Indian cities are being increasingly hit by caused by mosquito-borne diseases: dengue, malaria and chikungunya. With increased global travel, we now hear of the possibility of the zika and yellow fever viruses arriving and spreading in the country, a prospect fraught with nightmarish consequences. Cities are beginning up to wake up to the seriousness of the problem. Delhi and Visakhapatnam have declared an open war on the mosquito; many cities are set to follow suit. Yet, the proposed solutions rely on old, ineffective and even harmful short-term fixes such as insecticide fogging.
Fogging largely kills adult mosquitoes, leaving the larvae untouched – it may cause a short-term dip in mosquito densities but it soon bounces back to its original levels. Insecticide resistance, observed in some parts of Asia, can spread swiftly to India if we continue to indiscriminately use fogging as the primary means of mosquito control. The Aedes aegypti mosquito, which is responsible for the transmission of the majority of cases of dengue and chikangunya, is well adapted to densely populated urban environments. It breeds in sheltered habitats, such as pots, rain water harvesting containers and tyres.
The mosquito is aggressive, often biting multiple people in a day. It can spread many times faster in cities where people tend to move to multiple locations compared to rural areas. Originally active during the day, some reports suggest that it has adapted to artificial light and is active at night. The key to mosquito control lies in increased attention to larva. Mosquitoes, like other aquatic insects, avoid laying their eggs in lakes, rivers or wetland ecosystems where they face the risk of predation by aquatic fauna. Maintaining healthy water bodies helps reduce the risk of mosquito breeding.
Building over stormwater drains or blocking them with piles of garbage creates fertile breeding spaces for mosquitos in areas that are so polluted that they lack other natural predators like fish or tadpoles. Ironically, “sustainable” urban practices such as gardening and rainwater harvesting provide habitat for the mosquito to flourish uninterrupted even during the dry summer months where it would otherwise have diminished.
A relatively small number of urban containers can account for the majority of urban cases of infection. We need systematic urban monitoring and education campaigns to ensure that rain harvesting containers are completely covered, and pots and compost pits are well drained. Urban heat island effects, including the increase in daytime temperature and day-night temperature fluctuation, are known to play an important role. They increase mosquito breeding, promote dengue virus replication, and make patients more vulnerable to infection – a triple whammy that greatly increases chances of severe outcomes. Areas of cities with greater green surfaces and reduced urban heat islands have lower disease prevalence.
Managing urban biodiversity could help. Some studies suggest that as bird populations decrease due to environmental impacts near cities, mosquitoes shift from feeding on birds to feeding on humans. Dogs, pigs, goats and cattle can slow down disease transmission by acting as incompetent or dead-end reservoirs – they are bitten by mosquitoes, but do not permit virus replication. Urban ecological planning holds the key to the long-term management of mosquito borne diseases. Restoring ecologically diverse water bodies and extending green spaces has the added attraction of providing multiple co-benefits, including providing alternate biodiversity hosts, reducing air and water pollution, and making city climate more bearable.
The writer is Professor of Sustainability at Azim Premji University, Bengaluru.
