviviruses (Zika, Chikungunya) |
IgG Anti-Dengue | Primary infection: day 14 onward; Secondary infection: day 1 to 2 | Distinguish primary vs. secondary infection | Rapid early IgG rise suggests secondary infection (and therefore higher risk for severe dengue) |
RT-PCR | Days 1 to 5 | Most accurate test (gold standard for serotype identification) | Confirms diagnosis and identifies the serotype; often used in reference labs, not bedside |
CBC | Throughout illness | Monitor disease severity | Leukopenia (WBC < 5,000), thrombocytopenia (platelets < 100,000), hemoconcentration (hematocrit rise ≥ 20%) |
Liver Function Tests | Throughout illness | Detect hepatic involvement | Elevated AST typically more than ALT; AST/ALT ≥ 1000 indicates severe organ involvement |
Ultrasound (abdomen/chest) | Critical phase | Detect plasma leakage | Gallbladder wall thickening (> 3 mm), ascites, right-sided pleural effusion (earliest sign of leakage on imaging) |
The workup begins with a clinical suspicion based on the epidemiological context: a patient who lives in or recently traveled to a dengue-endemic area, presenting with acute high fever, headache, retro-orbital pain, and myalgias.
During the first 5 days of fever, the NS1 antigen is the best initial diagnostic test. It directly detects a viral protein in the serum and can return positive as early as day 1 of symptom onset. It does not require waiting for the immune response. If NS1 is positive in the right clinical setting, dengue is confirmed.
If the patient presents after day 5, NS1 sensitivity drops significantly, and you should pivot to IgM serology (MAC-ELISA). IgM typically becomes detectable between days 3 and 5 and peaks around day 14. A single positive IgM in an endemic area with compatible symptoms is considered a probable case. For definitive confirmation, a fourfold rise in IgG titers between acute and convalescent paired sera is used, though this is retrospective and not useful for acute management.
RT-PCR is the most accurate test and can identify the infecting serotype, but it is generally available only in reference laboratories and is most useful during the viremic phase (days 1 to 5). In practice, it is not the first test ordered in most clinical settings.
The complete blood count (CBC) is the cornerstone of monitoring disease severity. Serial measurements of the hematocrit are essential: a rise of ≥ 20% above baseline signals significant plasma leakage and entry into the critical phase. Thrombocytopenia (< 100,000/mm³) combined with hemoconcentration strongly suggests dengue hemorrhagic fever.
The tourniquet test (inflate a blood pressure cuff to the midpoint between systolic and diastolic for 5 minutes, then count petechiae) is a simple bedside screening tool. A result of > 20 petechiae per 1-inch² area is considered positive and supports the diagnosis, though it is not confirmatory on its own.
Abdominal ultrasound is highly valuable during the critical phase to detect early signs of plasma leakage, including gallbladder wall thickening, free fluid (ascites), and pleural effusion. A right-sided pleural effusion on lateral decubitus chest X-ray is often the earliest radiographic evidence of leakage.
04Management & Treatment
Scenario | Management |
|---|---|
Dengue Without Warning Signs (Group A) | Outpatient management: oral rehydration solution (ORS), adequate fluid intake (>5 glasses/day for adults), acetaminophen for fever (not exceeding 4 g/day in adults), daily follow-up for warning signs, serial platelet and hematocrit checks |
Dengue With Warning Signs (Group B) | Hospitalization: IV isotonic crystalloids (0.9% NS or Ringer's lactate) starting at 5 to 7 mL/kg/hr for 1 to 2 hours, then reduce to 3 to 5 mL/kg/hr for 2 to 4 hours, then 2 to 3 mL/kg/hr based on clinical response; monitor hematocrit every 4 to 6 hours; strict intake/output charting |
Severe Dengue / DSS (Group C) | Emergency resuscitation: IV crystalloid bolus at 10 to 20 mL/kg over 15 to 30 minutes, repeat if needed; if hematocrit is rising (plasma leakage), give additional crystalloid or switch to colloid (dextran 40 or starch); if hematocrit is falling with shock (hemorrhage), initiate blood transfusion (packed RBCs or fresh whole blood); consider ICU admission |
Severe Hemorrhage | Transfusion of packed red blood cells (10 mL/kg); fresh frozen plasma or platelet concentrate if DIC is present or platelets < 10,000 with active bleeding |
All patients | Avoid aspirin, ibuprofen, and all NSAIDs (increase bleeding risk); avoid intramuscular injections; avoid unnecessary IV procedures; use acetaminophen exclusively for pain and fever |
There is no antiviral therapy for dengue. Management is entirely supportive, and the central principle is judicious fluid resuscitation guided by serial hematocrit monitoring.
