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Dengue fever, historically called breakbone fever, is an acute viral illness caused by any of the four types of dengue virus (DENV-1 to DENV-4), members of the genus Orthoflavivirus in the family Flaviviridae. The virus is spread between people by day-biting female Aedes mosquitoes, chiefly Aedes aegypti and the more cold-tolerant Aedes albopictus.

Dengue is the fastest-spreading mosquito-borne viral disease in the world, with reported cases increasing many times over in recent decades. Roughly half the world's population lives in areas where transmission is possible, across more than 130 countries. Modelling and serological surveys suggest around 390 million infections each year, of which about 96 million cause illness, leading to hundreds of thousands of hospital admissions and tens of thousands of deaths. Cases rose sharply from 2023, with the Americas reporting about 4.6 million in 2023 and around 13 million in 2024, far above any previous year.

Infection produces a wide range of outcomes. Most infections cause no symptoms. Typical dengue fever involves sudden high fever, severe headache behind the eyes, muscle and joint pain, a rash, and falls in white cell and platelet counts. A minority of patients develop severe dengue, formerly called dengue haemorrhagic fever and dengue shock syndrome, in which small blood vessels briefly become leaky, allowing plasma to escape into the chest and abdomen and causing shock. The risk of this is substantially higher during a second infection with a different type of the virus, through a process called antibody-dependent enhancement, combined with direct damage to the lining of blood vessels by a viral protein, NS1.

Because the virus replicates inside a cold-blooded mosquito, and because mosquito breeding depends both on rainfall and on water stored during droughts, the spread of dengue is closely tied to climate change, to El Niño, to the warming of cities, and to unplanned urban growth.

Sylvatic Origins, Serotypes, and Genotypes

Dengue virus originated in forest cycles between monkeys and tree-dwelling mosquitoes, probably in Southeast Asia, with a separate ancient focus for DENV-2 in West Africa. The four types diverged from one another long before entering human populations. Each subsequently emerged independently into human transmission, probably within the past few centuries to a few thousand years, as people settled in and around forests and as the virus adapted first to Aedes albopictus and then to Aedes aegypti living in towns.

A fifth type was proposed in 2013 on the basis of an isolate from a patient in Sarawak, Malaysia, but this has not been accepted, and the variant is best regarded as belonging to a sylvatic lineage.

The four types share roughly 65 to 70% of their amino acid sequence and do not recombine with one another. Within each type, strains fall into distinct genotypes that differ in how readily they spread and in how severe the resulting illness is:

• DENV-1 comprises five genotypes, one of them sylvatic.
• DENV-2 comprises six, including Asian, Cosmopolitan, American, American/Asian, and sylvatic lineages. A clear demonstration of genotype differences occurred in the Americas. Before 1981, an indigenous American genotype circulated in Latin America and the Caribbean, causing mild illness even on second infection. In 1981, beginning in Cuba, a Southeast Asian genotype was introduced, spread rapidly, displaced the local one, and caused the first large epidemics of severe dengue in the region.
• DENV-3 comprises five genotypes; the introduction of one of them into Africa and the Americas was followed by severe epidemics.
• DENV-4 comprises four, one of them sylvatic.

Virion Structure and Thermal Expansion

A mature virus particle is roughly spherical, enveloped, and about 50 nanometres across. Electron microscopy, pioneered by Michael Rossmann, Richard Kuhn, Shee-Mei Lok, and colleagues, shows three layers:

1. A core containing the RNA genome bound to many copies of the capsid protein.
2. A lipid membrane taken from the host cell's endoplasmic reticulum.
3. An outer protein shell of 180 copies each of the envelope (E) and membrane (M) proteins.

On the mature particle, the E proteins lie flat against the membrane as 90 pairs, arranged in sets of three to give a smooth surface. Each E protein has three parts:

• Domain I, the central organizing region.
• Domain II, an elongated region that holds the pair together and carries at its tip the fusion loop, the hydrophobic segment that inserts into the host membrane during entry. In the mature particle this loop is tucked away against the partner protein. A sugar group on this domain binds the receptor DC-SIGN.
• Domain III, which projects from the surface, binds host receptors, and carries the main targets of type-specific neutralizing antibodies.

