March 2003 Dengue - An Emerging Mosquito-Borne Disease Breakbone fever, a delightful moniker for dengue fever, is an emerging mosquito-borne disease on a global scale. Dengue is a viral infection transmitted by mosquitoes to humans in tropical and sub-tropical areas of the globe. The disease occurs in 100 countries, infects 50 million people, hospitalizes half-a-million, and causes tens of thousands of deaths each year. Dengue fever is raging in Brazil, with over 770,000 cases in 2002.
There are 4 sereotypes or distinct strains of dengue virus creatively called dengue 1, 2, 3, and 4. Infection and recovery from one serotype provides lifelong immunity against that strain but no protection from the other three stereotypes. In fact, there is mounting evidence that an infection from a second, different serotype, after recovery from a first infection, can lead to a more serious disease response known as dengue haemorrhagic fever (DHF).
Classical dengue fever causes flu-like symptoms comprised of pain behind the eyes, muscle and joint pains, rash and fever of 102° - 105° F. Onset is sudden, symptoms last 3 - 7 days, recovery is usually complete and mortality is very low, less than one-half of 1%.
Dengue haemorrhagic fever is the severe form of dengue fever which is brought on by a second infection by a different strain of dengue. The second infection of a different strain of dengue seems to cause an exaggerated response by the patient’s immune system. The exaggerated response causes fever for 2 to 7 days, liver enlargement and tenderness, nausea, vomiting, abdominal pain, joint and backbone pain and hemorrhage - bleeding under the skin, from the nose, gums, blood in the vomit and blood in the stool. DHF can lead to dengue haemorrhagic shock or DHS. DHS occurs because of the fluid loss that occurs from capillary bleeding. The fluid loss lowers the blood pressure to the point of circulatory failure, at which time the patient goes into shock. There is no specific treatment for dengue, DHF or DHS. However, with appropriate, intensive supportive therapy, basically the maintenance of the circulating fluid volume, mortality is about 1%, without hospitalization and supportive care, fatality rates can exceed 20% from DHF. Most fatalities are children and young adults.
Vaccine development for dengue is difficult because the vaccine must provide immunity to all four serotypes with 1 immune response by the host. Immunization cannot be done for 1 serotype at a time because this would set up the exaggerated immune response that leads to DHF. Presently, vaccine development is ongoing, the costs of development and research is enormous. Thus far, progress has been gradual, but a vaccine for all types of dengue virus could be available within several years.
The risk of a dengue outbreak in the United States is increasing. Both of the mosquito species that transmit the disease, Aedes albopictus and Aedes aegypti, are present, hundreds of cases of dengue are imported each year, urban density is increasing, and because of no previous exposure, the U.S. population is highly susceptible. In fact, transmission has occurred 4 times, in 1980, 1986, 1995 and 2000 in south Texas in association with dengue epidemics in northern Mexico.
The reasons for the recent, dramatic, global emergence of classical dengue and DHF are complex and not well understood, but several important factors have been identified. First, effective mosquito control is virtually non-existent in most dengue endemic countries. Most programs in such areas are not integrated and rely solely on ultra-low-volume (ULV) insecticide sprays for adult mosquito control which is ineffective against Aedes aegypti and Aedes albopictus because they are container breeding mosquitoes. Second, rapid uncontrolled urbanization and population growth is occurring in endemic areas resulting in densely packed people, substandard housing, inadequate water systems, sewer systems and waste management systems. These circumstances cause a proliferation of trash and conditions capable of holding the water that the Aedes aegypti and Aedes albopictus mosquitoes utilize for production. Third, increased air travel provides the ideal means of transporting dengue viruses in infected people to other population centers around the world. Lastly, in most countries, the public health infrastructure has deteriorated because of a lack of funding, trained personnel, and other “governmental priorities.” The predominant governmental mentality is to respond to epidemics rather that being proactive and develop programs than prevent disease transmission, in other words, treat the symptoms instead of preventing the cause. The result is the global emergence of dengue/DHF epidemics in just the past 15 years.
Dengue is a disease that involves what is called a “direct cycle”, man is the host and mosquitoes are the vector or transmitting agent of the disease, with no other animal or organism involved in the cycle of transmission. The virus does not affect the mosquito, but an incubation period of 8 to 11 days is required before the mosquito can be infective. Once infected the mosquito remains infective for the rest of it’s life. Prevention is obvious, don’t get bitten by an infected mosquito. Use repellents, sleep under a mosquito net in endemic areas, wear protective clothing, avoid peak activity periods of the mosquito (early morning, early evening and shaded areas in the day time), and most importantly, reduce the number of the mosquitoes that transmit the disease. As mentioned earlier, ULV spraying is NOT effective against Aedes aegypti or Aedes albopictus, the key to controlling these mosquitoes is to eliminate the habitat they use to lay their eggs - water holding items, such as tires, trash, tarps, rain-gutters, buckets, pet watering dishes, boats, wheelbarrows, trays under potted plants, etc. Cleanup and vigilance is truly, the only way to control these mosquitoes... and in turn the disease. |