With the change in weather the threat of dengue fever, which claimed at least 10 lives in Karachi this year, seems to have abated. Although dengue fever is not specific to any one part of the country, the Karachi administration declared an ‘emergency’ when over 3,500 victims were affected.

Though the city administration carried out spraying campaigns, it was reported that expired insecticide, alpha cypermethrin, a toxic insecticide, was used for spraying. The US EPA (Environmental Protection Agency) has classified it as a possible human carcinogen (group C). Instead of depending on chemical sprays and other such interventions it would be better to educate the masses on preventive measures to combat such a threat.

Three species of Aedes mosquitoes are the vectors of the disease: Aedes aegypti, Aedes albopictus and Aedes scutellaris, with Aedes aegypti being more common. Aedes aegypti depend on water storage containers to lay their eggs. Only the female mosquito bites to obtain blood in order to lay eggs, and the eggs can survive long dry periods.

Clean and hygienic surroundings can control dengue to a great extent

Aedes aegypti bites primarily during the day. It is most active for two hours after sunrise and several hours before sunset. It can bite at night as well. The female mosquito approaches a person from behind, and bites on the ankles and elbows. Eggs are often laid on discarded, old tyres.

Aedes aegypti is a peri-domestic mosquito, and seldom ranges very far from its breeding place and has a relatively short flight range. Hence, if mosquitoes are found in a house, their breeding sites must be outside that house. Like most diseases, dengue infection is very much an environment-related disease.

Dengue control can be achieved through environmental management, though poor environmental conditions can make it difficult. Rapid population increase, poor housing conditions, development of slums, poor efficiency in collection of solid waste (in Karachi only 33 per cent of the solid waste generated is collected), poor hygienic conditions in the neighbourhoods and stagnant water, aids in mosquito breeding.

Environmental management focuses on reducing breeding habitats, biological control, genetic control and waste management.

In order to eliminate breeding habitats, water in containers, old tyres and tins around the house should be drained out. If, for some reason, water cannot be drained off, then the containers should be covered with tight lids. The insides of the containers should be scrubbed with a brush, as larvae and eggs may survive on the damp sides and, continue their development as soon as the receptacle is filled again.

In Cambodia, a new long-lasting insecticide-treated netting cover for household water storage containers is used. The cover, fitted over concrete rainwater storage tanks, is designed to prevent mosquito breeding in containers.

Biological control can be achieved by the introduction of predators, parasites or other living organisms. Copepods (small crustaceans) and larvivorous fish are effective in controlling the larvae of Aedes mosquito. Small ornamental fish introduced in water storage tanks can feed on the larvae. Regular monitoring of these organisms is important for sustained control.

Vietnam has achieved remarkable success in controlling Aedes aegypti through biological control. Using copepods in large water-storage tanks, combined with source reduction, Vietnam has successfully eliminated Aedes aegypti in many areas, thus preventing dengue transmission.

In Tanzania, ‘azolla,’ an aquatic plant that grows on the surface of water making it difficult for larvae to reach the surface to breathe, was successfully used to control malaria-carrying mosquitoes in rice fields.

Genetic control methods are aimed at suppressing the target populations. Sterile Insect Technique is a species-specific and environmentally benign method for suppression of target population. The technique is based on mass rearing, radiation mediated sterilisation and release of a large number of male insects.

Waste management and clean areas provide effective dengue control. In the urban centres of Pakistan, used tins, bottles, old tyres and discarded containers should be carted away by the municipal agencies on daily basis. Household waste and rubbish should be moved to landfills, or subjected to anaerobic digestion.

In addition to environmental interventions, personal protection is important as it is the first line of defence. Insect repellents can be applied on the exposed skin (neck, wrists and ankles). Loose, long-sleeved and light-coloured shirts and trousers will help protect against the dengue mosquito. People should avoid places and times when the vector is most active, by staying inside during peak biting hours (two hours after sunrise and several hours before sunset).

The writer has a Master’s degree in environmental engineering from the Asian Institute of Technology, Bangkok.

Published in Dawn, Sunday Magazine, December 13th, 2015



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