Group A (outpatient): Patients with dengue fever and no warning signs can be managed at home with oral rehydration. Encourage fluids such as ORS, coconut water, or rice water. Fever should be managed with acetaminophen (paracetamol) at standard dosing (500 to 1000 mg every 4 to 6 hours in adults, maximum 4 g/day). It is critical to avoid aspirin and NSAIDs because these drugs impair platelet function and increase the risk of hemorrhage. The patient must be counseled to return immediately if warning signs develop (persistent vomiting, abdominal pain, bleeding, lethargy). Daily follow-up with CBC is recommended during the critical phase window (days 3 to 7).
Group B (hospitalized): Patients with warning signs require admission and IV fluid therapy. Begin with isotonic crystalloids (0.9% normal saline or Ringer's lactate) at 5 to 7 mL/kg/hr. The goal is to maintain adequate perfusion while avoiding fluid overload. The rate is titrated down progressively over 24 to 48 hours as the patient stabilizes. Hematocrit is checked every 4 to 6 hours; a falling hematocrit with stable vital signs indicates successful volume replacement. A rising hematocrit despite fluid administration signals worsening leakage and may necessitate escalation to Group C management.
Group C (severe dengue): These patients are in or approaching shock and require emergency IV boluses. Start with 10 to 20 mL/kg of isotonic crystalloid over 15 to 30 minutes. Reassess clinically and with repeat hematocrit. If the hematocrit remains high and the patient is still in shock (indicating ongoing plasma leakage), a second crystalloid bolus or transition to colloid solutions (such as dextran 40) is appropriate. If the hematocrit drops during shock, suspect internal hemorrhage and initiate blood product transfusion. These patients belong in an ICU.
Recovery phase fluid management is equally important. Once the critical phase passes (usually 48 hours after defervescence), leaked plasma is reabsorbed back into the intravascular space. If IV fluids are not tapered promptly, the patient is at risk of pulmonary edema and respiratory distress from fluid overload. This is a commonly tested management pitfall.
Platelet transfusion is generally not indicated for thrombocytopenia alone. It is reserved for platelets < 10,000/mm³ with active hemorrhage, or for patients requiring invasive procedures with platelets below a safe threshold. Prophylactic platelet transfusion in dengue does not improve outcomes and is discouraged.
There is now a dengue vaccine (Dengvaxia / CYD-TDV) licensed in several countries. It is approved only for seropositive individuals (those with documented prior dengue infection), ages 9 to 45 years, living in endemic areas. Vaccinating seronegative individuals paradoxically increases the risk of severe dengue on first natural infection, mimicking the antibody-dependent enhancement mechanism. This is a high-yield vaccine concept.
05Differential Diagnosis & Distractors
Chikungunya
Why It Is Similar
Also transmitted by Aedes mosquitoes in endemic areas; presents with acute fever, arthralgia, rash
Key Discriminator
Chikungunya causes severe, debilitating polyarthralgia (often symmetric, small joints) that can persist for weeks to months; dengue has more prominent myalgia and retro-orbital pain; chikungunya does NOT cause significant thrombocytopenia or hemoconcentration
Zika Virus
Why It Is Similar
Same vector (Aedes), overlapping geographic distribution, fever with rash
Key Discriminator
Zika is typically milder with low-grade fever, prominent conjunctivitis (non-purulent), and the rash appears earlier; severe thrombocytopenia and plasma leakage are absent; key association is microcephaly in pregnancy
Malaria
Why It Is Similar
Febrile illness in tropical traveler, can cause thrombocytopenia and hepatosplenomegaly
Key Discriminator
Malaria has cyclical fevers (tertian or quartan), prominent splenomegaly, anemia (hemolytic), and is diagnosed by thick and thin blood smear or rapid diagnostic test (RDT); no retro-orbital pain or hemoconcentration
Typhoid Fever
Why It Is Similar
Prolonged fever in endemic area, can present with relative bradycardia and hepatosplenomegaly
Key Discriminator
Typhoid has a stepwise rising fever over the first week, relative bradycardia, rose spots on the abdomen, and constipation (not diarrhea initially); diagnosed by blood culture or Widal test; no thrombocytopenia or rash as in dengue
Leptospirosis
Why It Is Similar
Acute febrile illness in tropical setting with myalgia and hepatic involvement
Key Discriminator
Leptospirosis features conjunctival suffusion (redness without exudate), jaundice, and renal failure (Weil's disease); exposure history involves contact with animal urine or contaminated floodwater; diagnosed by MAT (microscopic agglutination test)
Meningococcemia
Why It Is Similar
Acute fever with petechial/purpuric rash
Key Discriminator
Meningococcemia progresses rapidly to purpura fulminans and septic shock; rash is purpuric (not just petechial); meningeal signs are often present; diagnosed by blood culture and CSF analysis; requires immediate antibiotics
