21/10/2019
Better environmental management for control of dengue
New scientific insights into dengue vector ecology and disease transmission patterns, together with more targeted use of environmental management strategies, may offer improved potential for combating dengue fever, the world's fastest growing vector-borne disease.
Dengue fever, together with associated dengue haemorrhagic fever (DHF), is the most important vector-borne viral disease affecting humans. Aedes aegypti, the urban yellow fever mosquito, is also the principal dengue-carrying vector. A secondary vector is Aedes albopictus.
Aedes aegypti was eliminated from large areas of the Americas as part of the yellow fever mosquito eradication campaign in the 1950s and 1960s, but later reinvaded those areas. Dengue has emerged or re-emerged in Asia, the Americas and elsewhere over the past three decades, and presently occurs in nearly 100 tropical and subtropical countries (1). Epidemics have become progressively larger. In the year 2002, the disease was responsible for an estimated 19 000 deaths, as well as the loss of 616 000 disability-adjusted life years (DALYs) (2).
This disease surge is of particular concern since there is no curative treatment for dengue, and in many settings space spray applications of insecticides, or generalized community clean-up campaigns of vector breeding sites have had only a transient and limited effect, if at all, on disease incidence.
The resurgence of dengue
Social and environmental factors – including increased urbanization (particularly of poor populations lacking basic health services) as well as expansion of international travel and trade – are linked to the resurgence of dengue disease (3). Climate change also may affect transmission, as dengue mosquitoes reproduce more quickly and bite more frequently at higher temperatures (4, 5).
The epidemiology and ecology of dengue are complicated by the fact that there are four virus serotypes, some or all of which may be circulating in a particular endemic region at a particular time. Within any given local population, levels of immunity to each of the four serotypes may vary over time as a function of natural population growth, past population exposure to other serotypes, etc. Depending on the local level of immunity, therefore, an epidemic may erupt at higher or lower thresholds of vector density.
While beginning as a flu-like illness, dengue can develop into a deadly fever (dengue haemorrhagic fever). Unlike most other diseases, sequential infection with different serotypes increases – rather than reduces – the risk of severe illness. Most cases of dengue haemorrhagic fever occur in children under the age of 15.
The importance of good environmental management
In proximity to human settlements, Aedes aegypti mosquitoes breed primarily in artificial water containers, and the mosquito’s life-cycle is closely associated with human activities. Larval habitats are increasing rapidly in urban areas (1). Since there is no curative treatment for dengue, targeted environmental and ecosystem management is increasingly relevant. In many settings, however, generalized community clean-up campaigns or space-spray application of insecticides, have had only a transient and limited effect – or even no measurable effect at all – on disease incidence.
New strategies for dengue prevention and control: identifying the most productive mosquito breeding sites
Careful local assessment of the ecology of Aedes aegypti larvae and pupae can help target environmental management and other control measures towards the most productive categories of breeding sites. This can be achieved using surveys to measure ‘pupal productivity’‘ to identify the categories from which the majority of adult mosquitoes emerge (6, 7). Similar tools, known as ‘key container’ and ‘key premise indices’, have been developed and tested in Viet Nam (8).
Predictive computer models have been developed to determine more accurately a ‘threshold’ limit for epidemic risk in a particular locale. One model considers local levels of population immunity to the four dengue serotypes, as well as vector densities and ambient temperature, to yield a calculation of the threshold number of Aedes pupae/per person or pupae/per area (6, 9, 10). Risk of dengue epidemic is considered low when the number is below the threshold; risk increases sharply above the threshold.
Together, such models and survey methods have triggered a growing awareness that relatively small numbers of ‘key containers’ (e.g. old tyres, water storage containers) may produce the great majority of the adult mosquitoes that trigger disease outbreaks. Once the most productive containers are identified, targeted control of dengue vectors becomes more affordable and feasible. At the same time, targeted vector control can help minimize the use of chemicals that may be costly and have other long-term health and environment impacts.
Community participation
Effective ‘bottom-up’ community participation increasingly is recognized as an important component of environmentally-sustainable control programmes – which make use of recent knowledge and more environmentally ‘friendly’ biological and chemical tools (3, 11).
In Viet Nam, biological control has been used with particular success in community participation programmes involving applications of a small crustacean, Mesocyclops (Copepoda), which feeds on the newly-hatched larvae of Aedes aegypti. Scientists, in collaboration with health workers, introduced Mesocyclops into household water tanks and water jars in rural provinces of northern and central Viet Nam, and monitored results. Local leaders, together with schoolchildren, conducted clean-up campaigns and awareness events. The strategy, which was gradually expanded by health authorities, eliminated the dengue fever vector in 40 of 45 communes in northern and central Viet Nam (more than 380 000 people) where the programme has been implemented so far. There have been no reported cases of dengue disease in the same area since at least 2002, while incidence in adjacent untreated areas has remained at rates of around 112 cases per 100 000 people (8, 12).
In Cambodia, the World Health Organization, together with national and local authorities, is testing a new long-lasting insecticide-treated netting cover for household water storage containers – using an insecticide treatment technology that has been developed for bednets in malaria prevention and control. The cover, fitted over concrete rainwater storage tanks, is designed both to prevent mosquito breeding in these key containers and to reduce adult vector densities and longevity (13).
Both examples illustrate how available scientific knowledge about vector ecology and disease epidemiology may potentially be harnessed to improved programmes of environmental management and community action, as a means for combating a most dreaded disease.
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