Immature particles inside the cell are larger, about 60 nm, and spiky, because the E proteins are arranged as 60 trimers paired with a precursor protein, prM, whose pr portion covers the fusion loop and prevents the particle from fusing prematurely inside the cell.

The mature particle is not rigid. For some strains, particularly of DENV-2, warming from mosquito temperatures to human body temperature causes the surface proteins to loosen and the particle to expand, exposing parts of the underlying membrane and briefly revealing normally hidden regions to antibodies. The extent of this varies between strains.

Genome Organization and Viral Proteins

The genome is a single strand of RNA of about 10,700 bases. It carries a cap at one end, allowing the host ribosome to translate it, but lacks the poly(A) tail found on host messenger RNA, ending instead in a stable folded structure.

Between short untranslated regions at either end lies a single reading frame encoding one long protein of about 3,400 amino acids, which is cut up by host and viral enzymes into three structural and seven non-structural proteins:

The dengue virus proteins

The untranslated region at the end of the genome contains knotted structures that block the host enzyme that would otherwise degrade the viral RNA. Degradation therefore stops partway, leaving an abundant short RNA fragment known as subgenomic flavivirus RNA. This fragment interferes with host sensing of viral RNA in human cells, and with the RNA interference defences of mosquitoes, and the amount produced is an important determinant of how well a strain spreads.

Receptor Attachment, Endosomal Fusion, and Furin Cleavage

An infected mosquito injects virus into the skin along with saliva that suppresses local immune responses.

The virus infects cells of the immune system: Langerhans cells in the epidermis, dendritic cells in the dermis, macrophages, and circulating monocytes. It attaches using several receptors, including the lectin DC-SIGN, which binds a sugar group on the E protein. The virus also exploits the fact that its envelope carries phosphatidylserine, a lipid that cells normally display when dying: receptors that clear dying cells therefore take up the virus, a trick known as apoptotic mimicry.

The particle is drawn into the cell inside a vesicle, which the cell then acidifies. The falling pH causes the E proteins to rearrange: the pairs come apart, the fusion loops swing out and insert into the vesicle membrane, and the proteins reassemble as upright trimers which then fold back, pulling the two membranes together until they fuse and release the genome into the cytoplasm.

The genome is translated, and the ends of the RNA then pair up to circularize it, which allows the viral polymerase to copy it. Replication occurs inside vesicles formed by bending the ER membrane. New particles bud into the ER and travel through the secretory pathway, where the acidic environment exposes prM to the host enzyme furin. After cleavage, the pr fragment remains attached as a cap until the particle reaches the neutral environment outside the cell, whereupon it falls away and the particle becomes infectious.

Incomplete Maturation and prM-Mediated Infectivity

Unlike related viruses, dengue has a poor furin cleavage site, so cleavage is inefficient. Infected cells therefore release a mixture of particles: fully mature and infectious ones, partly mature ones, and fully immature ones that cannot fuse and are not infectious. The proportions vary with the cell type and have been measured mainly in cell culture.

The immature particles matter because they are not permanently inert. If an antibody against prM binds one and carries it into a macrophage, the acidic interior of that cell allows furin to complete the cleavage, converting an otherwise harmless particle into an infectious one.

Systemic Viremia and the Mosquito Extrinsic Incubation Period

Infected dendritic cells migrate to the draining lymph nodes within a day, where the virus infects more immune cells and enters the bloodstream. Virus levels in the blood rise from a day or two before the fever begins and remain high for four to seven days.

For transmission to continue, a mosquito must feed during this window, and the virus level must be high enough to infect her. Soluble NS1 circulating in the patient's blood assists, by suppressing the mosquito's gut defences. Inside the mosquito, the virus infects the gut lining, escapes into the body cavity, and reaches the salivary glands after an interval that depends on temperature: roughly 7 to 12 days at 28 °C, and 5 to 6 days at 32 °C. Once the salivary glands are infected, the mosquito remains infectious for the rest of her life without apparent harm.

Clinical Progression: Febrile, Critical, and Recovery Phases

After an incubation period of four to seven days, symptomatic dengue passes through three phases, reflected in the WHO's classification into dengue without warning signs, dengue with warning signs, and severe dengue.

