Dengue

Dengue is caused by a virus of the genus Flavivirus, within the family Flaviviridae. It is spread by the bite of an infected Aedes spp. mosquito, which predominantly feed during daytime hours.

There are four distinct serotypes of dengue virus: DENV-1, DENV-2, DENV-3 and DENV-4. All have the potential to cause severe dengue, formerly known as dengue haemorrhagic fever (DHF). Severe dengue is more likely if a person has had a previous dengue infection.

According to the World Health Organization (WHO), the number of dengue cases reported worldwide has grown dramatically in recent decades. In 2000, there were 505,430 dengue cases reported to WHO, compared with 5.2 million in 2019. Factors associated with increasing risk of spread of dengue epidemic include climate change leading to increasing temperatures, high rainfall and humidity. Other factors include the increased movement of people and goods, urbanisation and pressure on water and sanitation.

Under reporting, misclassification of disease and the practice of reporting confirmed cases only, means the global burden of disease is likely to be far greater: approximately half of the world's population is at risk, and it is therefore estimated that the likely incidence of disease is between 100-400 million infections each year.

Risk areas

Dengue occurs in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas. Dengue is endemic in more than 100 countries in Africa, the Americas, the Eastern Mediterranean, Southeast Asia and the Western Pacific, with Asia accounting for 70% of the global burden of disease.

Outbreaks of dengue are under-reported in Africa due to limited laboratory capacity and less well-developed dengue surveillance systems. However, the presence of the disease and the high prevalence of antibodies to dengue viruses in serological (blood test) surveys in local populations suggest that dengue virus infection is endemic in many parts of the continent.

Increasing global temperatures aid the wider geographic distribution of some species of Aedes mosquitoes. Temperate regions are therefore increasingly at risk of becoming endemic for dengue. Dengue-infected travellers returning from endemic areas can generate local outbreaks in areas where there are suitable mosquito vectors, even if dengue is not usually found in that region.

Dengue is now an emerging disease outside tropical areas, including parts of Europe. Since 2010, several outbreaks have been reported in Europe in Croatia, France, Italy and Spain.

From September 2012 to March 2013, the autonomous Portuguese island of Madeira reported its first dengue outbreak, with 2,168 probable and 1,080 confirmed cases. Imported cases associated with this outbreak were also detected in travellers returning from Madeira to other countries in Europe, including the UK.

Risk for travellers

The likelihood of contracting dengue is determined by several factors, including destination, length of exposure, intensity of transmission and season of travel. Risk is thought to be higher during periods of intense mosquito feeding activity (two to three hours after dawn and during the early evening).

All travellers to dengue endemic countries are at risk, although determining individual risk is difficult. True dengue incidence in travellers is probably underestimated as in many countries dengue reporting is not obligatory. Also, due to absent or non-specific symptoms, dengue is probably under-diagnosed.

Travellers who spend long periods in endemic areas (such as expatriates or aid workers) are at increased risk. However, even short-term visitors may be exposed.

Severe dengue is rare in travellers. Individuals who are infected for the second time with dengue are at greater risk developing severe illness. Severe dengue is also more common in children, adolescents, and pregnant women. An increased risk of severe disease has also been described in older individuals and those with comorbidities such asthma, diabetes, chronic renal failure, obesity, hypertension, bleeding disorders and in those taking anticoagulants.

Transmission

Dengue is transmitted (spread) from human to human by different species of Aedes mosquito. Rarely blood-borne person to person transmission has been reported and pregnant women can pass the infection to their babies at the time of delivery. Reports of sexual transmission are very rare.

Preventing dengue

Prevention is by avoidance of mosquito bites. Particular vigilance with bite precautions should be taken around dawn and dusk. Those living in endemic areas should remove rubbish or water containers close to their home where possible as they can be breeding sites for mosquitoes.

Vaccine information

A vaccine, Qdenga®▼ has been licensed in the UK for the prevention of dengue disease in individuals from 4 years of age. The UK Joint Committee on Vaccination and Immunisation (JCVI) has advised that Qdenga®▼ vaccine can be considered for individuals aged 4 years of age and older who have had dengue infection in the past and are:

Planning to travel to dengue where there is a risk of dengue infection or areas with an ongoing outbreak of dengue.

Exposed to dengue virus through their work, for example, laboratory staff working with the virus.

Blood tests for previous dengue infection may not be 100 percent reliable [24] and assessment must be made of previous tests for dengue, likely exposure and clinical history, see Immunisation against infectious disease dengue chapter for further details on this and the use of Qdenga®▼.

Qdenga®▼ is a live vaccine (it contains live, attenuated dengue virus) and is contraindicated for anyone who is immunosuppressed, pregnant or breastfeeding. This vaccine is also contraindicated for those with hypersensitivity to any component of the vaccine and for children under four years of age.

All of this information is taken from the TravelHealthPro website