Mosquitoes spread to new areas in warmer India

April 6th, 2008 - 2:51 pm ICT by admin  

(April 7 is World Health Day)
By Atanu Sarkar
The incidence of mosquito-borne diseases such as malaria, dengue, and chikungunya is rising in India due to climate change. These diseases are no longer restricted to old areas. They are spreading, as mosquitoes are now able to live and breed in new areas. There is now evidence of rising temperature, change of rainfall (either high or low and its timing) and humidity in the country. Studies have found that while intensified rainfall favours mosquito breeding, rising temperature along with humidity increases its survival period during the adult phase and hence the disease transmission capacity of each mosquito magnifies several times.

There are reports of a rising number of vector-borne diseases in traditionally low endemic areas, such as Himachal Pradesh, Uttarakhand and Jammu and Kashmir.

People living in these states are not as immune to malaria as inhabitants of traditionally endemic states. Therefore, the fatality rate in newer areas is likely to be much higher.

But unfortunately due to poor record keeping and insufficient data it is very difficult to develop robust linkage between climate change and vector-borne disease.

There is a need to strengthen the health service to improve diagnostic and treatment facilities and proper mosquito control measures in order to reduce the number of cases and fatalities. Policymakers need early warning systems in the public health domain to predict any form of epidemics.

That may be one of the areas to look at on World Health Day Monday, which the World Health Organisation is observing this year on the theme ‘protect health from climate change’.

The ecology, development, behaviour and survival of mosquitoes and the transmission dynamics of the diseases they spread are strongly influenced by climatic factors. Temperature, rainfall, and humidity are especially important, but others, such as wind and the duration of daylight, can also be significant.

Change of climate is essentially creating a more conducive environment where the disease causing vectors and microbes can grow faster and survive longer. Eventually, that intensifies the infection and prolongs its duration.

Mosquitoes in particular are highly sensitive to temperature. The mosquitoes that can carry malaria (Anopheline) generally do not develop or breed below about 16 degrees Celsius and the variety that transmits dengue fever (Aedes aegypti) is limited by winter temperatures below 10 degrees Celsius.

With sufficient moisture, warmer temperatures will generally cause an increase in mosquito abundance, biting rates and activity level and will accelerate the incubation of the parasites and viruses within them. The maturation of the malaria parasite becomes faster in higher temperature.

Therefore, temperature rise will intensify the infection spread from mosquito to people. Higher temperatures increase the number of blood meals taken and the number of times eggs are laid by the mosquitoes.

Relative humidity also affects malaria transmission, for instance, if the average monthly relative humidity is below 60 percent, it is believed that the life of the mosquito is so shortened that there is no malaria transmission.

Apart from vector-borne diseases, other infectious diseases also rise due to abnormal climate variation. A study conducted in Bangladesh has shown increase in the number of diarrhoea cases following both high or low rainfall and rise in temperature. Though there is no similar study in India yet, the effects are likely to be similar.

Global climate change due to human action is now a scientifically proven fact and a developing country like India would bear the brunt of all possible outcomes. However, despite being the most serious consequence, health has received inadequate attention in the larger policy dimension.

(Atanu Sarkar is a doctor and a public health specialist at The Energy and Resources Institute (TERI). He can be contacted at

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