1. Fever (days 1 to about 5). Sudden high fever with flushing, severe headache behind the eyes, sensitivity to light, intense muscle and joint pain, nausea, and loss of appetite, followed after a few days by a rash in which small patches of normal skin remain within an area of redness. Tests show falling white cell and platelet counts, rising liver enzymes, and fragile capillaries, demonstrated by the tourniquet test.
2. The critical phase (24 to 48 hours, beginning as the fever falls). The dangerous phase of dengue occurs not at the height of the fever but as it subsides, when the amount of virus in the blood is already declining. In patients who develop severe dengue, small blood vessels briefly become leaky without the cells themselves dying. Plasma escapes into the chest and abdomen, producing effusions and ascites. Warning signs include severe abdominal pain, persistent vomiting, bleeding from the gums or gut, lethargy, an enlarging tender liver, and a rising haematocrit alongside a falling platelet count. If the lost volume is not replaced, shock follows, marked by a narrow pulse pressure, cold extremities, acidosis, bleeding, and failure of several organs.
3. Recovery (48 to 72 hours later). The vessel lining repairs itself, leakage stops, and the escaped fluid returns to the circulation. Stopping intravenous fluids at this point is essential, because continued infusion causes fluid overload in the lungs.

Antibody-Dependent Enhancement (ADE) and Original Antigenic Sin

Cohort studies across Southeast Asia and Latin America established that most cases of severe dengue occur either during a second infection with a different type of the virus, typically years after the first, or in infants aged around six to eleven months born to mothers who have had dengue. Severe disease does occur in first infections, and the proportions vary between settings and virus types.

In the 1970s, Scott Halstead proposed antibody-dependent enhancement to explain this pattern, later supported by long-term studies such as the Nicaraguan children's cohort of Eva Harris and Leah Katzelnick.

1. Too few antibodies to neutralize, enough to help. A first infection produces a small number of highly specific antibodies that neutralize that type for life, and a much larger number of cross-reactive antibodies of low affinity, directed against the fusion loop and prM. For a year or two, these are abundant enough to neutralize the other types as well, giving temporary protection. As they decline, they pass through a range of concentrations at which they bind the virus without neutralizing it. In the Nicaraguan cohort, children whose antibody titres fell in this intermediate band had around a sevenfold higher risk of severe disease. In infants, maternal antibodies decline through the same range over the first year of life.
2. Delivery into immune cells. Antibody-coated virus particles bind Fc receptors on monocytes and macrophages, which take them up. This bypasses the usual receptors, converts immature particles into infectious ones, and greatly increases the number of infected cells.
3. Suppression of the infected cell's defences. Uptake through these receptors has also been reported to trigger inhibitory signalling that dampens interferon responses and raises production of the suppressive signal interleukin 10, so that each infected cell produces more virus. The evidence for this intracellular component is more limited than for the uptake mechanism.
4. Misdirected memory and altered antibodies. A second infection reactivates memory cells directed against the first type rather than the new one, a phenomenon known as original antigenic sin, first described for dengue T cells by Gavin Screaton and Juthathip Mongkolsapaya. These cells kill infected cells poorly but release large amounts of inflammatory signals. In addition, work by Taia Wang and colleagues found that patients who develop severe disease produce antibodies lacking a particular sugar, which binds a different Fc receptor with roughly ten to twenty times higher affinity and contributes to platelet destruction and vessel damage.

NS1 Endothelial Glycocalyx Disruption and Vascular Leakage

Leakage was long attributed solely to inflammatory signals. From 2015, work by Eva Harris, Paul Young, Naphak Modhiran, and colleagues showed that soluble NS1, secreted in large amounts by infected cells, acts directly on blood vessels by two routes:

Mechanisms of vascular leakage

Comparison across related viruses shows that their NS1 proteins act on the blood vessels of different organs in ways that match the diseases they cause: dengue NS1 affects vessels throughout the body, yellow fever NS1 those of the liver, West Nile and Japanese encephalitis NS1 those of the brain, and Zika NS1 those of the placenta.

Multi-Serotype Epidemic Cycles and Age-Shift Dynamics

Where all four types circulate, the dynamics differ from those of a single pathogen. Infection with one type gives lifelong protection against that type, a year or two of partial protection against the others, and thereafter a period of increased vulnerability.

