Dengue Fever

Dengue fever is a debilitating and sometimes hemorrhagic fever (one that is associated with extensive internal bleeding). The disease is caused by four slightly different types of a virus from the genus Flavivirus that is designated as DEN. The four antigenic types are DEN-1, DEN-2, DEN-3, and DEN-4.

The dengue virus is transmitted to humans via the bite of a mosquito. The principle mosquito species is known as Aedes aegypti. This mosquito is found all over the world, and, throughout time, became adapted to urban environments. For example, the mosquito evolved so as to be capable of living year round in moist storage containers, rather than relying on the seasonal patterns of rainfall. Another species, Aedes albopictus (the "Tiger mosquito"), is widespread throughout Asia. Both mosquitoes are now well established in urban centers. Accordingly, dengue fever is now a disease of urbanized, developed areas, rather than rural, unpopulated regions.

The dengue virus is passed to humans exclusively by the bite of mosquito in search of a blood meal. This mode of transmission makes the dengue virus an arbovirus (that is, one that is transmitted by an arthropod). Studies have demonstrated that some species of monkey can harbor the virus. Thus, monkeys may serve as a reservoir of the virus. Mosquitoes who bite the monkey may acquire the virus and subsequently transfer the virus to humans.

The disease has been known for centuries. The first reported cases were in 1779–1780, occurring almost simultaneously in Asia, Africa, and North America. Since then, periodic outbreaks of the disease have occurred in all areas of the world where the mosquito resides. In particular, an outbreak that began in Asia after World War II, spread around the world, and has continued to plague southeast Asia even into 2002. As of 2001, dengue fever was the leading cause of hospitalization and death among children in southeast Asia.

Beginning in the 1980s, dengue fever began to increase in the Far East and Africa. Outbreaks were not related to economic conditions. For example, Singapore had an outbreak of dengue fever from 1990 to 1994, even after a mosquito control program that had kept the disease at minimal levels for over two decades. The example of Singapore illustrates the importance of an ongoing program of mosquito population control.

The disease is a serious problem in more than 100 countries in Africa, North and South America, the Eastern Mediterranean, South-East Asia, and the Western Pacific.

Unlike other bacterial or viral diseases, which can be controlled by vaccination, the four antigenic types of the dengue virus do not confer cross-protection. Thus, it is possible for an individual to be sickened with four separate bouts of dengue fever.

Following the transfer of the virus from mosquito to humans, the symptoms can be varied, ranging from nonspecific and relatively inconsequential ailments to severe and fatal hemorrhaging. The incubation period of the virus is typically 5 to 8 days, but symptoms may develop after as few as three days or as many as 15 days. The onset of symptoms is sudden and dramatic. Initially, chills tend to develop, followed by a headache. Pain with the movement of the eyes leads to more generalized and extreme pain in the legs and joints. A high fever can be produced, with temperatures reaching 104° F [40° C]. Also, a pale rash may appear transiently on the face.

These symptoms can persist for up to 96 hours. Often, the fever then rapidly eases. After a short period when symptoms disappear, the fever reappears. The temperature elevates rapidly but the fever is usually not as high as in previous episodes. The palms of the hands and soles of the feet may turn bright red and become very swollen.

In about 80% of those who are infected, recovery is complete after a convalescent period of several weeks with general weakness and lack of energy. However, in some 20% of those who are infected a severe form of dengue fever develops. This malady is characterized by the increased leakage of fluid from cells and by the abnormal clotting of the blood. These factors produce the hemorrhaging that can be a hallmark of the disease, which is called dengue hemorrhagic fever. Even then, recovery can be complete within a week. Finally, in some of those who are infected, a condition called dengue shock syndrome can result in convulsions. In addition, a failure of the circulatory system can occur, resulting in death.

The reasons for the varied degrees of severity and symptoms that the viral infection can elicit are still unclear. Not surprisingly, there is currently no cure for dengue, nor is there a vaccine. Treatment for those who are afflicted is palliative, that is, intended to ease the symptoms of the disease. Upon recovery, immunity to the particular antigenic type of the virus is in place for life. However, an infection with one antigenic type of dengue virus is not protective against the other three antigenic types. Currently, the only preventive measure that can be taken is to eradicate the mosquito vector of the virus.

See also Epidemics, viral; Zoonoses