Dengue is caused by a virus spread by Aedes (Stegomyia) mosquitoes. Over the past two decades there has been a dramatic global increase in the frequency of dengue fever (DF) dengue haemorrhagic fever (DHF), and dengue shock syndrome (DSS) and their epidemics, with a concomitant increase in disease incidence. The World Health Report 1996 stated, that the "re-emergence of infectious diseases is a warning that progress achieved so far towards global security in health and prosperity may be wasted." The report further indicated that "infectious diseases range from those occurring in tropical areas (such as malaria and DHF which are most common in developing countries) to diseases found worldwide (such as hepatitis and sexually transmitted diseases, including HIV/AIDS) and food-borne illnesses that affect large numbers of people in both the richer and poorer nations."
Dengue epidemics are known to have occurred over the last three centuries in tropical, subtropical and temperate areas of the world. The first epidemic of dengue was recorded in 1635 in the French West Indies, although a disease compatible with dengue had been reported in China as early as 992 AD. During the 18th, 19th and early 20th centuries, epidemics of dengue-like diseases were described globally in the tropics as well as in some temperate regions. Rush was probably describing dengue when he wrote of "break-bone fever" occurring in Philadelphia in 1780. Most of these epidemics were clinical dengue fever, although some were associated with the severe haemorrhagic form of the disease. Efforts to control Aedes aegypti and economic development have markedly reduced the threat of epidemic dengue in temperate countries during the past 50 years.
The first recorded outbreak of a dengue disease compatible with DHF occurred in Australia in 1897. A similar haemorrhagic disease was recorded in 1928 during an epidemic in Greece and again in Taiwan in 1931. The first confirmed epidemic of DHF was recorded in the Philippines in 1953-1954. Since then, major outbreaks of DHF with significant mortality have occurred in most countries of the South-East Asia Region, including India, Indonesia, Maldives, Myanmar, Sri Lanka, and Thailand, as well as in Singapore, Cambodia, China, Laos, Malaysia, New Caledonia, Palau, Philippines, Tahiti and Vietnam in the Western Pacific Region. Over the past 20 years, there has been a dramatic increase in the incidence and geographical distribution of DHF, and epidemics now occur each year in some South-East Asian countries.
The dengue viruses are members of the genus Flavivirus and family flaviviridae. These small (50 nm.) viruses contain single-strand RNA. The virion consists of a nucleocapsid with cubic symmetry enclosed in a lipoprotein envelope. The dengue virus genome is approximately 11,000 base pairs in length, and is composed of three structural protein genes encoding the nucleocaprid or core protein (C), a membrane-associated protein (M), an envelope protein (E), and seven nonstructural protein (NS) genes. The envelope glycoprotein is associated with viral haemagglutination and neutralization activity.
The dengue viruses form a distinct complex within the genus flavivirusbased on antigenic and biological characteristics. There are four virus serotypes which are designated as DEN-1, DEN-2, DEN-3 and DEN-4. Infection with any one serotype confers lifelong immunity to that virus serotype. Although all four serotypes are antigenically similar, they are different enough to elicit cross-protection for only a few months after infection by any one of them.
Dengue viruses of all four serotypes have been associated with epidemics of dengue fever in which there was little or no evidence of DHF. All four virus serotypes have also caused DHF epidemics associated with severe and fatal disease.
Dengue viruses are transmitted from person to person by Aedes (Ae.) mosquitoes of the subgenus Stegomyia. Ae. aegypti is the most important epidemic vector, but other species such as Ae. albopictus, Ae. polynesiensis, members of Ae. scutellaris complex, and Ae. (Finlaya) niveus have also been incriminated as secondary vectors. All except Ae. aegypti have their own restricted geographical distribution and, although they may be excellent hosts for dengue viruses, they are generally less efficient epidemic vectors than Ae. aegypti.
Dengue viruses infect humans and several species of lower primates. Humans are the main urban reservoir of the viruses. Studies in Malaysia and Africa have shown that monkeys are infected and are the likely reservoir hosts, although the epidemiological significance of this observation remains to be established. Dengue virus strains grow well in insect tissue cultures and on mammalian cell cultures after adaptation.
The female Aedes (Stegomyia) mosquito usually becomes infected with dengue virus when she takes blood from a person during the acute febrile (viraemic) phase of illness. After an extrinsic incubation period of 8 to 10 days, the salivary glands of the mosquito become infected and the virus is transmitted when the infective mosquito bites and injects the salivary fluid into the wound of another person. Following an incubation period in humans of 3-14 days (4-6 days average), there is often a sudden onset of the disease, with fever, headache, myalgias, loss of appetite, and a variety of nonspecific signs and symptoms, including nausea, vomiting and rash.
Viraemia is usually present at the time of or just before the onset of symptoms and lasts an average of five days after the onset of illness. This is the crucial period when the patient is most infective for the vector mosquito and contributes to maintaining the transmission cycle if the patient is not protected against vector mosquito bites.
There is evidence that the vertical transmission of dengue virus from infected female mosquitoes to the next generation occurs in several species including Ae. aegypti and Ae. albopictus. This may be an important mechanism for virus maintenance, but does not appear to be important in epidemics.
Dengue virus infection may be asymptomatic or may cause undifferentiated febrile illness (viral syndrome), dengue fever (DF), or dengue haemorrhagic fever (DHF) including dengue shock syndrome (DSS). Infection with one dengue serotype gives lifelong immunity to that particular serotype, but there is no cross-protection for the other serotypes. The clinical presentation depends on age, immune status of the host, and the virus strain.
