Dengue Virus
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Direct answer
Dengue virus is a single-stranded ribonucleic acid flavivirus with four serotypes, DEN-1 to DEN-4, transmitted by the day-biting Aedes aegypti mosquito breeding in domestic water containers. One serotype confers lifelong homotypic immunity only, and a second infection can trigger antibody-dependent enhancement, producing severe dengue with plasma leakage, haemoconcentration, haemorrhage and shock around the time fever settles. Management is meticulous fluid balance — there is no specific antiviral — with paracetamol alone for fever.
What you must remember
- Virus and vector: four serotypes circulate in India, transmitted mainly by Aedes aegypti, a day-biting peri-domestic mosquito breeding in clean stagnant water in containers; humans are the main amplifying host.
- Antibody-dependent enhancement: non-neutralising antibody from a first infection eases entry of a second serotype into Fc-receptor-bearing monocytes, which is why secondary dengue is severer.
- Clinical features: sudden high fever with severe headache, retro-orbital pain, myalgia and arthralgia — breakbone fever — with rash and a positive tourniquet test.
- Warning signs: abdominal pain, persistent vomiting, fluid accumulation, mucosal bleeding, lethargy, tender hepatomegaly and rising haematocrit with falling platelets — these demand admission.
- Severe dengue: plasma leakage with pleural effusion and ascites, haemoconcentration, severe thrombocytopenia, significant bleeding and dengue shock syndrome, with the critical phase around defervescence on days three to seven.
- Diagnosis: the NS1 antigen test is positive in the first days, immunoglobulin M appears after about five days, and a rapid high immunoglobulin G suggests secondary infection; polymerase chain reaction confirms when needed.
- Management: oral fluids with daily platelet and haematocrit monitoring for uncomplicated cases, careful intravenous crystalloids for the critical phase, avoidance of aspirin and non-steroidal anti-inflammatory drugs, and no routine platelet transfusion without significant bleeding.
Common confusion
Dengue is equated with a low platelet count alone, but severity is defined by plasma leakage — the rising haematocrit is the more important number, showing haemoconcentration. Test timing is also confused: NS1 is early and immunoglobulins later, so negative serology on day two does not exclude dengue. Remember the vector's habits — Aedes bites by day and breeds in fresh water in and around houses, unlike night-biting anopheles and culex, so source reduction of containers is the control strategy.
Exam-focused takeaway
In theory, describe the serotypes, vector and breeding habits, antibody-dependent enhancement, the phases of illness with warning signs, diagnosis by NS1 and serology, and fluid-based management — mirroring national guidelines scores well. In viva, expect why second infections are worse, when the critical phase occurs and why aspirin is avoided. At the practical, interpreting a dengue panel with platelet and haematocrit trends is standard.
Frequently asked questions
Why is secondary dengue more severe?
Antibody to one serotype enhances infection by another through antibody-dependent enhancement, raising viral load and driving the plasma leakage of severe dengue.
Which mosquito transmits dengue?
Mainly Aedes aegypti, day-biting and breeding in clean water in domestic containers such as coolers, tyres and tanks, making source reduction the key control measure.
What are the warning signs demanding admission?
Abdominal pain, persistent vomiting, fluid accumulation, mucosal bleeding, lethargy, tender hepatomegaly and a rising haematocrit with falling platelets.
Which tests diagnose dengue and when?
NS1 antigen in the first days, immunoglobulin M from about the fifth day and high immunoglobulin G in secondary infection; polymerase chain reaction confirms when available.
Why is haematocrit monitored so closely?
A rising haematocrit signals haemoconcentration from plasma leakage, the hallmark of severe dengue, and guides fluid replacement in the critical phase.
Is there specific treatment or a vaccine?
Treatment is supportive with fluids and paracetamol; a live vaccine is used selectively in highly endemic settings, so vector control remains the main tool.