Dengue Hemorrhagic Fever: How Dengue Becomes Severe
Dengue is often thought of as a miserable but temporary infection, causing several days of fever, intense muscle and joint pain, headache and exhaustion before the person gradually recovers.
Most infections do follow this course. However, in some patients, the infection can progress to a life-threatening illness involving internal bleeding, a dangerous loss of fluid from the bloodstream, shock or damage to vital organs.
That possibility has received renewed attention in Florida, where health officials recently confirmed the state's first dengue-related death of 2026. The death occurred in Hillsborough County, which has also been the center of an unusual increase in locally acquired dengue cases.
The illness historically known as dengue hemorrhagic fever is now generally called severe dengue. This change is important because severe dengue is not simply a case of dengue with heavy bleeding. Some patients become critically ill because fluid escapes from the bloodstream, while others develop serious bleeding or damage to organs such as the liver, heart or brain.
What is the difference between dengue and dengue hemorrhagic fever?
Dengue is caused by one of four closely related dengue viruses and is transmitted mainly by infected Aedes mosquitoes. Many infections cause no symptoms at all. When symptoms do occur, they commonly include a sudden fever, headache, pain behind the eyes, muscle and joint pain, nausea, vomiting and sometimes a rash.
Most people recover without serious complications.
The illness can, however, enter a more dangerous stage around the time the fever starts to disappear. During this period, some patients begin losing the liquid part of their blood into surrounding tissues. Others develop significant bleeding or problems affecting their organs.
This is why health authorities now use the broader term severe dengue. The CDC and World Health Organization classify severe disease according to three main complications:
- A major loss of fluid from the bloodstream, which can lead to shock or breathing problems
- Severe bleeding
- Serious damage to other organs
A previous dengue infection can also affect the risk of severe disease. There are four dengue virus types, and infection with one type provides long-term protection against that same type, but it does not provide the same protection against the others. In some people, a later infection with a different type can therefore increase the risk of severe dengue.
What are the warning signs of severe dengue?
The timing can be particularly confusing because the most dangerous period may begin just as the fever improves.
Warning signs include:
- Severe abdominal pain or tenderness
- Repeated or persistent vomiting
- Bleeding from the nose or gums
- Blood in vomit or stool
- Unusual sleepiness, extreme weakness or restlessness
- Difficulty breathing
- Fluid collecting in the abdomen or around the lungs
- An enlarged liver
These symptoms do not necessarily mean that severe dengue has already developed. They indicate that the person needs prompt medical assessment because the illness can deteriorate quickly.
How is severe dengue treated?
There is currently no medication that directly eliminates the dengue virus. Treatment therefore focuses on supporting the body while it recovers and preventing complications.
The main elements of treatment include:
- Fluid replacement: Intravenous fluids can help maintain circulation and blood pressure. Doctors adjust the amount according to the patient's condition rather than simply giving large amounts of fluid
- Close monitoring: Blood pressure, heart rate, urine production, blood tests and breathing are monitored to determine whether the patient is improving or developing complications
- Treatment for shock: If the loss of fluid becomes severe enough to cause shock, doctors use intravenous fluids and other supportive measures to restore adequate circulation and protect the organs
- Treatment of significant bleeding: Patients with serious bleeding may need blood transfusions or other supportive treatment
- Management of organ complications: Severe dengue can affect the kidneys, liver, heart and other organs. So, the treatment depends on which organs are affected and how severely
Fluid treatment has to be adjusted throughout the illness. During the dangerous phase, additional fluid may be necessary because fluid is leaving the bloodstream. Later, when the blood vessels recover, that fluid begins returning to the circulation. If a patient continues receiving large amounts of intravenous fluid at that point, the excess can accumulate in the lungs and make breathing difficult.
For people with uncomplicated dengue, treatment is much simpler. Rest and adequate hydration are important, and acetaminophen can be used for fever and pain. Aspirin and medications such as ibuprofen are generally avoided because they can increase the risk of bleeding.
What happens in the body during severe dengue?
The key change in severe dengue involves the lining of the body's smallest blood vessels. Under normal circumstances, this lining acts as a selective barrier, keeping most of the liquid portion of blood inside the circulation while allowing oxygen, nutrients and other substances to reach the tissues.
During severe dengue, this barrier temporarily becomes more permeable.
The dengue virus itself is not simply eating holes through the blood vessels. Instead, the infection triggers changes in the cells lining the vessels and in the immune system's response to the virus.
The consequences become more serious as more fluid leaves the circulation. Red blood cells largely remain inside the blood vessels, while part of the fluid surrounding them is lost. The blood therefore becomes more concentrated, while the amount of circulating fluid decreases. Doctors can detect some of these changes through blood tests, including changes in hematocrit, which measures the proportion of blood made up of red blood cells.
If enough fluid is lost, the heart has less blood available to pump. Blood pressure can eventually fall, reducing the amount of oxygen and nutrients reaching the body's tissues. This can lead to shock, and prolonged shock can damage organs.
Bleeding develops through a different but related set of problems. Dengue can cause platelet levels to fall, and platelets are essential for stopping bleeding. The infection can also interfere with other parts of the body's clotting system. As a result, some patients develop bleeding from the gums, gastrointestinal tract or other sites.
Fortunately, the changes in the blood vessels are usually temporary. Once the dangerous phase passes, the vessels begin to retain fluid normally again, and the fluid that had accumulated outside the bloodstream is gradually absorbed back into the circulation. This is why a patient who initially needed intravenous fluids can later be harmed by receiving too much.
Most people with dengue never develop these complications. For those who do, however, the transition from an ordinary dengue infection to severe disease can happen quickly, particularly around the time the fever disappears. Recognizing warning signs during this period is therefore an important part of preventing a dangerous illness from going unnoticed.
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