After an average incubation period of 4-6 days (range 3-14 days), various non-specific, undifferentiated prodomes, such as headache, backache and general malaise may develop. Typically, the onset of DF in adults is sudden, with a sharp rise in temperature occasionally accompanied by chillis, and is invariably associated with severe headache and flushed face(12). Within 24 hours there may be retro-orbital pain, particularly on eye movement or eye pressure, photophobia, backache and pain in the muscles and joints/bones of the extremities. The other common symptoms include anorexia and altered taste sensation, constipation, colicky pain and abdominal tenderness, dragging pains in the inguinal region, sore throat, and general depression. These symptoms vary in severity and usually persist for several days.
Typical cases of DHF are characterized by high fever, haemorrhagic phenomena, hepatomegaly, and often circulatory failure. Moderate to marked thrombocytopenia with concurrent haemoconcentration are distinctive clinical laboratory findings. The major pathophysiologic changes that determine the severity of the disease in DHF and differentiate it from DF are abnormal haemostasis and leakage of plasma as manifested by thrombocytopenia and rising haematocrit.
DHF commonly begins with a sudden rise in temperature which is accompanied by facial flush and other non-specific constitutional symptoms resembling dengue fever, such as anorexia, vomiting, headache, and muscle or joint pains.
Some DHF patients complain of sore throat, and an injected pharynx may be found on examination. Epigastric discomfort, tenderness at the right costal margin, and generalized abdominal pain are common. The temperature is typically high and in most cases continues for two to seven days, then falls to a normal or subnormal level. Occasionally the temperature may be as high as 40oC, and febrile convulsions may occur.
The most common haemorrhagic phenomenon is a positive tourniquet test. Easy bruising and bleeding at venipuncture sites are present in most cases. Fine petechiae scattered on the extremities, axillae, face and soft palate may be seen during the early febrile phase. A confluent petechial rash with characteristic small, round areas of normal skin is sometimes seen in convalescence after the temperature has returned to normal. A maculopapular or rubella-type rash may be observed early or late in the disease. Epistaxis and gum bleeding are less common. Mild gastrointestinal haemorrhage is occasionally observed. Haematuria is rarely observed.
Encephalitic signs such as convulsion and coma are rare in DHF. They may, however, occur as a complication in cases of prolonged shock with severe bleeding in various organs including the brain. Water intoxication, as a result of inappropriate use of hypotonic solution to treat DHF patients with hyponatraemia, is a relatively common iatrogenic complication that leads to encephalopathy. A subtle form of seizure is occasionally observed in infants under one year of age during the febrile phase and, in some cases, is considered to be febrile convulsions since the cerebrospinal fluid is normal. Subdural effusions have been observed in some cases.
In recent years there has been an increasing number of reports of DF or DHF with unusual manifestations. Unusual central nervous system manifestations, including convulsions, spasticity, change in consciousness and transient paresis, have been observed. Some of these cases may have encephalopathy as a complication of DHF with severe disseminated intravascular coagulation that may lead to focal occlusion or haemorrhage.
Fatal cases with encephalitic manifes-tations have been reported in Indonesia, Malaysia, Myanmar, India and Puerto Rico. However, in most cases there have been no autopsies to rule out bleeding or occlusion of the blood vessels. Although limited, there is some evidence that, on rare occasions, dengue viruses may cross the blood-brain barrier and infect the CNS. Further studies are needed to identify the factors contributing to these unusual manifestations. Attention should be given to the study of underlying host factors such as convulsive disorders and concurrent diseases.
Encephalopathy associated with acute liver failure is commonly observed and renal failure usually occurs at the terminal stage. Liver enzymes are markedly elevated in these cases, with serum aspartate aminotransferase about 2-3 times higher than serum alanine aminotransferase.
Other rarely observed, unusual manifes-tations of DF/DHF include acute renal failure and haemolytic uraemic syndrome. Some of these cases have been observed in patients with underlying host factors (e.g. G6P deficiency and haemoglobinopathy) that lead to intravascular haemolysis. Dual infections with other endemic diseases, such as leptospirosis, viral hepatitis B, and melioidosis, have been reported in cases with unusual manifestations.
No vaccine is available yet for the prevention of dengue infection and there are no specific drugs for its treatment. Hence DF/DHF control is primarily dependent on the control of Ae. aegypti. Dengue control programmes in the Region have in general not been very successful, primarily because they have relied almost exclusively on space spraying of insecticides for adult mosquito control. However, space spraying requires specific operations which were often not adhered to, and most countries found it cost prohibitive. In order to achieve sustainability of a successful DF/DHF vector control programme, it is essential to focus on larval source reduction and to have complete cooperation with non-health sectors, such as nongovernmental organizations, civic organizations and community groups, to ensure community understanding and involvement in implementation. There is, therefore, a need to adopt an integrated approach to mosquito control by including all appropriate methods (environ-mental, biological and chemical) which are safe, cost-effective and environmentally acceptable. A successful, sustainable Ae. aegypti control programme must involve a partnership between government control agencies and the community. The approaches described below are considered necessary to achieve long-term, sustainable control of Ae. aegypti.
Environmental manipulation including change in human habitat
Flower pots/vases and ant traps: Flower pots, flower vases and ant traps are common sources of Ae. aegypti breeding. They should be punctured to produce a drain hole. Alternatively, live flowers can be placed in a mixture of sand and water. Flowers should be removed and discarded weekly and vases scrubbed and cleaned before reuse. Brass flower pots, which make poor larval habitats, can be used in cemeteries in place of traditional glass containers. Ant traps to protect food storage cabinets can be treated with common salt or oil.
Aedes breeding in incidental water collections: Desert (evaporation) water coolers, condensation collection pans under refrigerators, and air conditioners should be regularly inspected, drained and cleaned. Desert water coolers generally employed in arid/semi-arid regions(44) of South-East Asia to cool houses during summer contain two manufacturing defects. These are as follows:
Building exteriors: The design of buildings is important to prevent Aedes breeding. Drainage pipes of rooftops sunshades/porticos often get blocked and become breeding sites for Aedes mosquitoes. There is a need for periodic inspection of buildings during the rainy season to locate potential breeding sites.