Immune Thrombocytopenic Purpura (ITP)
Why It Is Similar
Isolated thrombocytopenia with petechiae/bleeding
Key Discriminator
ITP typically presents in a well-appearing patient (no fever, no systemic illness); isolated thrombocytopenia without leukopenia or hemoconcentration; often follows a viral illness in children; no travel history or endemic exposure needed
Differential | Why It Is Similar | Key Discriminator |
|---|---|---|
Chikungunya | Also transmitted by Aedes mosquitoes in endemic areas; presents with acute fever, arthralgia, rash | Chikungunya causes severe, debilitating polyarthralgia (often symmetric, small joints) that can persist for weeks to months; dengue has more prominent myalgia and retro-orbital pain; chikungunya does NOT cause significant thrombocytopenia or hemoconcentration |
Zika Virus | Same vector (Aedes), overlapping geographic distribution, fever with rash | Zika is typically milder with low-grade fever, prominent conjunctivitis (non-purulent), and the rash appears earlier; severe thrombocytopenia and plasma leakage are absent; key association is microcephaly in pregnancy |
Malaria | Febrile illness in tropical traveler, can cause thrombocytopenia and hepatosplenomegaly | Malaria has cyclical fevers (tertian or quartan), prominent splenomegaly, anemia (hemolytic), and is diagnosed by thick and thin blood smear or rapid diagnostic test (RDT); no retro-orbital pain or hemoconcentration |
Typhoid Fever | Prolonged fever in endemic area, can present with relative bradycardia and hepatosplenomegaly | Typhoid has a stepwise rising fever over the first week, relative bradycardia, rose spots on the abdomen, and constipation (not diarrhea initially); diagnosed by blood culture or Widal test; no thrombocytopenia or rash as in dengue |
Leptospirosis | Acute febrile illness in tropical setting with myalgia and hepatic involvement | Leptospirosis features conjunctival suffusion (redness without exudate), jaundice, and renal failure (Weil's disease); exposure history involves contact with animal urine or contaminated floodwater; diagnosed by MAT (microscopic agglutination test) |
Meningococcemia | Acute fever with petechial/purpuric rash | Meningococcemia progresses rapidly to purpura fulminans and septic shock; rash is purpuric (not just petechial); meningeal signs are often present; diagnosed by blood culture and CSF analysis; requires immediate antibiotics |
Immune Thrombocytopenic Purpura (ITP) | Isolated thrombocytopenia with petechiae/bleeding | ITP typically presents in a well-appearing patient (no fever, no systemic illness); isolated thrombocytopenia without leukopenia or hemoconcentration; often follows a viral illness in children; no travel history or endemic exposure needed |
06Traps and High-Yield Pearls
The most common way students get questions about dengue wrong is misidentifying the critical phase. A vignette will describe a patient whose fever has broken and who "appears to be improving." The student, seeing resolution of the febrile symptoms, selects an answer choice that involves discharge or reduced monitoring. The correct answer is the opposite: defervescence marks the onset of the critical phase, when plasma leakage peaks and the risk of shock is at its highest. The "next best step" at this point is to intensify monitoring with serial hematocrit checks and prepare for IV fluid resuscitation if warning signs emerge.
A second common trap involves the use of NSAIDs or aspirin for pain management. A vignette may present a dengue patient with severe myalgia and offer ibuprofen or aspirin as answer choices alongside acetaminophen. Always choose acetaminophen. The bleeding risk from platelet dysfunction caused by NSAIDs on top of dengue-related thrombocytopenia is a tested concept.
Students also confuse dengue with chikungunya. The discriminator is straightforward: if the vignette emphasizes persistent, debilitating joint pain (especially involving the hands and feet), think chikungunya. If it emphasizes retro-orbital pain, thrombocytopenia, and hemoconcentration, think dengue.
Another pearl: a rising hematocrit with a falling platelet count is the hallmark laboratory combination of plasma leakage in dengue. If a vignette gives you these two lab trends together, recognize that the patient is entering or is in the critical phase and needs IV isotonic crystalloids.
Finally, remember the vaccine trap. Dengvaxia is only for seropositive patients. A question may describe a seronegative child in an endemic area and ask whether to vaccinate. The answer is no. Vaccination of seronegative individuals increases the risk of severe dengue through the same antibody-dependent enhancement mechanism that drives severe disease in secondary natural infection. The core competency being tested across dengue questions is your ability to recognize the timing of clinical deterioration, manage fluids appropriately without causing overload, and avoid iatrogenic harm from incorrect analgesics or misguided platelet transfusions.