Ask any Karachi homeowner about their water tank, and most will tell you it looks fine. The water appears clear, there is no visible growth, no smell \u2014 surely it is safe? Unfortunately, when it comes to dengue mosquito breeding, appearance is deeply misleading. A perfectly clear, odorless tank can harbour hundreds of mosquito larvae completely invisible to the untrained eye.<\/p>\n\n\n\n
This misconception is one of the primary reasons dengue continues to spread in Karachi despite widespread public awareness campaigns. Families believe they are safe because their tanks look clean. They are not checking for what they cannot see.<\/p>\n\n\n\n
The Aedes aegypti mosquito \u2014 responsible for transmitting dengue, chikungunya, and Zika \u2014 has a distinct preference: clean, stagnant water. Unlike many other insect pests, it does not seek out dirty, polluted, or biologically rich water. It actively prefers clear, still water with minimal organic content.<\/p>\n\n\n\n
This is precisely the water stored in Karachi’s overhead and underground tanks. The water that comes through municipal supply or tankers and sits in storage tanks is ideal for Aedes breeding. There is no turbulence to disturb eggs, the temperature is warm due to Karachi’s climate, and the water is clear \u2014 exactly what the mosquito needs.<\/p>\n\n\n\n
Aedes mosquitoes lay their eggs not floating on the water surface but along the waterline on container walls \u2014 just above the water level. These eggs are tiny, dark, and firmly attached. They are invisible without close inspection, and they survive for months in a dried state, hatching immediately when water returns.<\/p>\n\n\n\n
Karachi faces chronic water supply challenges. Most households \u2014 regardless of income level \u2014 maintain some form of water storage to cope with irregular supply. This is entirely rational given the infrastructure realities of the city. However, it creates a city-wide network of potential mosquito breeding sites.<\/p>\n\n\n\n
Plastic and concrete overhead tanks are ubiquitous across Karachi’s rooftops. The issues that make them high-risk include:<\/p>\n\n\n\n
Underground tanks and sumps present a separate challenge. Because they are below ground and often accessed only through small manholes, they are rarely inspected. Gaps or damaged covers allow mosquito entry, and the dark, enclosed space with its still, clean water is a perfect environment.<\/p>\n\n\n\n
Many Karachi homes discovered during outbreak investigations that their underground sump \u2014 fitted years ago and never serviced \u2014 was one of their largest breeding sites.<\/p>\n\n\n\n
Many homeowners do have their tanks cleaned \u2014 annually or semi-annually. This is good practice. However, incomplete cleaning creates a false sense of security and sometimes makes the problem worse. Here is why:<\/p>\n\n\n\n
Proper dengue-prevention tank maintenance requires physical scrubbing of all interior walls at and above the waterline, allowing the tank to dry completely if possible, and then refilling. In cases where tanks cannot be emptied, dengue prevention measures such as BTI (Bacillus thuringiensis israelensis) larvicide tablets \u2014 which are safe for drinking water and highly effective against mosquito larvae \u2014 can be used under professional guidance.<\/p>\n\n\n\n
Even without directly inspecting the tank interior, certain signs suggest a breeding problem:<\/p>\n\n\n\n
In Karachi’s high-rise buildings \u2014 common in Clifton, Defence, and newer developments in Bahria Town Karachi \u2014 a single contaminated rooftop tank can spread mosquitoes across multiple floors. Building management in such properties has a collective responsibility to ensure all shared water storage is maintained and treated.<\/p>\n\n\n\n
A significant proportion of Karachi’s population relies on private water tankers for their supply. These tankers are an underappreciated vector for dengue risk. The water transported by tankers:<\/p>\n\n\n\n
This does not mean tanker water is inherently unsafe to drink \u2014 most Karachi residents use it without incident. But it does mean that the tank receiving that water requires regular monitoring and cleaning, not just a one-time setup.<\/p>\n\n\n\n
A properly protected water tank in Karachi combines physical, chemical, and structural solutions:<\/p>\n\n\n\n