KEY FACTS
• Dengue is a mosquito-borne infection
that causes a severe flu-like illness, and sometimes a potentially
lethal complication called dengue haemorrhagic fever.
• Global incidence of dengue has grown dramatically in recent
decades.
• About two fifths of the world's population are now at
risk.
• Dengue is found in tropical and sub-tropical climates
worldwide, mostly in urban and semi-urban areas.
• Dengue haemorrhagic fever is a leading cause of serious
illness and death among children in some Asian countries.
• There is no specific treatment for dengue, but appropriate
medical care frequently saves the lives of patients with the more
serious dengue haemorrhagic fever.
• The only way to prevent dengue virus transmission is to
combat the disease-carrying mosquitoes.
Dengue is a mosquito-borne infection that
in recent decades 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 dengue epidemics in the
Philippines and Thailand. Today DHF affects most Asian countries
and has become a leading cause of hospitalization and death among
children in the region.
There are four distinct, but closely related, viruses that cause
dengue. Recovery from infection by one provides lifelong immunity
against that virus but confers only partial and transient protection
against subsequent infection by the other three viruses. There
is good evidence that sequential infection increases the risk
of developing DHF.
Global burden of dengue
The incidence of dengue has grown dramatically
around the world in recent decades. Some 2.5 billion people –
two fifths of the world's population – are now at risk from
dengue. WHO currently estimates there may be 50 million dengue
infections worldwide every year.
In 2007 alone, there were more than 890 000 reported cases of
dengue in the Americas, of which 26 000 cases were DHF.
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 the
most seriously affected. Before 1970 only nine countries had experienced
DHF epidemics, a number that had increased more than four-fold
by 1995.
Not only is the number of cases increasing as the disease is spreading
to new areas, but explosive outbreaks are occurring. In 2007,
Venezuela reported over 80 000 cases, including more than 6 000
cases of DHF.
Some other statistics:
• During epidemics of dengue, infection rates among those
who have not been previously exposed to the virus are often 40%
to 50%, but can reach 80% to 90%.
• An estimated 500 000 people with DHF require hospitalization
each year, a very large proportion of whom are children. About
2.5% of those affected die.
• Without proper treatment, DHF fatality rates can exceed
20%. Wider access to medical care from health providers with knowledge
about DHF - physicians and nurses who recognize its symptoms and
know how to treat its effects - can reduce death rates to less
than 1%.
The spread of dengue is attributed to expanding geographic distribution
of the four dengue viruses and their mosquito vectors, the most
important of which is the predominantly urban species Aedes aegypti.
A rapid rise in urban mosquito 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 eight to 10 days, an infected mosquito is
capable, during probing and blood feeding, of transmitting the
virus 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 the virus to humans has not yet been defined.
Infected humans are the main carriers and multipliers of the virus,
serving as a source of the virus for uninfected mosquitoes. The
virus circulates in the blood of infected humans for two to seven
days, at approximately the same time that they have a fever; Aedes
mosquitoes may acquire the virus when they feed on an individual
during this period. Some studies have shown that monkeys in some
parts of the world play a similar role in transmission.
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 fever with
rash. Older children and adults may have either a mild fever 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 (DHF) is a potentially deadly complication
that is characterized by high fever, often with enlargement of
the liver, and in severe cases circulatory failure. The illness
often begins with a sudden rise in temperature accompanied by
facial flush and other flu-like symptoms. The fever usually continues
for two to seven days and can be as high as 41°C, possibly
with convulsions and other complications.
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 to
24 hours, or quickly recover following appropriate medical treatment.
Treatment
There is no specific treatment for dengue fever.
For DHF, medical care by physicians and nurses experienced with
the effects and progression of the complicating haemorrhagic fever
can frequently save lives - decreasing mortality rates from more
than 20% to less than 1%. Maintenance of the patient's circulating
fluid volume is the central feature of DHF care.
Immunization
There is no vaccine to protect against dengue. Although progress
is underway, developing a vaccine against the disease - in either
its mild or severe form - is challenging.
• With four closely related viruses that can cause the disease,
the vaccine must immunize against all four types to be effective.
• There is limited understanding of how the disease typically
behaves and how the virus interacts with the immune system.
• There is a lack of laboratory animal models available
to test immune responses to potential vaccines.
Despite these challenges, two vaccine candidates have advanced
to evaluation in human subjects in countries with endemic disease,
and several potential vaccines are in earlier stages of development.
WHO provides technical advice and guidance to countries and private
partners to support vaccine research and evaluation.
Prevention and control
At present, the only method of controlling or preventing dengue
virus transmission 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 the mosquito also breeds extensively in natural
habitats such as tree holes, and leaves that gather to form "cups"
and catch water.
In recent years, Aedes albopictus, a secondary dengue vector in
Asia, has become established in the United States, several Latin
American and Caribbean countries, parts of Europe and Africa.
The rapid geographic spread of this species is largely attributed
to the international trade in used tyres, a breeding habitat.
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 that are useful in households, e.g. water storage
vessels, prevents 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 vector control measures can also include
broad application of insecticides as space sprays using portable
or truck-mounted machines or even aircraft. However, the mosquito-killing
effect is 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 difficult. Regular monitoring of the vectors'
susceptibility to 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
to determine programme effectiveness.
For more information contact:
WHO Media centre
Telephone: +41 22 791 2222
E-mail: mediainquiries@who.int