Mandatory water storage for fire fighting: Fire prevention regulations may require mandatory water storage. Such storage tanks need to be kept mosquito-proofed. In some municipalities in India(45), timber merchants are required to maintain two metal drums (50 gallons) full of water for fire fighting. These drums should be kept covered with tight lids. Also, metal drums used for water storage at construction sites should be mosquito-proofed.
Solid waste disposal: Solid wastes, namely tins, bottles, buckets or any other waste material scattered around houses, should be removed and buried in land fills. Scrap material in factories and warehouses should be stored appropriately until disposal. Household and garden utensils (buckets, bowls and watering devices) should be turned upside down to prevent the accumulation of rain water. Similarly, canoes and small boats should be emptied of water and turned upside down when not in use. Plant waste (coconut shells, cocoa husks) should be disposed of properly and without delay.
Tyre management: Used automobile tyres are of major importance as breeding sites for urban Aedes, and are therefore a significant public health problem. Imported used tyres are believed responsible for the introduction of Ae. albopictus into the United States, Europe and Africa. Tyre depots should always be kept under cover to prevent the collection of rain water. New technologies for tyre recycling and disposal are continually coming into use, but most of them have proved to be of limited application or cost-effectiveness. Used tyres can be filled with earth or concrete and used for planters or traffic/crash barriers. They may also be used as soil erosion barriers, or used to create artificial reefs and reduce beach erosion by wave action. Tyres can also be recycled for sandals, floor mats, industrial washers, gaskets, buckets, garbage pails and carpet backing, while truck tyres have been made into durable, low-cost refuse containers.
Filling of cavities of fences: Fences and fence posts made from hollow trees such as bamboo should be cut down to the node, and concrete blocks should be filled with packed sand, crushed glass, or concrete to eliminate potential Aedes larval habitats.
Glass bottles and cans: Glass bottles, cans and other small containers should be buried in land fills or crushed and recycled for industrial use.
Protective clothing: Clothing reduces the risk of mosquito biting if the cloth is sufficiently thick or loosely fitting. Long sleeves and trousers with stockings may protect the arms and legs, the preferred sites for mosquito bites. Schoolchildren should adhere to these practices whenever possible. Impregnating clothing with chemicals such as permethrin can be especially effective in preventing mosquito bites.
Mats, coils and aerosols: Household insecticidal products, namely mosquito coils, pyrethrum space spray and aerosols have been used extensively for personal protection against mosquitoes. Electric vaporizer mats and liquid vaporizers are more recent additions which are marketed in practically all urban areas.
Repellents: Repellents are a common means of personal protection against mosquitoes and other biting insects. These are broadly classified into two categories, natural repellents and chemical repellents. Essential oils from plant extracts are the main natural repellent ingredients, i.e. citronella oil, lemongrass oil and neem oil. Chemical repellents such as DEET (N, N-Diethyl-m-Toluamide) can provide protection against Ae. albopictus, Ae. aegypti and anopheline species for several hours. Permethrin is an effective repellant when impregnated in cloth.
Insecticide-treated mosquito nets and curtains: Insecticide-treated mosquito nets (ITMN) have limited utility in dengue control programmes, since the vector species bites during the day. However, treated nets can be effectively utilized to protect infants and night workers who sleep by day. They can also be effective for people who generally have an afternoon sleep. "Olyset net", a wide mesh net woven from polyethylene thread containing 2% permethrin, is yet another improvement in ITMN technology. This net has two advantages over traditional nets in that the wide mesh permits better ventilation and light, and the treated thread enables a slow release of permethrin to the fibre surface, ensuring a long residual effect (over a year). In studies carried out in Malaysia, four washings with soap and water did not diminish the efficacy and the mortality of Ae. aegypti was 86.7%. For control of DF/DHF in Vietnam, Olyset net curtains were hung on the inside against doors/windows; Ae. aegypti was adversely affected and dengue virus transmission was interrupted. Further studies on impregnated fabrics appear warranted.
The application of biological control agents which are directed against the larval stages of dengue vectors in South-East Asia has been somewhat restricted to small-scale field operations.
Fish: Larvivorus fish (Gambusia affinis and Poecilia reticulata) have been extensively used for the control of An. stephensi and/or Ae. aegypti in large water bodies or large water containers in many countries in South-East Asia. The applicability and efficiency of this control measure depend on the type of containers.
Bacteria: Two species of endotoxin-producing bacteria, Bacillus thuringiensis serotype H-14 (Bt.H-14) and Bacillus sphaericus (Bs) are effective mosquito control agents. They do not affect non-target species. Bt.H-14 has been found to be most effective against An. stephensi and Ae. aegypti, while Bs is the most effective against Culex quinquefasciatus which breeds in polluted waters. There is a whole range of formulated Bti products produced by several major companies for control of vector mosquitoes. Such products include wettable powders and various slow-release formulations including briquettes, tablets and pellets. Further developments are expected in slow-release formulations. Bt.H-14 has an extremely low-level mammalian toxicity and has been accepted for the control of mosquitoes in containers storing water for household use.
Cyclopoids: The predatory role of copepod crustaceans was documented between 1930-50, but scientific evaluation was taken up only in 1980 in Tahiti, French Polynesia, where it was found that Mesocyclops aspericornis could effect a 99.3% mortality rate among Aedes (Stegomyia) larvae and 9.7% and 1.9%, respectively among Cx. quinquefasciatus and Toxorhynchities amboinensis larvae. Trials in crab burrows against Ae. polynesiensis and in water tanks, drums, and covered wells met with mixed results. In Queensland, Australia, out of seven species evaluated in the laboratory, all but M. notius were found to be effective predators of both Ae. aegypti and An. farauti but not against Cx. quinquifasciatus. Field releases in both northern and southern Queensland, however, showed mixed results. In Thailand, results were also mixed, but in Vietnam, results were more successful, contributing to the eradication of Ae. aegypti from one village. Although the lack of nutrients and frequent cleaning of some containers can prevent the sustainability of copepods, they could be suitable for large containers which cannot be cleaned regularly (wells, concrete tanks and tyres). They can also be used in conjunction with Bt.H-14. Copepods have a role in dengue vector control, but more research is required on the feasibility of operational use.
Autocidal ovitraps: Autocidal ovitraps were successfully used in Singapore as a control device in the eradication of Ae. aegypti from the Changgi international airport. In Thailand, this autocidal trap was further modified as an auto-larval trap using plastic material available locally. Unfortunately, under the local conditions of water storage practices in Thailand, the technique was not very efficient in reducing natural populations of Ae. aegypti. Better results can be expected if the number of existing potential larval habitats is reduced, or more autocidal traps are placed in the area under control, or both activities are carried out simultaneously. It is believed that, under certain conditions, this technique could be an economical and rapid means of reducing the natural density of adult females as well as serve as a device for monitoring infestations in areas where some reduction in population densities of the vector have already taken place. However, the successful application of autocidal ovitraps/larval traps depends on the number placed, the location of placement, and their attractiveness as Ae. aegypti female oviposition sites.
Chemicals have been used to control Ae. aegypti since the turn of the century. In the first campaigns against the yellow fever vector in Cuba and Panama, in conjunction with widespread clean-up campaigns, Aedes larval habitats were treated with oil and houses were fumigated with pyrethrins. When the insecticidal properties of DDT were discovered in the 1940s, this compound became a principal method of Ae. aegypti eradication programmes in the Americas. When resistance to DDT emerged in the early 1960s, organophosphate insecticides, including fenthion, malathion and fenitrothion were used for Ae. aegypti adult control and temephos as a larvicide. Current methods for applying insecticides include larvicide application and space spraying.
Chemical Larviciding: Larviciding or "focal" control of Ae. aegypti is usually limited to domestic-use containers that cannot be destroyed, eliminated, or otherwise managed. It is difficult and expensive to apply chemical larvicides on a long-term basis. Therefore chemical larvicides are best used in situations where the disease and vector surveillance indicate the existence of certain periods of high risk and in localities where outbreaks might occur. Establishing the precise timing and location are essential for maximum effectiveness. Control personnel distributing the larvicide should always encourage house occupants to control larvae by environmental sanitation. There are three insecticides that can be used for treating containers that hold drinking water.
Temephos 1% sand granules: One per cent temephos sand granules are applied to containers using a calibrated plastic spoon to administer a dosage of 1 ppm. This dosage has been found to be effective for 8-12 weeks, especially in porous earthen jars, under normal water use patterns. Although resistance to temephos in Ae. aegypti and Ae. albopictus populations has not been reported from the South-East Asia Region, the susceptibility level of Aedes mosquitoes should be monitored regularly in order to ensure the effective use of the insecticide.
Insect growth regulators: Insect growth regulators (IGRs) interfere with the development of the immature stages of the mosquito by interference of chitin synthesis during the molting process in larvae or disruption of pupal and adult transformation processes. Most IGRs have extremely low mammalian toxicity (LD50 value of acute oral toxicity for methoprene (Altosid) is 34 600 mg/kg). In general, IGRs may provide long-term residual effects (three to six months) at relatively low dosages when used in porous earthen jars. Because IGRs do not cause immediate mortality of the immature mosquitoes, countries with legislation stipulating that the breeding of Aedes larvae is an offense, will require some alteration of the law, so as not to penalize home owners who use these compounds.
Space sprays: Space spraying involves the application of small droplets of insecticide into the air in an attempt to kill adult mosquitoes. It has been the principal method of DF/DHF control used by most countries in the Region for 25 years. Unfortunately, it has not been effective, as illustrated by the dramatic increase in DHF incidence in these countries during the same period of time. Recent studies have demonstrated that the method has little effect on the mosquito population, and thus on dengue transmission. Moreover, when space spraying is conducted in a community, it creates a false sense of security among residents, which has a detrimental effect on community-based source reduction programmes. From a political point of view, however, it is a desirable approach because it is highly visible and conveys the message that the government is doing something about the disease. This, however, is poor justification for using space sprays. The current recommendations are that space spraying of insecticides (fogging) should not be used except in the most extreme conditions during a major DHF epidemic. However, the operations should be carried out at the right time, at the right place, and according to the prescribed instructions with maximum coverage, so that the fog penetration effect is complete enough to achieve the desired results.
When space sprays are employed, it is important to follow the instructions on both the application equipment and the insecticide label and to make sure the application equipment is well maintained and properly calibrated. Droplets that are too small tend to drift beyond the target area, while large droplets fall out rapidly. Nozzles for ultra-low volume ground equipment should be capable of producing droplets in the 5 to 27 micron range and the mass median diameter should not exceed the droplet size recommended by the manufacturer. Desirable spray characteristics include a sufficient period of suspension in the air with suitable drift and penetration into target areas with the ultimate aim of impacting adult mosquitoes. Generally, there are two forms of space-spray that have been used for Ae. aegypti control, namely "thermal fogs" and "cold fogs". Both can be dispensed by vehicle-mounted or hand-operated machines.
Thermal fogs: Thermal fogs containing insecticides are normally produced when a suitable formulation condenses after being vaporized at a high temperature. Generally, a thermal fogging machine employs the resonant pulse principle to generate hot gas (over 200oC) at high velocity. These gases atomize the insecticide formulation instantly so that it is vaporized and condensed rapidly with only negligible formulation breakdown. Thermal fogging formulations can be oil-based or water-based. The oil(diesel)-based formulations produce dense clouds of white smoke, whereas water-based formulations produce a colorless fine mist. The droplet (particle) size of a thermal fog is usually less than 15 microns in diameter. The exact droplet size depends on the type of machine and operational conditions. However, uniform droplet size is difficult to achieve in normal fogging operations.
Ultra-low volume (ULV), aerosols (cold fogs) and mists: ULV involves the application of a small quantity of concentrated liquid insecticides. The use of less than 4.6 litres/ha of an insecticide concentrate is usually considered as an ULV application. ULV is directly related to the application volume and not to the droplet size. Nevertheless, droplet size is important and the equipment used should be capable of producing droplets in the 10 to 15 micron range, although the effectiveness changes little when the droplet size range is extended to 5-25 microns. The droplet size should be monitored by exposure on teflon or silocone-coated slides and examined under a microscope. Aerosols, mists and fogs may be applied by portable machines, vehicle-mounted generators or aircraft equipment.
House-to-house application using portable equipment: Portable spray units can be used when the area to be treated is not very large or in areas where vehicle-mounted equipment cannot be used effectively. This equipment is meant for restricted outdoor use and for enclosed spaces (buildings) of not less than 14m3.Portable application can be made in congested low-income housing areas, multistoried buildings, godowns and warehouses, covered drains, sewer tanks and residential or commercial premises. Operators can treat an average of 80 houses per day, but the weight of the machine and the vibrations caused by the engine make it necessary to allow the operators to rest, so that two or three operators are required per machine.
Vehicle-mounted fogging: Vehicle-mounted aerosol generators can be used in urban or suburban areas with a good road system. One machine can cover up to 1500-2000 houses (or approximately 80 ha) per day. It is necessary to calibrate the equipment, vehicle speed, and swath width (60-90m) to determine the coverage obtained by a single pass. A good map of the area showing all roads is of great help in undertaking the application. An educational effort may be required to persuade the residents to cooperate by opening doors and windows. The speed of the vehicle and the time of day of application are important factors to consider when insecticides are applied by ground vehicles. The vehicle should not travel faster than 16 kph (10 mph). When the wind speed is greater than 16 kph or when the ambient air temperature is greater than 28oC (82oF), the insecticide should not be applied(25). The best time for application is in the early morning (approximately 0600-0830 hours) or late afternoon (approximately 1700-1930 hours).
Insecticide formulations for space sprays: Organophosphate insecticides, such as malathion, fenitrothion and pirimiphos methyl have been used for the control of adult Aedes vectors. Undiluted technical grade malathion (active ingredient 95%+) or one part technical grade diluted with 24 parts of diesel have been used for ULV spraying and thermal fogging respectively. For undiluted technical grade ULV malathion applications from vehicles, the dosage on an area basis is 0.5 liters per hectare. Apart from the above-mentioned formulations, a number of companies produce pyrethroid formulations containing either permethrin, deltamethrin, lambda-cyhalothin or other compounds which can be used for space spray applications. It is important not to under-dose during operational conditions. Low dosages of pyrethroid insecticides are usually more effective indoors than outdoors.
Integrated Control Approach: The use of insecticides for the prevention and control of dengue vectors should be integrated into environmental methods wherever possible. During periods of little or no dengue virus activity, the routine source reduction measures described earlier can be integrated into larvicide application in containers that cannot be eliminated, covered, filled or otherwise managed. For emergency control to suppress a dengue virus epidemic or to prevent an imminent outbreak, a programme of rapid and massive destruction of the Ae. aegypti population should be undertaken with both insecticides and source reduction, using the techniques described in these guidelines in an integrated manner.
Insecticide susceptibility monitoring: During the past 40 years, chemicals have been widely used to control mosquitoes and other insects from spreading diseases of public health importance. As a result, Ae. aegypti and other dengue vectors in several countries have developed resistance to commonly-used insecticides, including temephos, malathion, fenthion, permethrin, propoxur and fenitrothion. It is therefore advisable to obtain baseline data on insecticide susceptibility before insecticidal control operations are started, and to continue monitoring susceptibility levels periodically. WHO kits are available for testing the susceptibility of adult and larval mosquitoes and other arthropod vectors to commonly-used insecticides. These can be obtained from the Communicable Diseases Cluster, World Health Organization, 1211 Geneva 27, Switzerland, or through WHO Regional Offices or WHO Representatives in the countries.
Community participation (CP) has been defined "as a process whereby individuals, families and communities are involved in the planning and conduct of local vector control activities so as to ensure that the programme meets the local needs and priorities of the people who live in the community, and promotes community's self-reliance in respect to development." In short, CP entails the creation of opportunities that enable all members of the community and extended society to actively contribute to, influence the development of, and share equitably in the fruits of accrued benefits. Objectives of community participation in dengue prevention and control:
How to invoke community participation
Community level: People should not only be provided with knowledge and skills on vector control, but education materials should empower them with the knowledge that allows them to make positive health choices and gives them the ability to act individually and collectively.
Systems level: To enable people to mobilize local actions and societal forces beyond a single community, i.e. health, development and social services.
Political level: Mechanisms must be made available to allow people to articulate their health priorities to political authorities. This will facilitate placing vector control high on the priority agenda and effectively lobby for policies and actions.
