The World Health Organization (WHO) has updated its fact sheet on dengue and severe dengue.
Dengue is a mosquito-borne viral disease that is transmitted by female mosquitoes mainly of the species Aedes aegypti and, to a lesser extent, Ae. albopictus.
The Aedes aegypti mosquito is the primary vector of dengue. The virus is transmitted to humans through the bites of infected female mosquitoes. After virus incubation for 4–10 days, an infected mosquito is capable of transmitting the virus for the rest of its life.
Infected symptomatic or asymptomatic humans are the main carriers and multipliers of the virus, serving as a source of the virus for uninfected mosquitoes. Patients who are already infected with the dengue virus can transmit the infection (for 4–5 days; maximum 12) via Aedes mosquitoes after their first symptoms appear.
The Aedes aegypti mosquito lives in urban habitats and breeds mostly in man-made containers. Unlike other mosquitoes Ae. aegypti is a day-time feeder; its peak biting periods are early in the morning and in the evening before dusk. Female Ae. aegypti bites multiple people during each feeding period. Aedes eggs can remain dry for over a year in their breeding habitat and hatch when in contact with water.
Aedes albopictus, a secondary dengue vector in Asia, has spread to North America and more than 25 countries in the European Region, largely due to the international trade in used tyres (a breeding habitat) and other goods (e.g. lucky bamboo). Ae. albopictus is highly adaptive and, therefore, can survive in cooler temperate regions of Europe. Its spread is due to its tolerance to temperatures below freezing, hibernation, and ability to shelter in microhabitats.
Dengue is caused by a virus of the Flaviviridae family and there are 4 distinct, but closely related, serotypes of the virus that cause dengue (DEN-1, DEN-2, DEN-3 and DEN-4). Recovery from infection by one provides lifelong immunity against that particular serotype. However, cross-immunity to the other serotypes after recovery is only partial and temporary. Subsequent infections (secondary infection) by other serotypes increase the risk of developing severe dengue.
The global incidence of dengue has grown dramatically in recent decades. About half of the world’s population is now at risk– one study estimates that 3.9 billion people, in 128 countries, are at risk of infection with dengue viruses.
Member States in three WHO regions regularly report the annual number of cases. The number of cases reported increased from 2.2 million in 2010 to over 3.34 million in 2016. Although the full global burden of the disease is uncertain, the initiation of activities to record all dengue cases partly explains the sharp increase in the number of cases reported in recent years.
Dengue virus is transported from one place to another by infected travelers.
Before 1970, only 9 countries had experienced severe dengue epidemics. The disease is now endemic in more than 100 countries in the WHO regions of Africa, the Americas, the Eastern Mediterranean, South-East Asia and the Western Pacific. The America, South-East Asia and Western Pacific regions are the most seriously affected.
Not only is the number of cases increasing as the disease spreads to new areas, but explosive outbreaks are occurring. The threat of a possible outbreak of dengue fever now exists in Europe as local transmission was reported for the first time in France and Croatia in 2010 and imported cases were detected in 3 other European countries.
Among travellers returning from low- and middle-income countries, dengue is the second most diagnosed cause of fever after malaria.
In 2015, Delhi, India, recorded its worst outbreak since 2006 with over 15 000 cases.
The year 2016 was characterized by large dengue outbreaks worldwide. The Region of the Americas region reported more than 2.38 million cases in 2016, where Brazil alone contributed slightly less than 1.5 million cases, approximately 3 times higher than in 2014. 1032 dengue deaths were also reported in the region.
In 2017, a significant reduction was reported in the number of dengue cases in the Americas – from 2 177 171 cases in 2016 to 584 263 cases in 2017. This represents a reduction of 73%. The post Zika outbreak period (after 2016) has seen a decline of cases of dengue and the exact factors leading to this fall decrease is still unknown.
After a drop in the number of cases in 2017-18, sharp increase in cases is being observed in 2019.
An estimated 500 000 people with severe dengue require hospitalization each year, and with an estimated 2.5% case fatality, annually. However, many countries have reduced the case fatality rate to less than 1% and globally, 28% decline in case fatality have been recorded between 2010 and 2016 with significant improvement in case management through capacity building at country level.
Dengue fever is a severe, flu-like illness that affects infants, young children and adults, but seldom causes death.
Dengue should be suspected when a high fever (40°C/104°F) is accompanied by 2 of the following symptoms:
- severe headache,
- pain behind the eyes,
- muscle and joint pains,
- swollen glands or
Symptoms usually last for 2–7 days, after an incubation period of 4–10 days after the bite from an infected mosquito.
Severe dengue is a potentially deadly complication due to plasma leaking, fluid accumulation, respiratory distress, severe bleeding, or organ impairment. Warning signs occur 3–7 days after the first symptoms in conjunction with a decrease in temperature (below 38°C/100°F) and include:
- severe abdominal pain,
- persistent vomiting,
- rapid breathing,
- bleeding gums,
- restlessness and
- blood in vomit.
The next 24–48 hours of the critical stage can be lethal; proper medical care is needed to avoid complications and risk of death.
There is no specific treatment for dengue fever.
For severe dengue, medical care by physicians and nurses experienced with the effects and progression of the disease can save lives – decreasing mortality rates from more than 20% to less than 1%. Maintenance of the patient’s body fluid volume is critical to severe dengue care.
Prevention and Control
The live attenuated dengue vaccine CYD-TDV has been shown in clinical trials to be efficacious and safe in persons who have had a previous dengue virus infection (seropositive individuals), but carries an increased risk of severe dengue in those who experience their first natural dengue infection after vaccination (seronegative individuals).
For countries considering vaccination as part of their dengue control programme, pre-vaccination screening is the recommended strategy. With this strategy, only persons with evidence of a past dengue infection would be vaccinated (based on an antibody test, or on a documented laboratory confirmed dengue infection in the past).
At present, the main method to control or prevent the transmission of dengue virus is to combat vector mosquitoes through:
- preventing mosquitoes from accessing egg-laying habitats by environmental management and modification;
- disposing of solid waste properly and removing artificial man-made habitats;
- covering, emptying and cleaning of domestic water storage containers on a weekly basis;
- applying appropriate insecticides to water storage outdoor containers;
- using of personal household protection measures, such as window screens, long-sleeved clothes, repellents, insecticide treated materials, coils and vaporizers (These measures have to be observed during the day both at home and place of work since the mosquito bites during the day);
- improving community participation and mobilization for sustained vector control;
- applying insecticides as space spraying during outbreaks as one of the emergency vector-control measures;
- active monitoring and surveillance of vectors should be carried out to determine effectiveness of control interventions.
Careful clinical detection and management of dengue patients can significantly reduce mortality rates from severe dengue.
Link to the updated fact sheet:
Link to WHO page on Dengue control:
Link to WHO’s Global Strategy for Dengue Prevention and Control 2012-2020: