Dengue is a mosquito-borne infection which in recent years has become a major
international public health concern. Dengue is found in tropical and sub-tropical regions
around the world, predominately in urban and peri-urban areas. Dengue haemorrhagic fever
(DHF), a potentially lethal complication, was first recognized during the 1950s and is
today a leading cause of childhood mortality in several Asian countries. There are four
distinct, but closely related, viruses which cause dengue. Recovery from infection by one
provides lifelong immunity against that serotype but confers only partial and transient
protection against subsequent infection by the other three. Indeed, there is good evidence
that sequential infection increases the risk of more serious disease resulting in DHF.
The global prevalence of dengue has grown dramatically in recent decades. The
disease is now endemic in more than 100 countries in Africa, the Americas, the Eastern
Mediterranean, South-East Asia and the Western Pacific (see Table 1). South-East Asia and
the Western Pacific are most seriously affected. Before 1970 only nine countries had
experienced DHF epidemics, a number which had increased more than four-fold by 1995. Some
2500 million people - two fifths of the world's population - are now at risk from
dengue. WHO currently estimates there may be 50 million cases of dengue infection
worldwide every year. In 1998 alone, there were more than 616,000 cases of dengue in the
Americas, of which 11,000 cases were DHF. This is greater than double the number of dengue
cases which were recorded in the same region in 1995. Not only is the number of cases
increasing as the disease is spreading to new areas, but explosive outbreaks are
occurring. In Brazil nearly 475,000 cases were reported between January and October 1998
more than were reported from the entire continent in previous years.
Some other statistics:
- During epidemics of dengue, attack rates among susceptibles are often 40 50%, but
may reach 80 90%.
- An estimated 500 000 cases of DHF require hospitalisation each year, of whom a very
large proportion are children and roughly 5% die.
- Without proper treatment, DHF case fatality rates can exceed 20%. With modern intensive
supportive therapy, can be reduced to less than 1%.
The spread of dengue is attributed to expanding geographic distribution of the four
dengue viruses and of their mosquito vectors, the most important of which is the
predominantly urban species Aedes aegypti. A rapid rise in urban populations is
bringing ever greater numbers of people into contact with this vector, especially in areas
which are favourable for mosquito breeding e.g., where household water storage is common
and where solid waste disposal services are inadequate.
Dengue viruses are transmitted to humans through the bites of infective female Aedes
mosquitoes. Mosquitoes generally acquire the virus while feeding on the blood of an
infected person. Once infective a mosquito is capable of transmitting the virus to
susceptible individuals for the rest of its life, during probing and blood feeding.
Infected female mosquitoes may also transmit the virus to the next generation of
mosquitoes by transovarial transmission i.e. via its eggs, but the role of this in
sustaining transmission of virus to humans has not yet been delineated. Humans are the
main amplifying host of the virus, although studies have shown that in some parts of the
world monkeys may become infected and perhaps serve as a source of virus for uninfected
mosquitoes. The virus circulates in the blood of infected humans for 2-7 days, at
approximately the same time as they have fever; Aedes mosquitoes may acquire the
virus when they feed on an individual at this time.
Dengue fever is a severe, flu-like illness that affects infants, young children
and adults but rarely causes death. The clinical features of dengue fever vary according
to the age of the patient. Infants and young children may have a non-specific febrile
illness with rash. Older children and adults may have either a mild febrile syndrome or
the classical incapacitating disease with abrupt onset and high fever, severe headache,
pain behind the eyes, muscle and joint pains, and rash. Dengue haemorrhagic fever is a
potentially deadly complication that is characterized by high fever, haemorrhagic
phenomenaoften with enlargement of the liverand in severe cases, circulatory
failure. The illness commonly begins with a sudden rise in temperature accompanied by
facial flush and other non-specific constitutional symptoms of dengue fever. The fever
usually continues for 2-7 days and can be as high as 40-41° C,
possibly with febrile convulsions and haemorrhagic phenomena. In moderate DHF cases, all
signs and symptoms abate after the fever subsides. In severe cases, the patient's
condition may suddenly deteriorate after a few days of fever; the temperature drops,
followed by signs of circulatory failure, and the patient may rapidly go into a critical
state of shock and die within 12-24 hours, or quickly recover following appropriate volume
There is no specific treatment for dengue fever. However, careful clinical
management by experienced physicians and nurses frequently save the lives of DHF patients.
With appropriate intensive supportive therapy, mortality may be reduced to less than 1%.
Maintenance of the circulating fluid volume is the central feature of DHF case
management.[for detailed advice on managing patients with DHF see:ref.1997 DHF manual]
Vaccine development for dengue and DHF is difficult because any of four
different viruses may cause disease, and because protection against only one or two dengue
viruses could actually increase the risk of more serious disease. Nonetheless, progress is
gradually being made in the development of vaccines that may protect against all four
dengue viruses. Such products could be commercially available within several years.
Prevention and Control
At present, the only method of controlling or preventing dengue and DHF is to
combat the vector mosquitoes. In Asia and the Americas, Aedes aegypti breeds
primarily in man-made containers like earthenware jars, metal drums and concrete cisterns
used for domestic water storage, as well as discarded plastic food containers, used
automobile tyres and other items that collect rainwater In Africa it also breeds
extensively in natural habitats such as tree holes and leaf axils. In recent years, Aedes
albopictus, a secondary dengue vector in Asia, has become established in the United
States and several Latin American and Caribbean countries as well as two European and one
African state. The rapid geographic spread of this species has been largely attributed to
the international trade in used tyres.
Vector control is implemented using environmental management and chemical methods.
Proper solid waste disposal and improved water storage practices, including covering
containers to prevent access by egg laying female mosquitoes are among methods which are
encouraged through community-based programmes. The application of appropriate insecticides
to larval habitats, particularly those which are considered useful by the householders,
e.g. water storage vessels, prevent mosquito breeding for several weeks but must be
re-applied periodically. Small, mosquito-eating fish have also been used with some
success. During outbreaks, emergency control measures may also include the application of
insecticides as space sprays to kill adult mosquitoes using portable or truck-mounted
machines or even aircraft. However, the killing effect is only transient, variable in its
effectiveness because the aerosol droplets may not penetrate indoors to microhabitats
where adult mosquitoes are sequestered, and the procedure is costly and operationally very
demanding. Regular monitoring of the vectors' susceptibility to the most widely used
insecticides is necessary to ensure the appropriate choice of chemicals. Active monitoring
and surveillance of the natural mosquito population should accompany control efforts in
order to determine the impact of the programme.
For more information on Dengue, including a list of those countries
where outbreaks have occurred, please consult the subject at the