Defining community actions: For sustaining DF/DHF prevention and control programmes, the following community actions are essential:
Developing economies in countries of the South-East Asia Region have recognized many social, economic and environmental problems which promote mosquito breeding. The dengue problem thus exceeds the capabilities of ministries of health. The prevention and control of dengue requires close collaboration and partnerships between the health and non-health sectors (both government and private), nongovernmental organizations (NGOs) and local communities. During epidemics such cooperation becomes even more critical, since it requires pooling of resources from all groups to check the spread of the disease. Intersectoral cooperation involves at least two components:
Resource sharing should be sought wherever the dengue control coordinator can make use of underutilized human resources, e.g. for local manufacture of needed tools, seasonal government labourers for water supply improvement activities, or community and youth groups to clean up discarded tyres and containers in neighbourhoods. The dengue control programme should seek the accommodation or adjustment of existing policies and practices of other ministries, sectors, and municipal governments to include public health as a central focus for their goals. For instance, the public works sector could be encouraged to adjust its policies to give first priority to water supply improvements for communities at highest risk of dengue. In return, the Ministry of Health could authorize the use of some of its field staff to assist the ministry responsible for public works to repair water supply and sewerage systems in other urban areas. The following examples show how several government ministries may contribute to dengue vector control efforts.
Role of the ministry responsible for public works: The ministry responsible for public works and its municipal counterparts should play a key role in dengue control. They can contribute to source reduction by providing a safe, dependable water supply, adequate sanitation, and effective solid waste management. In addition, through the adoption and enforcement of housing and building codes, a municipality may mandate the provision of utilities such as individual household piped water supplies or sewerage connections, and rainwater (storm water) run-off control for new housing developments, or forbid open surface wells.
Role of the Ministry of Education: The Ministry of Health should work closely with the Ministry of Education to develop a health education (health communication) component targeted at school children, and devise and communicate appropriate health messages. Health education models can be jointly developed, tested, implemented and evaluated for various age groups. Research programmes in universities and colleges can be encouraged to include components that produce information of direct importance (e.g. vector biology and control, case management) or indirect importance (e.g. improved water supply, educational inter-ventions to promote community sanitation, waste characterization studies) to dengue control programmes.
Role of the ministry responsible for the environment: The Ministry of Environment can help the Ministry of Health collect data and information on ecosystems and habitats in or around cities at high risk of dengue. Data and information on local geology and climate, land usages, forest cover, surface waters, and human populations are useful in planning control measures for specific ecosystems and habitats. The Ministry of Environment may also be helpful in determining the beneficial and adverse impacts of various Ae. aegypti control tactics (chemical, environmental and biological).
Role of the ministry responsible for information, communication and the mass media: Information directed at the community at large is best achieved through the mass media, such as television, radio and newspapers. Therefore, the ministry responsible for information, communication and the mass media should be approached to coordinate the release of messages on the prevention and control of dengue developed by public health specialists.
Role of Nongovernmental organization (NGOs): NGOs can play an important role in promoting community participation and implementing environmental management for dengue vector control. This will most often involve health education, source reduction, and housing improvement related to vector control. Community NGOs may be informal neighbourhood groups or formal private voluntary organizations, service clubs, churches or other religious groups, or environmental and social action groups.
After proper training by the Ministry of Health staff in source reduction methods, NGOs can collect discarded containers (tyres, bottles, tins, etc.), clean drains and culverts, fill depressions, remove abandoned cars and roadside junk, and distribute sand or cement to fill tree holes. NGOs may also play a key role in the development of recycling activities to remove discarded containers from yards and streets. Such activities must be coordinated with the environmental sanitation service.
NGOs may also be able to play a specific, but as yet unexplored, role in environmental management during epidemic control. Under guidance from the Ministry of Health, NGOs could concentrate on the physical control of locally identified, key breeding sites such as water drums, waste tyre piles, and cemetery flower vases.
Service clubs such as Rotary International have supported DF/DHF prevention and control programmes in the American Region for over 15 years. In Asia and the Pacific, programmes have been initiated in Sri Lanka, Philippines, Indonesia and Australia to provide economic and political support for successful community-based campaigns. A new grant from the Rotary Foundation of Rotary International \as been awarded to study the possibility of up scaling this project to a global programme. Women's clubs have contributed to Ae. aegypti control by conducting household inspections for foci and carrying out source reduction. There are many opportunities, mostly untapped, for environmental organizations and religious service groups to play similar roles in each Ae. aegypti-infested community.
The formulation of legislate/on on dengue/Ae.aegypti control should, therefore, take into consideration the following points:
Dengue infection is caused by a virus. It occurs commonly as dengue fever. Occasionally the patient suffering from dengue may develop bleeding. Common sites for bleeding are nose, gums or skin. Sometimes, the patient may have coffee ground vomiting or black stools. This indicates bleeding in gastro intestinal tracts and it is serious. The patient with dengue who has bleeding has dengue haemorrhagic fever (DHF). Rarely the patient suffering from dengue may develop shock, then it is called dengue shock syndrome (DSS).
Dengue infection is caused by a virus. It occurs commonly as dengue fever. Occasionally the patient suffering from dengue may develop bleeding. Common sites for bleeding are nose, gums or skin. Sometimes, the patient may have coffee ground vomiting or black stools. This indicates bleeding in gastro intestinal tracts and it is serious. The patient with dengue who has bleeding has dengue haemorrhagic fever (DHF). Rarely the patient suffering from dengue may develop shock, then it is called dengue shock syndrome (DSS).
Dengue should be suspected when you have sudden onset of fever. The fever is high 103-105 degrees F or 39-40 degrees C. It is accompanied with severe headache (mostly in the forehead), pain behind the eyes, body aches and pains, rash on the skin and nausea or vomiting. The fever lasts for 5-7 days. In some patients, fever comes down on 3rd or 4th day but comes back. All the above symptoms and signs may not be present in the patient. The patient feels much discomfort after the illness.
