Dengue virus

Dengue virus is the most important arthropod-borne virus of humans, causing an estimated 390 million infections a year across the tropics and subtropics. It is a mosquito-borne flavivirus with four distinct serotypes, spread chiefly by the urban mosquito Aedes aegypti, and it produces a spectrum of illness from a silent or mild febrile infection to a life-threatening syndrome of plasma leakage and shock.

The feature that sets dengue apart from most other viral infections is that prior immunity can make the next infection worse. Infection with one serotype gives lasting protection against that serotype but only brief protection against the others, and a later infection with a different serotype carries the highest risk of severe disease. This phenomenon, antibody-dependent enhancement, shapes the epidemiology, the clinical picture and the difficulty of making a safe vaccine.

Severe disease is not caused by the virus destroying tissue but by a sudden, transient increase in the leakiness of small blood vessels, which appears paradoxically just as the fever settles. Recognising that critical window, and managing it with careful fluid replacement, is what separates a survivable illness from a fatal one, and it is why dengue is as much a problem of clinical timing as of virology.

Discovery and historical significance

A dengue-like illness was described early. Benjamin Rush gave the classic account of “breakbone fever” in Philadelphia in 1789, capturing the severe musculoskeletal pain that still names the disease in many languages.

The virus itself was isolated only in the 1940s, when Susumu Hotta and Ren Kimura in Japan and, independently, Albert Sabin recovered it from febrile patients and began to distinguish its serotypes. The recognition that a second, heterologous infection underlies the severe haemorrhagic form came from careful epidemiology in South East Asia in the 1950s and 1960s, as dengue haemorrhagic fever emerged as a major cause of childhood death, and it framed the antibody-enhancement model that dominates dengue research to this day.

Classification, structure, and genome

Classification

Dengue virus is the species Orthoflavivirus denguei in the genus Orthoflavivirus (recently renamed from Flavivirus), family Flaviviridae. It is a member of the mosquito-borne flavivirus group, alongside yellow fever, Zika, West Nile and Japanese encephalitis viruses.

Its defining internal division is into four serotypes, DENV-1 to DENV-4, which differ in their envelope protein by roughly a quarter to a third of amino acid sequence. The four are close enough to cross-react serologically but distant enough that immunity to one does not durably protect against the others, the biological fact on which antibody-dependent enhancement depends. Each serotype contains multiple genotypes, and all four now co-circulate across much of the tropics.

Virion structure

The virion is a small enveloped particle about 50 nanometres across. Beneath the lipid envelope, an icosahedral shell of 180 copies each of the envelope (E) and membrane (M) proteins lies flat in a smooth herringbone arrangement. The E protein carries out both receptor binding and, as a class II fusion protein, membrane fusion, and it is the dominant target of neutralising antibody.

Virion maturity is central to dengue biology. Immature particles carry the precursor premembrane (prM) protein, which is cleaved by the host protease furin as the particle exits the cell. Because this cleavage is frequently incomplete, released virions are a mixture of fully mature, partially mature and immature particles, and the poorly neutralised immature particles are important carriers of antibody-dependent enhancement.

Genome organisation

The genome is a single strand of positive-sense RNA of about 11 kilobases, with one open reading frame flanked by structured untranslated regions and no poly-A tail. It is translated as a single polyprotein that host and viral proteases cut into three structural proteins (capsid, prM and E) and seven nonstructural proteins.

Several nonstructural proteins recur through the virus biology. NS5 is the RNA-dependent RNA polymerase and methyltransferase; NS3 is the protease and helicase; and NS1 is secreted from infected cells as a hexamer that circulates in the blood, where it both serves as an early diagnostic antigen and contributes directly to the vascular leak of severe disease.

Replication cycle

Dengue enters cells by receptor-mediated endocytosis. Rather than depending on one defined receptor, the virus uses a range of attachment factors, including the lectin DC-SIGN on dendritic cells, which helps explain its tropism for these early target cells. Bound virus is taken up in a clathrin-coated vesicle.

