Travel Health Information Sheets
Dengue fever (DF) is caused by an arthropod borne virus of the genus Flavivirus, and within the family Flaviviridae. Other flaviviruses include Japanese encephalitis and yellow fever. There are four distinct serotypes of dengue virus (DEN 1, DEN 2, DEN 3 and DEN 4) all of which have the potential to cause either classic DF or the more serious form of the disease, dengue haemorrhagic fever (DHF). Dengue is transmitted by the bite of an infected Aedes sp. mosquito.
Dengue has been reported since the 18th century. During that time major epidemics occurred at intervals of 10 to 40 years in Asia, Africa, and North America. The Aedes mosquito and the dengue virus were dependent on sailing vessels to transport them from one population to another, and when a new serotype was introduced, new epidemics occurred . The epidemiology of dengue changed after the Second World War, due to increasing economic growth and the urbanisation of South East (SE) Asia in particular, where millions of people moved to the cities. The dengue virus spread rapidly and the disease developed into pandemic proportions .
The World Health Organization (WHO) estimates that 2.5 billion people live in over 100 endemic countries and areas where dengue viruses can be transmitted . A recent study estimated that there were more people at risk of dengue infection calculating that up to 3.97 billion people are at risk in 128 countries . Approximately 50 million infections occur annually with 500 000 cases of DHF. It is estimated that there are 22,000 deaths annually, mainly among children .
The main vector, Aedes aegypti, is found worldwide between latitudes 35ºN and 35ºS .
The principle areas affected include the Caribbean, South and Central America, Mexico, Africa, the Pacific Islands, SE Asia, Indian sub-continent, Hawaii, and Australia (see Figure 1).
Epidemic dengue increased in East Africa in the 1980s, and all four serotypes have now been documented throughout the African continent. The same pattern has also emerged in the Americas, notably Central and South America.
Figure 1. Dengue, countries or areas at risk, 2011.
This map is reproduced with acknowledgment to the World Health Organization.
The highest burden of disease occurs in SE Asia and the Western Pacific (Figure 1), but over the last few years there has been a rising trend in South America and the Caribbean .
Cases are also increasingly being reported outside tropical areas. The continued increase in urbanisation, population growth and global travel introduces the different serotypes into new populations.
DF is an emerging disease in parts of Europe; during 2010 locally acquired cases were reported in Croatia  and France . In October 2012 the first locally acquired cases were reported in the Autonomous Region of Madeira .
To standardise the reporting of dengue and improve the quality and accuracy of dengue statistics, the World Health Organization (WHO) created DengueNet . This is an online database for sharing current surveillance data in order to detect and monitor incidence and trends of DF and DHF. The number of cases of DF reported to the WHO has increased annually (See Figure 2).
Figure 2. Average annual number of DF/DHF cases reported to WHO and average annual number of countries reporting dengue .
Dengue fever in UK travellers
DF does not naturally occur in the United Kingdom (UK) but is imported in small numbers. In 2012, 343 cases of DF were reported in England, Wales and Northern Ireland (EWNI). The top five reported countries of travel in 2012 were India (75), Thailand (58), Jamaica (33), Portugal (Madeira) (20), and Sri Lanka (15) ; (for dengue cases in EWNI 2010-April 2013 see Figure 3.).
Figure 3. Laboratory-confirmed and probable cases of dengue fever by month and country of travel, England, Wales and Northern Ireland: 2010-April 2013
Data courtesy of Rare and Imported Pathogens Laboratory and Travel and Migrant Health Section, Public Health England
The chance of contracting DF is determined by several factors including travel destination, length of exposure in endemic areas, the intensity of dengue transmission, and the 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 tropical countries where dengue is endemic are at risk of infection, although determining the actual level of risk is difficult. Travellers who spend a long period in endemic areas (such as expatriates or aid workers) are at increased risk; however, even short-term visitors may be exposed to dengue [12-14]. A study by the GeoSentinel surveillance network ( a global network of travel and tropical medicine clinics in six countries) indicated that the regions at highest risk for dengue were SE Asia and the Caribbean . The true incidence of DF travellers is probably underestimated because in many countries reporting is not obligatory and, due to its non-specific symptoms, it is probably under-diagnosed .