Models developed by Neil Ferguson, Derek Cummings, Bryan Grenfell, Isabel Rodriguez-Barraquer, and others reproduce the characteristic epidemic cycles of three to five years:

• A large epidemic of one type leaves most of the population temporarily protected against all four.
• Two or three years later, as that cross-protection fades, the whole cohort becomes vulnerable at once. A different type arriving at that moment spreads explosively and produces a wave of severe cases.
• Successful control can have a counterintuitive effect. As transmission falls, people are infected later in life. Because adults tolerate the illness less well than young children, and because fewer infections occur under the protection of maternal antibodies in infancy, a partial reduction in transmission can temporarily increase the number of cases requiring hospital care, before further reductions eventually bring transmission to an end. This pattern has been documented in Thailand and Singapore.

Temperature-Dependent Extrinsic Incubation and Vector Thresholds

Every stage of transmission depends on temperature, and the two vectors differ:

• Aedes aegypti with dengue: transmission becomes possible at about 17.8 °C (95% confidence interval 14.6 to 21.2), peaks at 29.1 °C (28.4 to 29.8), and ceases at about 34.6 °C (34.1 to 35.6).
• Aedes albopictus with dengue: transmission becomes possible at about 16.2 °C (13.5 to 18.2), peaks at 26.4 °C (25.4 to 27.6), and ceases at about 31.5 °C (30.4 to 32.8).

The time the virus needs to develop inside Aedes aegypti falls steeply with temperature, and can be approximated over the range 18 to 33 °C by:

giving about 29 days at 18 °C, 23 days at 21 °C, 15 days at 28 °C, and 11 days at 33 °C. Because the proportion of mosquitoes surviving this period falls exponentially with its length, warming a city from 24 to 29 °C both shortens this interval substantially and speeds up the feeding cycle, so that the biting rate roughly doubles. Since the biting rate enters the transmission equation squared, the combined effect is large.

Urban Heat, Altitudinal Shifts, and El Niño Amplification

Warming, the strong El Niño of 2023 to 2024, and renewed travel after the COVID-19 pandemic together drove a sharp rise in cases from 2023. The Americas recorded about 4.6 million cases in 2023 and around 13 million in 2024, with record epidemics in Brazil, Argentina, Paraguay, Peru, Colombia, and Mexico, alongside large outbreaks in Bangladesh, Indonesia, Thailand, Sri Lanka, and several African countries.

Four processes underlie the longer-term expansion:

1. Cities are moving toward the optimum. Because dengue transmission peaks at a higher temperature than that of any other major vector-borne disease, warming combined with the additional two to five degrees of urban heat moves tropical and subtropical cities into the range of maximum transmission and lengthens the season by one to four months.
2. Spread to higher elevations. Because temperature falls by about 6.5 °C per kilometre of altitude, the lower limit for transmission has moved several hundred metres upslope. Dengue now occurs in cities formerly considered too cool, including Kathmandu at around 1,400 m, which had large epidemics from 2022, as well as highland cities in Bolivia, Colombia, Ecuador, and Mexico.
3. Spread toward the poles. In southern South America, transmission has extended across central Argentina and into Uruguay. In Europe, where Aedes albopictus is established in more than twenty countries, hot summers and infected travellers have produced locally acquired cases in Italy, France, and Spain. In the United States, local transmission has recurred in Florida, Texas, Hawaii, and, recently, southern California.
4. Both rain and drought help the mosquito. Heavy rainfall fills outdoor containers. Drought has the opposite cause but the same effect: when piped water fails, households store water indoors in open containers, which are warm, free of predators, and constantly refilled.

Stage-Specific Laboratory Diagnostics

Because the virus and the antibody response follow opposite time courses, the right test depends on how long the patient has been ill.

Diagnostic tests by stage of illness

Supportive Clinical Management and Fluid Resuscitation

No antiviral drug is approved for dengue. Several candidates have been abandoned, and an NS4B inhibitor that reached clinical trials was paused in 2024, though related compounds remain in development.

