Dengue is transmitted by the bite of an Aedesmosquito infected with any one of the four dengue viruses. It occurs in tropical and sub-tropical areas of the world. Symptoms appear 3—14 days after the infective bite. Dengue fever is a febrile illness that affects infants, young children and adults.
Symptoms range from a mild fever, to incapacitating high fever, with severe headache, pain behind the eyes, muscle and joint pain, and rash. There are no specific antiviral medicines for dengue. It is important to maintain hydration. Use of acetylsalicylic acid (e.g. aspirin) and non steroidal anti-inflammatory drugs (e.g. Ibuprofen) is not recommended.
Dengue haemorrhagic fever (fever, abdominal pain, vomiting, bleeding) is a potentially lethal complication, affecting mainly children. Early clinical diagnosis and careful clinical management by experienced physicians and nurses increase survival of patients.
- 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%.
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.
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.
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.
Epidemiological and laboratory-based surveillance is required to monitor and guide dengue/DHF prevention and control programmes regardless of whether these are based on mosquito control or possible vaccination if an effective and safe vaccine becomes available. However, though there are standard case definitions for dengue and dengue haemorrhagic fever (DHF), the reporting of dengue/DHF is not standardized. Epidemiological and laboratory data are often collected by different institutions and reported in different formats, and are therefore difficult to collate. This results in delayed collection and analysis at the regional/global levels, vital for epidemic prediction and preparedness.
To address these problems, WHO has created DengueNet as a central data management system to collect and analyse standardized epidemiological and virological data in a timely manner, and to present epidemiological trends, as soon as new data are entered and to provide both historical and real-time data. DengueNet currently houses data from 1995-2001.
The DengueNet system responds to the WHO resolution on dengue fever/DHF prevention and control adopted at the 55th World Health Assembly in May 2002, asking Member States to strengthen surveillance and emphasizing the critical importance of strengthening laboratory diagnosis in affected countries.
Building on the existing network of dengue laboratories in the Americas, WHO is now launching a pilot test of DengueNet focusing on data quality and active participation of national programmes. Forty epidemiologists and virologists participated in the first meeting on DengueNet implementation in the Americas region. The group considered the need for global epidemiological and laboratory surveillance of dengue and DHF; the national laboratory capacities in the Americas region; and the epidemiological data and reporting requirements for DengueNet. Implementation of DengueNet is planned for South-East Asia and the Western Pacific regions in 2003.
When fully in place, DengueNet will provide public health authorities and the general public with immediate real time data on dengue/DHF cases and deaths and circulating dengue virus serotypes