Dengue fever — mosquito-borne viral illness affecting many in Sri Lanka
Dengue fever is caused by the dengue virus transmitted through Aedes mosquito bites. Early recognition of warning signs can prevent serious complications.
Dr. Seneth Gajasinghe, MBBS MD
Written & Reviewed by
MBBS (Col) — MD (Col) — SLMC No. 27329 — Medical Director, Sineth Hospitals
✅ Medically Reviewed 📅 June 2026 ⏱ 8 min read
🦟 Overview

What Is Dengue Fever?

Dengue fever is a viral infection spread through the bite of infected Aedes mosquitoes. The disease is caused by the dengue virus and is common in tropical countries, including Sri Lanka, where outbreaks occur regularly throughout the year.

The virus is transmitted when an Aedes mosquito bites an infected person and then bites another person. Dengue is not contagious from person to person — it can only be spread through infected mosquito bites.

How dengue spreads — Aedes mosquito bites infected person, carries virus, bites another person
Dengue spreads when an Aedes mosquito bites an infected person and later bites another. The mosquito — not direct contact — is the carrier.

Many patients recover completely with supportive care. However, some develop serious complications, particularly during the critical phase, that require close medical monitoring and hospital care.

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Key Facts
  • 🦟Dengue is caused by the dengue virus — there are four different serotypes (DENV-1 to DENV-4).
  • 🇱🇰Sri Lanka reports thousands of dengue cases annually. The illness is common and should be taken seriously.
  • ⚠️A second dengue infection (with a different serotype) can carry a higher risk of severe disease.
🌡️ Symptoms

Symptoms of Dengue Fever

Symptoms usually begin 4–10 days after being bitten by an infected mosquito. The onset is typically sudden, with fever developing rapidly.

Some patients develop only mild symptoms, while others can become seriously ill. The severity of symptoms can change during the course of the illness, especially around the critical phase.

Common symptoms of dengue fever — high fever, severe headache, pain behind eyes, muscle and joint pain, rash
Dengue fever typically presents with high fever, severe headache, pain behind the eyes, body aches and fatigue.
🌡️ High fever (often above 38.5°C)
🤕 Severe headache
👁️ Pain behind the eyes
💪 Muscle and joint pain
🤢 Nausea and vomiting
😖 Extreme tiredness and weakness
🩸 Skin rash (appears around days 3–5)
🦴 Bone and back pain
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Important: Not all patients develop all symptoms. Some patients may have only mild fever and fatigue, while others develop the full range. The absence of a rash does not rule out dengue.
🔍 Comparison

Dengue vs Common Viral Fever

Early dengue can look very similar to many other viral illnesses. This is one reason why dengue is sometimes missed or diagnosed late. The following features may help distinguish dengue from a general viral fever, though only a doctor can make a diagnosis.

Dengue fever compared to common viral fever — differences in symptoms and severity
Dengue shares features with many viral illnesses. Key distinguishing signs include severe body aches, pain behind the eyes, skin rash and falling platelet count.
Feature Dengue Fever Common Viral Fever
Fever High, often sudden onset Mild to moderate
Body pain Severe — muscles, joints, bones Mild body aches
Pain behind eyes Common — a characteristic feature Uncommon
Rash Skin rash often appears days 3–5 Rash uncommon
Warning signs Can develop — require urgent review Not typically present
Blood count changes Falling platelets and white cells Usually normal or mildly abnormal
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If you are uncertain whether your fever is dengue or another illness, seek medical assessment early rather than waiting. Blood tests can help clarify the diagnosis.
📅 Disease Course

The Three Phases of Dengue

Understanding the three phases of dengue is essential for patients and their families. The course of dengue follows a predictable pattern — and knowing what to expect in each phase helps you act at the right time.

Three phases of dengue fever — febrile phase days 1-3, critical phase days 4-6, recovery phase day 7 onwards
Dengue progresses through three distinct phases. The critical phase — when fever may appear to be settling — is the most dangerous period.
Phase 1
Days 1–3
Febrile Phase
High fever, body aches, severe headache. Fever may reach 39–40°C. The patient feels very unwell.
Phase 2
Days 4–6
Critical Phase
Fever may improve — but this is the most dangerous period. Fluid leakage, falling platelets and serious complications can begin.
Phase 3
Day 7+
Recovery Phase
Gradual improvement. Appetite returns. Platelet count begins to rise. Fatigue may persist for weeks.
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The transition from febrile to critical phase is the highest-risk period. Patients and families must understand that feeling better when the fever comes down does not mean the illness is over.
🚨 Critical Phase

Why the Critical Phase Is Dangerous

The critical phase of dengue is the period when serious complications are most likely to develop. This usually occurs around days 4–6 of illness.