The characteristics of dengue that make it different from other causes of fever are the pain behind the eyes, severe pains in the muscles, severe joint pains, and skin rashes. These features make the diagnosis of suspected Dengue likely. The severe joint pains caused by DF is the reason why DF is also called break-bone fever.
If a patient suspected to be having dengue has reduced platelets or an increase in blood haematocrit, then the patient has probable dengue. These additional findings make dengue more likely. Patients with dengue may not have a high haematocrit if the person was anaemic to start with.
It is possible to get dengue more than once. Dengue can occur because of 4 different but related strains of dengue virus. If a person has suffered from one virus, there can be a repeat occurrence of dengue if a different strain is involved subsequently. Being affected by one strain offers no protection against the others. A person could suffer from dengue more than once in her/his lifetime.
There are laboratory tests that provide direct or indirect evidence for dengue fever. These tests provide evidence for the occurrence of dengue infection. There are some additional tests that can help to identify the type of dengue infection. The tests for confirmation of Dengue should be done in reliable laboratories.
Dengue fever occurs following the bite of an infected mosquito Aedes aegypti.This type of mosquito has a peculiar white spotted body and legs and is easy to recognize even by laymen. It breeds in clean water (see Question 20) and has a flight range of only 100 - 200 metres. The mosquito gets the Dengue virus after biting a human being infected with dengue virus.
After the entry of the virus in the person, it multiplies in the lymph glands in the body. The symptoms develop when the virus has multiplied in sufficient numbers to cause the symptoms. This happens generally about 4-6 days (average) after getting infected with the virus.
Yes, There are many people who are infected with the virus and do not suffer from any signs or symptoms of the disease. For every patient with symptoms and signs there may be 4-5 persons with no symptoms or with very mild symptoms.
Most patients with dengue fever can be treated at home. They should take rest, drink plenty of fluids that are available at home and eat nutritious diet. Whenever available, Oral Rehydration Salt/ORS (commoin treating diarrhoea) is preferable. Sufficient fluid intake is very important and becomes more important in case DF progresses into DHF or DSS where loss of body fluid / blood is the most salient feature. It is important to look for danger signs and contact the doctor as soon as any one or more of these are found.
Like most viral diseases there is no specific cure for dengue fever. Antibiotics do not help. Paracetamol (can be purchased without prescription) is the drug of choice to bring down fever and joint pain. Other medicines such as Aspirin and Brufen should be avoided since they can increase the risk of bleeding. Doctors should be very careful when prescribing medicines. Any medicines that decrease platelets should be avoided.
The infection can become dangerous since it may cause damage to the blood vessels. The damage may range from increased permeability of the blood vessels, causing leakage of blood fluid/plasma into various organs to completely broken blood vessels that causes bleeding. The symptoms and signs of dengue hemorrhagic fever and dengue shock syndrome are related to damage to the blood vessels and derangement in functioning in components of blood that help it to clot.
People who suffer from dengue fever have no risk of death but some of them develop Dengue Hemorrhagic Fever or Dengue Shock Syndrome. In some of these cases death can occur. With proper treatment, the patients with Dengue hemorrhagic fever and dengue shock syndrome can recover fully. Good treatment provided in time can save most lives.
Generally the progress towards dengue hemorrhagic fever or dengue shock syndrome occur after 3-5 days of fever. At this time, fever has often come down. This may mislead many of us to believe that the patient is heading towards recovery. In fact, this is the most dangerous period that requires high vigilance from care-givers. The signs and symptoms that should be looked for are severe pain in the abdomen, persistent vomiting, bleeding from any site like, bleeding in the skin appearing as small red or purplish spots, nose bleed, bleeding from gums, passage of black stools like coal tar. Bring the patient to the hospital whenever the first two signs, namely, severe pain in the abdomen and persistent vomiting are detected. Usually it is too late if we wait until bleeding has occurred. The most dangerous type of dengue is the dengue shock syndrome. It is recognized by signs like excessive thirst, pale and cold skin (due to very low blood pressure), restlessness and a feeling of weakness.
A vaccine has been developed to prevent dengue fever but it is still under trial. It is not yet available in the market. Scientific progress is likely to help in prevention of dengue fever by vaccination in the years to come.
Most people who suffer from dengue fever recover in 1-2 weeks time. Some may feel tired for several weeks. However, if symptoms persist after this period, consult a doctor.
The highly domestic mosquito Aedes aegypti rests indoors, in closets and other dark places. Outside it rests where it is cool and shaded. The female mosquito lays her eggs in water containers in and around the homes, and other dwellings. These eggs will develop, become larvae, and further develop into adults in about 10 days.
Dengue mosquitoes breed in stored, exposed water collections. Favoured places for breeding are barrels, drums, jars, pots, buckets, flower vases, plant saucers, tanks, discarded bottles, tins, tyres, water coolers etc. To prevent the mosquitoes from multiplying, drain out the water from desert coolers/window air coolers (when not in use), tanks, barrels, drums, buckets etc. Remove all objects containing water (e.g. plant saucers etc.) from the house. Collect and destroy discarded containers in which water collects e.g. bottles, plastic bags, tins, used tyres etc. In case it is not possible to drain out various water collections or to fully cover them, use TEMEPHOS, an insecticide, ( brand name Abate1 part per million according to the local guidelines to prevent larvae from developing into adults.
There is no way to tell if a mosquito is carrying the dengue virus. Therefore, people must protect themselves from all mosquito bites. Dengue mosquitoes bite during the day time throughout the day. Highest biting intensity is about 2 hours after sunrise and before sunset.Wear full sleeves clothes and long dresses to cover as much of your body as possible. Use repellents- be careful in using them in young children and old people. Use mosquito coils and electric vapour mats during the daytime also to prevent dengue.Use mosquito nets to protect children, old people and others who may rest during the day. The effectiveness of these nets can be improved by treating them with permethrin (pyrethroid insecticide). This bed-net is called Insecticide Treated Nets and are widely used in the prevention of malaria.
