Dengue Virus Infection
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Direct answer
Dengue virus is a mosquito-borne flavivirus with four serotypes, transmitted mainly by Aedes aegypti. Its NS1 antigen can be detected in early illness before IgM appears, and severe disease is driven by plasma leakage. Secondary infection with a different serotype raises the risk of severe dengue through antibody-dependent enhancement.
What you must remember
- Serotypes: DENV-1 to DENV-4, and infection with one serotype gives lasting protection only against that serotype.
- Vectors: Aedes aegypti is the principal vector in India, with Aedes albopictus as a secondary vector, and both bite mainly during the day.
- NS1 antigen: Detectable in the first few days of fever, making it a useful early test.
- IgM and IgG: IgM usually becomes detectable from about the fifth day, and a rising IgG pattern supports recent infection.
- Warning signs: Abdominal pain or tenderness, persistent vomiting, mucosal bleeding, lethargy, liver enlargement and rising haematocrit with falling platelets.
- Plasma leakage: Capillary leak typically begins around the time the fever falls, which is the most dangerous phase.
- Antibody-dependent enhancement: Cross-reactive antibodies from a previous infection help the virus enter monocytes and macrophages.
- Management: Supportive care with careful fluid management, and NSAIDs and aspirin are avoided because of bleeding risk.
Following a case from fever to recovery
Dengue follows a predictable course, and clinicians act differently in each phase. The steps below track a typical case, and the timing is approximate.
- Febrile phase: High fever with headache, retro-orbital pain and myalgia appears in the first few days, and the virus circulates in the blood.
- Critical phase: Around the time the fever falls, capillary leak begins, so haematocrit rises and platelets fall, and warning signs need urgent review. Clinicians monitor haematocrit and platelet counts at short intervals and adjust fluids to the trend.
- Recovery phase: Capillary permeability returns to normal and fluid is reabsorbed, so over-hydration becomes a risk. Improved appetite and urine output usually signal that the patient has entered this phase.
- Laboratory confirmation: NS1 antigen testing or RT-PCR is used early, and IgM capture ELISA becomes useful from about the fifth day. A negative early result does not exclude dengue, so testing is repeated if the illness continues.
- Severity classification: The WHO 2009 scheme grades patients as having dengue without or with warning signs, or as having severe dengue with plasma leakage, bleeding or organ impairment. The grade guides whether the patient can be managed at home or needs hospital and intensive care.
Where the exam tries to catch you
Dengue questions often test the drug to avoid, the timing of tests and the reason second infections are worse.
- Trap: Using aspirin or ibuprofen for dengue fever. Correct: These impair platelet function and can worsen bleeding, so paracetamol is preferred for fever.
- Trap: Believing a positive IgM always proves current dengue. Correct: Cross-reactive flavivirus antibodies, such as those after Zika or Japanese encephalitis infection, can give false positives, so timing and clinical context matter.
- Trap: Assuming a second infection is milder. Correct: Secondary infection with a different serotype is the classic risk factor for severe dengue because of antibody-dependent enhancement.
Frequently asked questions
Why is secondary dengue infection more dangerous?
Secondary infection with a different serotype carries a higher risk of severe dengue because of antibody-dependent enhancement. Cross-reactive antibodies from the first infection bind the new virus without neutralising it and help it enter monocytes. This mechanism is why the second infection can cause marked plasma leakage.
When is NS1 antigen detection most useful in dengue?
NS1 antigen detection is most useful in the first few days of fever, when IgM antibodies are usually not yet detectable. It supports an early diagnosis, and IgM testing can be added later in the illness.
Which warning signs indicate a risk of severe dengue?
Abdominal pain or tenderness, persistent vomiting, mucosal bleeding, lethargy, liver enlargement and a rising haematocrit with falling platelets are the warning signs of dengue. Their appearance around the time the fever falls calls for urgent fluid management and close monitoring.
Which mosquito transmits dengue virus in India?
Aedes aegypti is the principal vector of dengue virus in India, and Aedes albopictus contributes as a secondary vector. Both bite mainly during the day, so vector control must target breeding in household water containers.
Why are NSAIDs avoided in dengue?
NSAIDs and aspirin are avoided in dengue because they impair platelet function and can worsen bleeding. Paracetamol is the usual antipyretic, and its dosing should follow standard guidance.