WHAT IS DENGUE
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
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 –
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, the rate can be reduced to less than 1%.
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 population 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
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 femalemosquitoes 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.
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 phenomena—often with
enlargement of the liver—and 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 replacement therapy.
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.
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.
Source: WHO Press
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