Many patients and families make a dangerous mistake during this time: they think the patient is getting better because the fever starts to come down. In reality, this is precisely when the patient needs the closest monitoring.

The dangerous period of dengue — fever settling marks the start of the critical phase, not recovery
When the fever settles around day 4–6, it does not mean recovery has begun. The critical phase is starting — this is when fluid leakage and complications occur.
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The most dangerous period of dengue often begins when the fever starts to come down. Do not reduce monitoring or stop medical follow-up when the fever settles.

During the critical phase, the following can occur:

  • 💧 Plasma leakage — fluid leaks from blood vessels into surrounding tissues, causing complications including respiratory distress
  • 📉 Falling platelet count — increases the risk of bleeding
  • 🩸 Bleeding — from gums, nose, skin or internal organs
  • 🫀 Shock — severe plasma leakage can lead to dengue shock syndrome, which is life-threatening
  • 🫁 Fluid accumulation — in the lungs or abdomen

Most patients pass through the critical phase without serious complications when they are monitored appropriately and receive supportive care. The key is early recognition and appropriate medical review.

🚩 Warning Signs

Warning Signs of Severe Dengue

Warning signs indicate that the patient may be developing serious complications and requires immediate medical assessment. Do not wait to see if these signs improve on their own.

Warning signs of severe dengue — abdominal pain, persistent vomiting, bleeding, breathing difficulty, confusion
Any of these warning signs requires immediate medical review. Do not manage warning signs at home.
  • 🔴 Severe abdominal pain or tenderness
  • 🔴 Persistent vomiting — unable to keep fluids down
  • 🔴 Difficulty breathing or rapid breathing
  • 🔴 Bleeding from gums
  • 🔴 Nose bleeding that does not stop easily
  • 🔴 Blood in vomit or dark-coloured vomit
  • 🔴 Blood in stool or black, tarry stools
  • 🔴 Extreme weakness or inability to stand or walk
  • 🔴 Confusion, irritability or drowsiness — altered level of consciousness
  • 🔴 Skin that is cold, pale or clammy
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Patients with any warning sign require immediate medical assessment. Go to the nearest hospital or emergency department without delay. Do not attempt to manage warning signs at home.
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Learn each warning sign in detail — what it looks like, why it happens, and what to do: Dengue Warning Signs Explained →
🧪 Blood Tests

Blood Tests Used in Dengue

Blood tests are an essential part of diagnosing dengue and monitoring the illness. Your doctor will advise which tests are needed and how often to repeat them based on your clinical situation.

Blood tests used in dengue fever — FBC, NS1 antigen test, IgM and IgG antibodies
Blood tests in dengue serve two purposes: confirming the diagnosis and monitoring the illness for complications.

Full Blood Count (FBC)

The Full Blood Count (FBC) is the most important monitoring test in dengue. It is used to track:

🩸 Platelet count — falling platelets signal increasing severity
⬜ White blood cell count — often falls in dengue
📊 Haematocrit — a rising haematocrit suggests plasma leakage
🔁 Trend over time — daily monitoring is often needed in the critical phase

NS1 Antigen Test

The NS1 antigen test detects a dengue virus protein directly in the blood. It is most useful in the first 1–5 days of illness (febrile phase), before the body has produced antibodies. A positive NS1 test confirms dengue infection. A negative result in the early days does not completely rule it out.

Dengue IgM and IgG Antibody Tests

These antibody tests become useful from around day 5 of illness onwards. IgM antibodies appear first and indicate recent infection. IgG antibodies appear later and can indicate a previous dengue infection. A positive IgM confirms dengue. Rising IgG alongside IgM can suggest a second dengue infection, which carries a higher risk of complications.

