Dengue fever outbreak

As of 2010 dengue fever is believed to infect 50 to 100 million people worldwide a year with 1/2 million life-threatening infections. It dramatically increased in frequency between 1960 and 2010, by 30 fold. This increase is believed to be due to a combination of urbanization, population growth, increased international travel, and global warming. The geographical distribution is around the equator with 70% of the total 2.5 billion people living in endemic areas from Asia and the Pacific. Many people in outbreaks are not virally tested, therefore their infections may also be due to chikungunya, a coinfection of both, or even other similar viruses.

The first recognized dengue epidemics occurred almost simultaneously in Asia, Africa, and North America in the 1780s, shortly after the identification and naming of the disease in 1779. A pandemic began in Southeast Asia in the 1950s, and by 1975 DHF had become a leading cause of death among children in the region. The first case of DHF was reported in Manila, Philippines around 1953 through 1954. A small child with dengue fever started to bleed uncontrollably. Other children then became victims to the new symptom. Epidemic dengue has become more common since the 1980s. By the late 1990s, dengue was the most important mosquito-borne disease affecting humans after malaria, with around 40 million cases of dengue fever and several hundred thousand cases of dengue hemorrhagic fever each year. Significant outbreaks of dengue fever tend to occur every five or six months. The cyclical rise and fall in numbers of dengue cases is thought to be the result of seasonal cycles interacting with a short-lived cross-immunity for all four strains in people who have had dengue. When the cross-immunity wears off the population is more susceptible to transmission whenever the next seasonal peak occurs. Thus over time there remain large numbers of susceptible people in affected populations despite previous outbreaks due to the four different serotypes of dengue virus and the presence of unexposed individuals from childbirth or immigration.

There is significant evidence, originally suggested by S.B. Halstead in the 1970s, that dengue hemorrhagic fever is more likely to occur in people who have secondary infections by another one of dengue fever's four serotypes. One model to explain this process is known as antibody-dependent enhancement (ADE), which allows for increased uptake and virion replication during a secondary infection with a different strain. Through an immunological phenomenon, known as original antigenic sin, the immune system is not able to adequately respond to the stronger infection, and the secondary infection becomes far more serious.

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