Acidification of the endosome then drives the reaction at the heart of entry: a conformational change in the E protein exposes its fusion loop, which inserts into the endosomal membrane and fuses it with the viral envelope, releasing the genome into the cytoplasm.

The positive-sense genome is translated directly into the polyprotein, which is processed at the endoplasmic reticulum membrane. Replication then proceeds inside virus-induced invaginations of the endoplasmic reticulum, membranous compartments that concentrate the replication machinery and hide the double-stranded RNA intermediate from innate immune sensors, with NS5 copying the genome.

Assembly buds immature particles into the endoplasmic reticulum lumen. As these travel through the secretory pathway, the mildly acidic trans-Golgi network lets furin cleave prM to its mature form, priming the particle for the next round of infection, and the virion is released by exocytosis while secreted NS1 accumulates in the circulation.

Pathogenesis

The first cells infected are dendritic cells, monocytes and macrophages in the skin, from which the virus spreads to lymph nodes and the bloodstream. Because these phagocytes bear Fc receptors, they are also the cells at the centre of antibody-dependent enhancement.

Antibody-dependent enhancement is the defining mechanism of severe dengue. After a first infection, antibody against that serotype wanes to a level that no longer neutralises but still binds a different serotype on later exposure. Antibody-coated virus is then taken up more efficiently into Fc-receptor-bearing cells, raising the viral load and provoking a stronger inflammatory response. The single greatest risk factor for severe dengue is therefore a second infection with a different serotype; the risk is real but modest in absolute terms, on the order of ~0.5% of secondary infections. The same mechanism explains why infants of dengue-immune mothers can develop severe primary dengue as maternal antibody declines through the first year of life.

The amplification is not only antibody-driven. Cross-reactive memory T cells raised against the first serotype respond to the second but bind it poorly, and instead of clearing it efficiently they release a disproportionate burst of inflammatory mediators, a phenomenon termed original antigenic sin in the T-cell compartment. The resulting cytokine storm, with tumour necrosis factor and several interleukins prominent, is what tips endothelial permeability from a controlled response into frank plasma leakage.

The injury itself is a transient, reversible increase in vascular permeability rather than structural damage to the vessels. A surge of inflammatory cytokines, together with the direct action of secreted NS1 on the endothelial surface layer, loosens the barrier and lets plasma leak into the tissues and body cavities. Because the change is functional, survivors recover without vascular scarring once the critical phase passes. Thrombocytopenia, from both reduced production and increased consumption, and a coagulopathy compound the picture and account for the bleeding tendency.

Epidemiology

Dengue is the commonest and fastest-spreading arboviral disease in the world, with an estimated 390 million infections each year, of which roughly a quarter are clinically apparent, and about half the global population living in areas at risk. It is endemic across more than 100 countries in Asia, the Americas, Africa and the Western Pacific, with Asia carrying the largest burden.

Transmission is overwhelmingly urban and peri-urban. The principal vector, Aedes aegypti, is a container-breeding, day-biting mosquito exquisitely adapted to human dwellings, and Aedes albopictus is a secondary vector with a wider temperate range. Humans are the amplifying host in this urban cycle, reaching viraemia high enough to infect feeding mosquitoes, so the virus needs no animal reservoir to sustain an epidemic.

The disease is expanding. Urbanisation, population growth, global travel and a warming climate are extending the range of the vector and lengthening transmission seasons, and the co-circulation of all four serotypes in many regions increases the pool of people exposed to the heterologous second infection that drives severe disease.

Natural history

After an incubation of about four to seven days (range 3 to 14 days), symptomatic dengue classically runs through three phases. Most infections, however, never reach clinical attention, being either asymptomatic or an undifferentiated mild fever.