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Which test is right for you depends on when you are tested. NS1 is most useful in the first few days; antibody tests become more reliable from day 5 onwards. Your doctor will advise the appropriate tests based on how many days you have been unwell.
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For a detailed explanation of the NS1 test, its timing, and what positive and negative results mean: Dengue NS1 Test Explained: Timing, Accuracy, Positive and Negative Results →
🩸 Platelets

What Happens to Platelets in Dengue?

Platelets are blood components that help stop bleeding by forming clots at the site of injury. In dengue, platelet counts often fall — sometimes significantly — during the critical phase.

Platelet count changes in dengue fever — falling during critical phase, rising during recovery
Platelet counts typically fall during the critical phase (days 4–6) and begin to recover as the patient enters the recovery phase. The trend matters more than a single reading.

The normal platelet count in adults is approximately 150,000 to 400,000 per microlitre. In dengue, counts can fall well below 100,000 — or even below 20,000 in severe cases.

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A low platelet count alone does not always mean severe disease. Doctors assess the platelet count together with clinical symptoms, rate of fall and other blood test results. Many patients with low platelet counts recover without needing a platelet transfusion.

What matters most is the rate of change — how rapidly the count is falling — and whether the patient has any bleeding symptoms or warning signs. Your doctor will advise how frequently to recheck your blood count based on your situation.

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Platelet transfusions are not routinely given for a low count alone. They are reserved for patients who are actively bleeding or whose count has fallen to a critically low level. Do not request transfusions without medical advice.
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For a detailed guide on platelet counts in dengue — including why they fall, what the numbers mean and when transfusions are needed — read: Platelets in Dengue: What Do the Numbers Mean? →
💊 Treatment

Treatment of Dengue Fever

There is no specific antiviral medicine that cures dengue. Treatment is supportive — meaning it focuses on managing symptoms, maintaining hydration and monitoring closely for complications.

🛌 Rest — allow your body to fight the infection
💧 Adequate fluid intake — oral rehydration or intravenous fluids if needed
💊 Paracetamol — for fever and pain relief
🩺 Regular medical monitoring — especially blood counts
📋 Follow-up as advised by your doctor
🏥 Hospital admission if warning signs develop

Maintaining good fluid intake is one of the most important aspects of dengue management. Adequate hydration helps the body cope with the illness and reduces the risk of complications from plasma leakage.

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There is no specific antiviral treatment for dengue. Supportive care, adequate hydration, careful monitoring and early recognition of warning signs are the cornerstones of management.
🚫 What to Avoid

What Should Be Avoided in Dengue?

Certain medicines and substances can make dengue worse and must be strictly avoided. This is one area where the wrong decision can have serious consequences.

What to avoid in dengue fever — ibuprofen, aspirin, diclofenac, self-medication, alcohol
These medicines and substances can worsen dengue complications. Always consult your doctor before taking any medicine.
  • 🚫 Ibuprofen — a common painkiller that increases the risk of bleeding in dengue
  • 🚫 Diclofenac (Voltaren) — an anti-inflammatory medicine that also increases bleeding risk
  • 🚫 Aspirin — thins the blood and significantly increases bleeding risk in dengue
  • 🚫 Self-medication — taking any medicines not prescribed by your doctor
  • 🚫 Alcohol — can worsen dehydration and affect liver function
  • 🚫 Steroids — not indicated in dengue and can cause harm
  • 🚫 Antibiotics — dengue is viral; antibiotics are not effective and should not be taken unless a specific bacterial complication is diagnosed
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Only take medicines recommended by your healthcare professional. Ibuprofen, aspirin and diclofenac are commonly available over the counter but are dangerous in dengue. Paracetamol is the only safe choice for fever and pain.
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For the full medicine guide — including antibiotics, steroids, herbal remedies and supplements: Medicines to Avoid in Dengue Fever →
🏥 Seek Help

When Should You Go to Hospital?

