Dengue and chikungunya are amongst common mosquito-borne infections in India where prevention from disease is totally based on reducing bites by infected mosquitoes. As of now, neither a vaccine, nor a drug is available against either. It is therefore important that mosquito control measures applied should be those that have a proven impact on human infection and disease.
Both diseases are spread by the bite of an infected Aedes aegypti mosquito, and to a lesser extent, the Aedes albopictus. Both share the same breeding and biting habits and consequently, both have similar control measures. The menace of dengue has grown rapidly — approximately four billion people living in more than 128 countries are at risk of dengue. Every year, an estimated 400 million get infected globally and about 100 million get the disease. The number of dengue cases in the world has jumped by almost four times since 1990. One report estimates that India has about six million laboratory confirmed dengue cases per year, resulting in an annual economic loss of about $1.11 billion. The official figure for India in 2015 is about a lakh cases.
The geographic spread is multi-factorial but importantly, some provide conducive environments for faster reproduction and disease transmission (including increased urbanisation, population movement, erratic water supply, etc.). Mosquito control measures are well-known and applied at various stages of the mosquito’s life cycle; to prevent breeding (by avoiding water collection in and around houses, etc.), target larvae and pupae (using chemicals like temephos or biological methods like fishes) and focus on adult mosquitoes (through spraying and with personal protection using mosquito repellents, coils, bed-nets, etc.).
On paper, dengue control measures appear straightforward. But their large-scale application has been difficult, if not impossible, and harder to sustain. Singapore and Cuba used these methods to reduce dengue — but it returned after 15-20 years in both these countries. In Iquitos, Peru, indoor ULV insecticide application was used successfully to reduce dengue. Similarly, dengue epidemics were controlled in Brazil and Hawaii, using spraying with residual insecticides. These successes were possible as control measures were instituted when cases of dengue just started to appear and implementation was fast, comprehensive and sustained. However, the challenge is to have an effective and sensitive surveillance system to report early cases, immediately followed by large-scale application of control measures. This may be difficult, if not impossible, to replicate in India, where surveillance is weak or non-existent and the public health system is slow to respond.
Surprisingly, studies have found that most existing dengue vector control strategies (larvicidal, outdoor and indoor insecticide and space spraying) have not been robustly evaluated for their impact on reducing human infection and disease. Several studies evaluated the effect on larvae and/or mosquitoe density, but reduction in mosquito population does not correlate well with predictable reduction in dengue disease. This lack of information explains why, many times, these control measures have failed.
Decreasing dengue will require a better understanding of how well existing tools can reduce dengue transmission, not just mosquito numbers. A sensitive and effective surveillance system is a priority. This system should trigger action. Rigorously designed field trials of existing tools should be undertaken, using epidemiological and entomological outcomes as end-points. Mathematical models should be used to know in which phase of the outbreak interventions like fogging should be initiated to minimise cases of dengue. Laboratory studies of successful interventions should be scaled up to see feasibility and impact. Limited resources must be efficiently targeted to only those essential activities that will decrease the number of dengue cases.