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DENGUE
AND DENGUE HAEMORRHAGIC FEVER
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,
predominantly in urban and semi-urban
areas.
Dengue
haemorrhagic fever (DHF), a
potentially lethal complication, was
first recognized in the 1950s during
the dengue epidemics in the
Philippines and Thailand, but today
DHF affects most Asian countries and
has become a leading cause of
hospitalisation and death among
children in several of them.
There
are four distinct, but closely
related, viruses that 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. There is good evidence that
sequential infection increases the
risk of more serious disease resulting
in DHF.
Prevalence
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.
South-east Asia and the Western
Pacific are most seriously affected.
Before 1970 only nine countries had
experienced DHF epidemics, a number
that 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
2001 alone, there were more than 609
000 reported cases of dengue in the
Americas, of which 15 000 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 2001, Brazil reported
over 390 000 cases including
more than 670 cases of DHF.
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. At least 2.5% of
cases die, although case fatality
could be twice as high.
- Without
proper treatment, DHF case
fatality rates can exceed 20%.
With modern intensive supportive
therapy, such rates 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 that are favourable for
mosquito breeding, e.g. where
household water storage is common and
where solid waste disposal services
are inadequate.
Transmission
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. After
virus incubation for 8-10 days, an
infected mosquito is capable, during
probing and blood feeding, of
transmitting the virus, to susceptible
individuals for the rest of its life.
Infected female mosquitoes may also
transmit the virus to their offspring
by transovarial (via the eggs)
transmission, 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 two to seven days, at
approximately the same time as they
have fever; Aedes mosquitoes
may acquire the virus when they feed
on an individual during this period.
Characteristics
Dengue
fever is a severe, flu-like illness
that affects infants, young children
and adults, but seldom 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
two to seven 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.
Treatment
There
is no specific treatment for dengue
fever. However, careful clinical
management by experienced physicians
and nurses frequently saves 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 http://www.who.int/emc/diseases/ebola/Denguepublication/index.html]
Immunization
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 being made in the development of
vaccines that may protect against all
four dengue viruses. Such products may
become available for public health use
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, several Latin American and
Caribbean countries, in parts of
Europe and in one African country. 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 that 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 and
copepods (tiny crustaceans) 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.
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