Not every dengue patient needs hospital admission, but you must seek immediate medical review if any of the following apply:

When to go to hospital with dengue fever — warning signs, inability to drink, bleeding, breathing difficulty
These situations require immediate hospital assessment. Do not delay or wait to see if the condition improves at home.
  • 🚨 Any warning signs listed above
  • 🚨 Unable to drink fluids due to vomiting or severe nausea
  • 🚨 Persistent vomiting for more than 24 hours
  • 🚨 Difficulty breathing
  • 🚨 Any bleeding — from gums, nose, skin, urine or stool
  • 🚨 Severe weakness — unable to walk or stand
  • 🚨 Pregnancy — pregnant women with dengue need closer monitoring
  • 🚨 Infants and young children — they can deteriorate rapidly
  • 🚨 Elderly patients or those with other medical conditions such as diabetes, kidney or heart disease
  • 🚨 Rapidly falling platelet count or significantly abnormal blood results
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If in doubt, seek medical assessment. It is far better to be assessed and told you are safe to go home than to wait at home during a dengue complication. Do not self-manage warning signs.
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Urgent assessment does not always mean admission. For how doctors decide between home monitoring and hospital admission: When Does a Dengue Patient Need Hospital Admission? →
❓ FAQ

Frequently Asked Questions

Mild dengue can often be managed at home with rest, adequate fluid intake and paracetamol for fever. However, patients must be monitored daily — ideally with blood count checks as advised by the doctor. If any warning signs develop — such as severe abdominal pain, persistent vomiting, bleeding or extreme weakness — hospital assessment is needed immediately. Do not attempt to manage warning signs at home.
Dengue typically lasts 7–10 days in total. The febrile phase lasts 3–4 days; the critical phase occurs around days 4–6; the recovery phase begins around day 7. Many patients feel significant fatigue for several weeks after recovery. If fever or symptoms persist beyond 10 days without improvement, consult your doctor.
No. Many patients with mild dengue recover at home with careful monitoring. Hospital admission is generally needed if warning signs develop, if the patient cannot take fluids orally, if blood tests show significant or rapid changes, or if the patient belongs to a higher-risk group — including very young children, elderly patients, pregnant women, or those with diabetes, heart disease or kidney disease.
The normal platelet count in adults is approximately 150,000 to 400,000 per microlitre of blood. In dengue, platelet counts can fall significantly — sometimes below 50,000 or even lower. A low platelet count alone does not always mean severe disease; doctors assess the count together with the patient's symptoms and other blood results.
Platelet counts typically start to decline around days 3–5 of illness, coinciding with the onset of the critical phase. The lowest counts are often seen around days 5–7. As the patient enters the recovery phase (day 7 onwards), platelet counts usually begin to rise again. Daily monitoring of the blood count is often recommended during this period.
Yes. There are four dengue virus serotypes (DENV-1 to DENV-4). Infection with one serotype gives lifelong immunity only to that serotype. A second dengue infection with a different serotype can actually carry a higher risk of severe dengue, including dengue haemorrhagic fever. This is why dengue should always be taken seriously, even if you have had it before.
No. Dengue cannot spread directly from person to person. It is transmitted only through the bite of an infected Aedes mosquito. You cannot catch dengue by touching, caring for, or being near someone who has it. However, if a mosquito bites a dengue patient and then bites you, that mosquito can transmit the virus.
Your doctor will advise how frequently to repeat blood tests based on your condition and how many days into the illness you are. In the critical phase (days 4–6), daily full blood count monitoring is commonly recommended. More frequent testing — sometimes every 12 hours — may be needed if the platelet count is falling rapidly or if warning signs develop.
No. Ibuprofen, diclofenac and aspirin should generally be avoided during dengue unless specifically advised by a doctor, because they may increase the risk of bleeding. Paracetamol is usually preferred for fever, but the dose should follow medical advice.
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Key Takeaways
  • 🦟Dengue is a mosquito-borne viral illness — it cannot spread from person to person.
  • 🌡️High fever, severe headache, pain behind the eyes and body aches are common symptoms.
  • ⚠️The critical phase often begins when the fever starts to settle.
  • 🚩Warning signs such as severe abdominal pain, persistent vomiting, bleeding or drowsiness require urgent medical review.
  • 🧪Blood tests help diagnose dengue and monitor platelet count, white cells and haematocrit.
  • 🚫Ibuprofen, diclofenac and aspirin should generally be avoided unless advised by a doctor.
  • ✅Most patients recover completely with proper monitoring and supportive care.

For parents and caregivers, read our guide to dengue in children, including symptoms, monitoring and when to seek medical help.

For pregnancy-specific information, read our guide to dengue in pregnancy, including possible risks, warning signs and when to seek medical care.