Treatment is therefore supportive, and depends on avoiding harm and on careful management of fluids during the critical phase:

• Avoiding drugs that promote bleeding. Paracetamol may be used for fever and pain. Aspirin, ibuprofen, and other anti-inflammatory drugs must be avoided, because they impair platelet function and irritate the stomach, and can precipitate serious bleeding in a patient whose platelet count is already low. Aspirin additionally risks Reye's syndrome in children. Trials have shown no benefit from corticosteroids or from transfusing platelets in the absence of bleeding.
• Fluid replacement guided by haematocrit. When a patient enters the critical phase with warning signs or shock, survival depends on prompt but carefully measured intravenous fluids, using isotonic solutions such as Ringer's lactate or saline.
• Unlike septic shock, leakage in dengue lasts a defined period of 24 to 48 hours. Infusion rates must therefore be reduced progressively, guided by repeated measurements of haematocrit, urine output, and pulse pressure.
• If the haematocrit stays high despite fluids, a colloid solution may be given. Hydroxyethyl starch, formerly used for this purpose, is now avoided in many countries because of concerns about kidney injury.
• If the haematocrit falls sharply while the patient remains in shock, the cause is usually internal bleeding rather than recovery, and blood transfusion is required.
• Once recovery begins, intravenous fluids must be stopped promptly to avoid fluid overload in the lungs.
• With experienced management, deaths from dengue shock syndrome can be reduced to well under 1%, although outcomes depend heavily on the resources available.

Tetravalent Dengue Vaccines

A dengue vaccine presents an unusual difficulty: because partial immunity to one type can worsen infection with another, a vaccine must protect against all four simultaneously, and must do so durably. Three live vaccines have completed large trials, and the differences between them illustrate the importance of including the virus's non-structural proteins.

Licensed tetravalent dengue vaccines

Integrated Vector Control and Wolbachia Deployment

Because vaccines give only partial protection, control of the mosquito remains essential.

1. Wolbachia. Introducing this bacterium into Aedes aegypti populations has two effects: it spreads by itself, because infected females out-reproduce uninfected ones, and it strongly reduces the mosquito's ability to transmit dengue. In a randomized trial in Yogyakarta, Indonesia, it reduced dengue cases by 77% and hospital admissions by 86%. Deployments in Brazil, Colombia, and Australia have reported large reductions, though most were not randomized. A variant approach releases only infected males, which sterilize wild females; this has suppressed mosquito numbers and dengue in Singapore.
2. Targeted indoor spraying. Outdoor fogging is largely ineffective, because the mosquitoes rest indoors and the spray does not reach eggs or larvae. Spraying the lower walls, closets, and furniture where the mosquitoes actually rest, using insecticides other than pyrethroids, is far more effective, as are screens on windows and doors and devices releasing repellent vapour indoors.
3. Removing and treating breeding sites. Clearing discarded containers, covering stored water, treating cisterns with bacterial larvicides, and using traps that dust visiting females with an insect growth regulator which they then carry to hidden breeding sites.

• Dengue virus
• Aedes aegypti
• Antibody-dependent enhancement
• Mosquito-borne disease
• Vector control

Why does severe dengue plasma leakage occur when the fever is subsiding?

Vascular permeability in severe dengue peaks during defervescence (days 4–6 of illness) rather than at peak viremia because plasma leakage is driven by the cumulative endothelial toxicity of circulating hexameric NS1 protein and an acute adaptive immune recall cascade. Secreted NS1 activates endothelial cathepsin L and heparanase to strip the luminal glycocalyx, while cross-reactive memory T cells and afucosylated IgG antibodies trigger intense cytokine release and platelet activation across microvascular beds.

How does antibody-dependent enhancement (ADE) increase the risk of severe dengue?

Primary infection with one dengue serotype elicits lifelong homotypic neutralizing antibodies alongside low-affinity, cross-reactive antibodies against the conserved E fusion loop and prM protein. Once heterologous antibody titers wane into a sub-neutralizing concentration window roughly 1–3 years later, they bind a second infecting serotype without blocking fusion and facilitate Fcγ-receptor-mediated endocytosis into monocytes and macrophages, amplifying systemic viral burden and downstream vascular permeability.

Why must aspirin and NSAIDs be avoided in suspected dengue fever?

Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen inhibit cyclooxygenase-1 (COX-1) and thromboxane A₂ synthesis, irreversibly or transiently impairing platelet aggregation while simultaneously irritating the gastric mucosa. In dengue patients—who already suffer severe thrombocytopenia, NS1-mediated platelet activation, and microvascular fragility—NSAID administration can precipitate life-threatening gastrointestinal hemorrhage during the critical phase, making paracetamol (acetaminophen) the only safe antipyretic.