A common mistake is to think that the illness is getting better when the fever goes down. With dengue, the most dangerous phase often comes exactly when the temperature falls. This article is based on Chapters 9 and 10 of the book Dengue fever (Vietnam Association of Preventive Medicine and Vietnam General Association of Medicine; printing sponsored by Takeda Vietnam), following the Ministry of Health classification (updated in 2023).

The three phases of the illness

After a bite from an infected mosquito, the incubation period is about 5–7 days (sometimes 3–10 days). The illness then starts suddenly and goes through three phases.

Febrile phase (2–7 days):

Sudden high fever of 39–40°C, flushed face and body.
Muscle and joint pain, pain behind the eyes, headache, nausea.
There may be small bleeding spots under the skin, bleeding gums or nosebleeds.

Critical phase (usually days 4–7, lasting 24–48 hours):

Most patients gradually improve after the fever goes.
Some develop severe plasma leakage because the blood vessel walls become more permeable. Without timely fluid replacement, this can lead to shock.

Recovery phase (days 7–10):

The fever is gone, appetite improves, urine output increases.
The skin may itch.

Most people infected with dengue virus have no symptoms or only a mild fever. In an outbreak, fewer than 5% of cases become severe. Because there is no way of knowing in advance who will become severely ill, every patient needs to be monitored.

Source: Dengue fever (Vietnam Association of Preventive Medicine and Vietnam General Association of Medicine; printing sponsored by Takeda Vietnam), Chapter 9, section 1, pp. 169–171.

Warning signs: hospital admission required

A person diagnosed with dengue who has at least one of the following signs must be admitted to hospital:

Restlessness, lethargy, drowsiness.
Severe, continuous abdominal pain, or increasing pain over the liver (below the right ribs).
Repeated vomiting: 3 or more times in 1 hour, or 4 or more times in 6 hours.
Mucosal bleeding: bleeding gums, nosebleeds, vomiting blood, black or bloody stools, abnormal vaginal bleeding, blood in the urine.
Liver enlarged more than 2 cm below the rib margin.
Passing little urine.
Tests: rising haematocrit with a rapid fall in platelets; liver enzymes AST/ALT of 400 U/L or more; fluid around the lungs or in the abdomen on ultrasound or X-ray.

According to the book, repeated vomiting and increasing abdominal pain are early signs of plasma leakage.

Source: Dengue fever (Vietnam Association of Preventive Medicine and Vietnam General Association of Medicine; printing sponsored by Takeda Vietnam), Chapter 9, sections 1.2 and 2.2, pp. 170, 172.

Signs of shock: emergency care immediately

Restlessness, agitation or drowsiness.
Cold hands and feet, cold skin, mottled bluish skin.
A fast weak pulse, narrowed pulse pressure (difference between systolic and diastolic pressure of 20 mmHg or less) or blood pressure that cannot be measured.
Passing little urine.

Severe bleeding may also occur: nosebleeds that need packing, gastrointestinal bleeding, internal bleeding. Severe signs can occur even without shock.

Source: Dengue fever (Vietnam Association of Preventive Medicine and Vietnam General Association of Medicine; printing sponsored by Takeda Vietnam), Chapter 9, sections 1.2 and 2.3, pp. 170, 172.

Who should be monitored in hospital from the start

Patients without warning signs are usually treated as outpatients. However, Ministry of Health guidance advises hospital admission for:

Infants.
People who are overweight or obese.
Pregnant women.
People aged 60 and over.
People with chronic diseases: kidney, heart or liver disease, asthma, COPD, diabetes, haemolytic anaemia…
People who live alone, live far from a health facility, or whose family cannot monitor them closely.

Source: Dengue fever (Vietnam Association of Preventive Medicine and Vietnam General Association of Medicine; printing sponsored by Takeda Vietnam), Chapter 10, section 2.1, p. 197.

Home care

Bring down a fever of 38.5°C or higher with paracetamol 10–15 mg/kg per dose, 4–6 hours apart. The total must not exceed 60 mg/kg in 24 hours.
Do not use aspirin, analgin (metamizole) or ibuprofen, as they can cause bleeding and acidosis.
Give plenty of fluids: oral rehydration solution, fruit juice (coconut water, orange, lemon), thin rice porridge with a little salt.
Do not eat or drink brown or red foods and drinks (sarsaparilla soda, chocolate…), as they can be mistaken for vomiting blood or bloody stools.
Go back for check-ups and tests every day as scheduled.
If any warning sign appears, go to hospital immediately, even if the fever has gone.

Source: Dengue fever (Vietnam Association of Preventive Medicine and Vietnam General Association of Medicine; printing sponsored by Takeda Vietnam), Chapter 10, section 2.1, p. 197.