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Los Angeles County West Vector & Vector-Borne Disease Control District
Dengue Fever
Dengue (déng gee, déng
gŕy) 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.
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 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%.
By 1997, dengue has become the most important mosquito-borne viral disease affecting humans; its global distribution is comparable to that of malaria, and an estimated 2.5 billion people live in areas at risk for epidemic transmission . Each year, tens of millions of cases of dengue fever occur and, depending on the year, up to hundreds of thousands of cases of DHF. The case-fatality rate of DHF in most countries is about 5%; most fatal cases are among children and young adults.
There is an increasing risk for dengue outbreaks in the continental United States. Two competent mosquito vectors, Aedes aegypti and Aedes albopictus, are present and, under certain circumstances, each could transmit dengue viruses. This type of transmission has been detected 4 times in the last 20 years in south Texas (1980, 1986, 1995 and 2000) and has been associated with dengue epidemics in northern Mexico. Moreover, numerous viruses are introduced annually by travelers returning from tropical areas where dengue viruses are endemic. From 1977 to 1994, a total of 2,248 suspected cases of imported dengue were reported in the United States. Although some specimens collected were not adequate for laboratory diagnosis, 481(21%) cases were confirmed as dengue. Many more cases probably go unreported each year because surveillance in the United States is passive and relies on physicians to recognize the disease, inquire about the patient's travel history, obtain proper diagnostic samples, and report the case. These data suggest that southern Texas and the southeastern United States, where Ae. aegypti and Ae. albopictus are found, are at risk for dengue transmission and sporadic outbreaks.
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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. 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.
Characteristics
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 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.
Treatment
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]
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 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
The reasons for this dramatic global emergence of dengue/DHF as a major public health problem are complex and not well understood. However, several important factors can be identified. First, effective mosquito control is virtually nonexistent in most dengue-endemic countries. Considerable emphasis for the past 20 years has been placed on ultra-low-volume insecticide space sprays for adult mosquito control, a relatively ineffective approach for controlling Ae. aegypti. Second, major global demographic changes have occurred, the most important of which have been uncontrolled urbanization and concurrent population growth. These demographic changes have resulted in substandard housing and inadequate water, sewer, and waste management systems, all of which increase Ae. aegypti population densities and facilitate transmission of Ae. aegypti-borne disease. Third, increased travel by airplane provides the ideal mechanism for transporting dengue viruses between population centers of the tropics, resulting in a constant exchange of dengue viruses and other pathogens. Lastly, in most countries the public health infrastructure has deteriorated. Limited financial and human resources and competing priorities have resulted in a "crisis mentality" with emphasis on implementing so-called emergency control methods in response to epidemics rather than on developing programs to prevent epidemic transmission. This approach has been particularly detrimental to dengue control because, in most countries, surveillance is very inadequate; the system to detect increased transmission normally relies on reports by local physicians who often do not consider dengue in their differential diagnoses. As a result, an epidemic has often reached or passed transmission before it is detected.
(Information provided by the Centers for Disease Control and Prevention ; World Health Organization)