The spread of dengue from a patient to others is possible. The patient should be protected from contact with the mosquito. This can be achieved by ensuring that the patient sleeps under a bed-net. Effective mosquito repellents are used where the patient is being provided care. This will prevent the mosquito from biting the patient and from getting infected and spreading it to others.
There is no travel restriction. However, you should be aware of what to do if you are traveling an area where dengue has been reported. This includes observing prevention measures described in these frequently asked questions and answers and reporting to the doctor if you have fever and are worried that it might be dengue fever.
Patients suspected to be suffering from dengue haemorrhagic fever or dengue shock syndrome should be admitted to a hospital without delay.The progress of these patients should be monitored regularly at 1-2 hours interval.Platelet counts and haematocrits should be monitored repeatedly to review the progress of patients.If the haematocrit levels fall dangerously then a blood transfusion should be considered. A fall of more than 20 % as compared to previous levels may be an indication for transfusion. If the haematocrit values rise the patient should be given fluids intravenously and the fluids carefully monitored to ensure that the patient does not get excess fluids. A rise of more than 20 % as compared to previous levels may be an indication for IV fluids. The doctor should decide based on best judgement of patient's condition.
Do not prescribe aspirin and brufen or any other medicine that reduces the platelets or increases the tendency to bleed.Avoid giving IV fluids unless the patient is bleeding or the haematocrit level is rising progressively.Avoid rushing into giving blood transfusion unless the haematocrit is falling dangerously. Do not give platelet transfusion unless the platelet count is very low or unless there is bleeding.
All suspected or probable dengue cases should be reported to the health authorities. Further confirmation of the cases is done by the health authorities. Seek their guidance on blood collection and transportation of samples from suspected/probable cases of dengue.
In fact, the community is the key to dengue prevention. As elaborated above, prevention of dengue relies heavily on preventing the mosquito (Aedes aegypti) that transmits dengue from breeding inside and in the vicinity of homes. Every household can undertake the very simple measures to prevent existing water collections from becoming places for breeding of A.aegypti by draining out water from various containers, by regular changing of water plus cleaning flower vases and other items or, in the case of unused items, by discarding/destroying them.Since the mosquito does not travel far, "house cleaning" by all members of a community will ensure that no breeding places exist, preventing dengue from occurring.The main strategy in the prevention and control of dengue is "source reduction", or prevention of breeding places, mentioned above.
Prevention of mosquito breeding places remains our mainstay. However to stop or to slow down the transmission it may be supplemented by "thermal fogging", using fogging machines. In fogging,we use an insecticide that has an immediate knock-down effect on adult mosquitos. When fogging is undertaken after an epidemic occurs, it is unfortunately too late. Fogging, to be effective, should be done at about 3-4 days interval. It is expensive and time consuming. Therefore, measures undertaken by the community, for the community, to prevent the breeding of mosquitoes is far more cost-effective than containment measures once an outbreak occurs.
Dengue is an acute fever caused by a virus. It occurs in two forms:
Dengue fever is marked by the onset of sudden high fever, severe headache and pain behind the eyes, muscles and joints.
Dengue Haemorrhagic fever (DHF) is a more severe form, in which bleeding and sometimes shock occurs - leading to death. It is most serious in children. Symptoms of bleeding usually occur after 3-5 days of fever.
The high fever continues for five to six days (103 - 105 OF or 39- 40 OC). It comes down on the third or the fourth day but rises again. The patient feels much discomfort and is very weak after the illness.
Dengue spreads rapidly and may affect large number of people during an epidemic resulting in reduced work productivity, but most importantly causing the loss of lives.
Recognition of Dengue Fever
Recognition of Dengue Hemorrhagic Fever and Shock
Symptoms similar to dengue fever plus, any one of the following:
There is no specific medicine for the treatment of the disease. However proper and early treatment can relieve the symptoms and prevent complications and death. Aspirin and Brufen should be avoided in dengue fever, as it is known to increase the bleeding tendency and also it increases the stomach pain. Paracetamol can be given on medical advice. If one or more signs of Dengue Haemorrhagic fever are seen, take the patient to the hospital immediately. Give fluids to drink while transferring the patient to the hospital.
Basic Facts on Dengue
Dengue is spread by the bite of an infected mosquito Aedes aegypti. The mosquito gets the virus by biting the infected persons. The first symptoms of the disease occur about 5-7 days after an infected bite. There is no way to tell if a mosquito is carrying the dengue virus. Therefore, people must protect themselves from all mosquito bites.
The mosquito rests indoors, in closets and other dark places. Outside, they rest where it is cool and shaded. The female mosquito lays her eggs in water containers in and around homes, schools and other areas in towns or villages. These eggs become adult in about 10 days.
Dengue mosquitoes breed in stored exposed water collections. Favoured breeding places are:
Barrels, drums, jars, pots, buckets, flower vases, plant saucers, tanks, discarded bottles, tins, tyres, water cooler, etc. and a lot more places where rain-water collects or is stored.
Prevention of Dengue
All efforts of control should be directed against the mosquitoes. It is important to take control measures to eliminate the mosquitoes and their breeding places. However, the efforts should be intensified before the transmission season (during and after the rainy season) and at the time of the epidemic.
Prevent Mosquito Bites:
Dengue mosquitoes bite during the daytime. Protect yourself from the bite:
Prevention Multiplication of Mosquitoes:
Mosquitoes which spread dengue live and breed in and around houses.
For detailed update on dengue in Gujarat and floods in Mumbai, check out the following links.