The febrile phase lasts two to seven days, with high fever and the systemic symptoms of the acute illness. The critical phase begins around defervescence, typically days 4 to 6, and lasts 24 to 48 hours: in the small minority who progress, this is when plasma leakage peaks and shock can develop, precisely when the falling fever might wrongly suggest improvement. Those who pass through it enter the recovery phase, during which leaked fluid is reabsorbed over the following week, a stage at which over-vigorous earlier fluid replacement can now cause overload.

Clinical presentations and complications

Most symptomatic dengue is an acute febrile illness: sudden high fever with severe headache, retro-orbital pain, marked myalgia and arthralgia (the “breakbone” pain), and a rash. It is self-limiting in the great majority.

The World Health Organization (WHO) classifies disease by severity, a scheme that replaced the older dengue fever, dengue haemorrhagic fever and dengue shock syndrome grading.

Category	Defining features
Dengue without warning signs	Fever plus two of: nausea or vomiting, rash, aches and pains, leukopenia, positive tourniquet test
Dengue with warning signs	Any of: abdominal pain or tenderness, persistent vomiting, clinical fluid accumulation, mucosal bleeding, lethargy, liver enlargement, rising haematocrit with a rapid fall in platelets
Severe dengue	Severe plasma leakage (shock or respiratory distress from fluid accumulation), severe bleeding, or severe organ impairment

The warning signs cluster around defervescence and mark the entry to the critical phase, which is why a patient whose fever is settling but who develops abdominal pain, vomiting or bleeding needs closer observation, not reassurance. Severe plasma leakage produces dengue shock, recognised by a narrowing pulse pressure before hypotension appears; severe bleeding and severe organ involvement (hepatitis with very high transaminases, myocarditis, or encephalopathy) define the other routes to severe disease. Severe organ impairment can occur even without marked plasma leakage.

The tourniquet test, inflating a blood-pressure cuff to midway between systolic and diastolic pressure for five minutes and counting the petechiae that appear over a patch of forearm skin, is a simple bedside marker of capillary fragility and forms part of the case definition where laboratory support is limited.

Beyond the plasma-leakage picture, dengue can injure individual organs, a pattern termed expanded dengue syndrome. The liver is commonly involved, from mild transaminase rises to, rarely, fulminant hepatic failure with very high aspartate and alanine transaminases. Dengue myocarditis can depress cardiac function and cause arrhythmia, adding a cardiogenic element to shock. Neurological disease arises by several routes: an encephalopathy that is usually metabolic, secondary to shock, hepatic failure or electrolyte disturbance, and, less often, a genuine viral encephalitis, alongside post-infectious Guillain–Barré syndrome.

Dengue in pregnancy carries added risk. Maternal infection raises the chance of severe disease, and vertical transmission around delivery can cause neonatal dengue, presenting with fever, thrombocytopenia and bleeding in the first days of life. Infection near term is also associated with preterm birth, low birth weight and peripartum haemorrhage.

Diagnosis

Diagnosis is anchored to the phase of illness. In the first five days, while the patient is viraemic, the secreted NS1 antigen test and reverse-transcriptase polymerase chain reaction (RT-PCR) detect the virus directly, and RT-PCR can also identify the serotype.

From about day 5, as viraemia falls and antibody rises, serology takes over: immunoglobulin M (IgM) indicates recent infection, and a rising immunoglobulin G (IgG) titre confirms it, with a very high IgG early in illness suggesting a secondary infection.

Serology must be read with care. Antibody against dengue cross-reacts with other flaviviruses, including after yellow fever or Japanese encephalitis vaccination, so a single positive IgM often needs confirmation, and paired acute and convalescent samples may be required. The full blood count supports the clinical assessment: leukopenia is common, and a falling platelet count with a rising haematocrit is the laboratory signature of the critical phase.

Management

There is no specific antiviral for dengue; management is supportive, and its central discipline is fluid balance. Uncomplicated dengue is managed with oral fluids, rest and paracetamol, with the patient advised of the warning signs and the timing of the critical phase.