Transmission occurs following a bite from an infected Aedes mosquito. It is most widely transmitted by Ae. aegypti and Ae. albopictus (Asia, Philippines and Japan), other Aedes species also transmit disease in specific areas; Ae. polynesiensis, Ae. scutellaris and Ae. pseudoscutallaris (Pacific Islands and New Guinea), Ae. polynesiensis (Society Islands) and Ae. niveus (Philippines) .
The cycle of transmission typically involves humans and mosquitoes. The virus is spread from an infected human to a mosquito and then to another human, often in areas where there are dense human populations. In parts of SE Asia and Africa, the transmission cycle may also involve jungle primates that act as a reservoir for the virus.
The Aedes mosquito prefers to breed in water-filled receptacles, usually close to human habitation. They often rest in dark rooms (e.g. in bathrooms and under beds) and breed in small pools that collect in discarded human waste. Although they are most active during daylight hours, biting from dawn to dusk, mosquitoes will feed throughout the day indoors and during overcast weather. The mosquito becomes infectious 8-10 days after feeding and remains infectious for life (2-3 months).
The disease can be classified into five presentations; non-specific febrile illness, classic dengue, DHF, DHF with dengue shock syndrome, and other unusual syndromes such as encephalopathy and fulminant liver damage. Clinical features vary with the age of the patient. Between 15% and 90% of cases are asymptomatic or sub-clinical .
The incubation period is 5 to 8 days. In non-immune persons, dengue begins with a fever lasting 1 to 5 days.
Young children with dengue often have an undifferentiated febrile illness with a maculopapular rash, which typically spreads from the trunk to include the limbs and face and which occurs between days 3 and 5 of the illness. Upper respiratory tract infections are common. Most infections in children are asymptomatic or minimally symptomatic.
Classic dengue is more common amongst older children, adolescents, and adults. It has an abrupt onset with a high fever which is often accompanied by a severe headache, myalgia, arthralgia, nausea, and vomiting. Most infections are self-limiting with improvement in symptoms and rapid recovery occurring 3 to 4 days after the onset of the rash.DHF is primarily a disease of children under 15 years in hyperendemic areas. It is characterised by increased capillary permeability and haemostatic changes e.g. bleeding under the skin (purpura), from the gums and gastrointestinal tract. Mortality can be as high as 10-20% if left untreated.
DHF with shock is accompanied by respiratory and/or renal failure. Mortality, if left untreated, can be 40%. Survival rates are significantly higher if the patient is treated in hospital by experienced staff, leading to low mortality rates of 1% to 2%. It is not certain what causes progression to the severe form, although it has been suggested that previous infections with a different serotype of the virus predisposes to DHF when a person becomes re-infected. This form of dengue is rarely seen in travellers.
Rare presentations of infection include severe haemorrhage, jaundice, parotitis, and cardiomyopathy. Neurological symptoms can include encephalitis, polyneuropathies and transverse myelitis. Guillain-Barré syndrome has also been reported.
Lifelong immunity to the infecting virus serotype occurs in those who recover, however, infection with one serotype does not confer immunity to the other three serotypes or to other flaviviruses.
Health professionals should be alert to the possibility of dengue in those who have recently returned from an endemic area presenting with a flu-like illness. Advice should be sought in the first instance from a local microbiology, virology or infectious disease consultant.
There is no specific antiviral treatment for either classic dengue or DHF. Supportive nursing care and careful management of fever, fluid balance, electrolytes, and clotting parameters are the standard treatment.
There is no vaccine to prevent DF. Prevention is by minimising mosquito bites especially during daylight hours. Particular vigilance with bite precautions should be taken around dawn and dusk.
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Updated May 2013
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