The decisive intervention in severe disease is judicious intravenous fluid replacement through the critical phase, titrated to clinical status and haematocrit rather than given by fixed volume, because both under-replacement (shock) and over-replacement (fluid overload in the recovery phase) are dangerous. Isotonic crystalloids are first line, with colloids reserved for refractory shock.

Two supportive points are important. Aspirin and other non-steroidal anti-inflammatory drugs are avoided because they worsen the bleeding tendency, and paracetamol is preferred for fever and pain. Platelet transfusion is reserved for significant bleeding rather than given for a low count alone. There is no role for corticosteroids.

Prevention and public health

With no specific treatment available, dengue prevention rests on controlling the mosquito vector and, increasingly, on vaccination.

Vector control

Controlling the Aedes aegypti vector is the foundation of dengue prevention where no vaccine is in use. It relies on source reduction, the removal or covering of the small artificial water containers in which the mosquito breeds, supported by larvicides, adulticide spraying during outbreaks, and personal protection with repellents and screening. The most significant recent advance is the release of mosquitoes carrying the bacterium Wolbachia, which reduces their ability to transmit dengue and has lowered transmission substantially in several field trials.

Vaccination

Dengue vaccination is complicated by antibody-dependent enhancement, because a vaccine that behaves like a first infection can prime a dengue-naive recipient for severe disease on later natural exposure.

The first vaccine, Dengvaxia (CYD-TDV), is restricted to people with confirmed prior dengue infection for exactly this reason: it raised the risk of severe dengue in seronegative vaccinees. The newer tetravalent live-attenuated vaccine Qdenga (TAK-003) is WHO-recommended for children in high-transmission settings and can be given without pre-vaccination screening, a considerable practical advantage, with sustained efficacy demonstrated over several years of follow-up.

Surveillance and notification

Dengue is a notifiable condition in many countries and is under global surveillance by the WHO, which tracks its geographic expansion and serotype circulation. Surveillance guides vector-control campaigns, anticipates outbreaks from serotype shifts, and underpins the transmission-intensity thresholds that determine where vaccination is worthwhile.

South African context

Dengue is not endemic in South Africa, and almost all diagnosed cases are in travellers returning from endemic areas in Asia, the Indian Ocean islands, the Americas and elsewhere in Africa. It should be considered in any traveller with a febrile illness after visiting a dengue-endemic region, particularly in the first two weeks after return.

Dengue is a Category 3 notifiable medical condition, reported through routine channels rather than the 24-hour pathway reserved for the haemorrhagic-fever arboviruses. Confirmatory serology and molecular testing are provided by the National Institute for Communicable Diseases (NICD) Arbovirus Reference Laboratory, to which specimens are submitted with a structured case-investigation form recording travel and exposure history. Because the viraemic window is short, a convalescent sample is often needed to confirm or exclude infection when the first specimen is taken late.

References and recommended reading

• Pierson TC, Lazear HM, Diamond MS. Flaviviruses. In: Fields Virology, 7th edition, Chapter 9. Philadelphia: Wolters Kluwer; 2023. The principal source for dengue clinical disease, antibody-dependent enhancement, pathogenesis and epidemiology.
• Lindenbach BD, Randall G, Bartenschlager R, Rice CM. Flaviviridae: The Viruses and Their Replication. In: Fields Virology, 7th edition, Chapter 7. Philadelphia: Wolters Kluwer; 2023. The source for virion structure, genome organisation and the replication cycle.
• Petersen LR, Barrett ADT. Arthropod-Borne Flaviviruses. In: Richman DD, Whitley RJ, Hayden FG (eds.), Clinical Virology, 4th edition, Chapter 53. Washington: ASM Press; 2016. The foundational account of the mosquito-borne flaviviruses.
• World Health Organization. Dengue and severe dengue. WHO fact sheet; 2025. The source for the severity classification, warning signs and current vaccine recommendations.
• National Institute for Communicable Diseases. Arboviral Disease. NICD; 2025. The source for the South African epidemiology, notifiable-condition status and the reference-laboratory pathway.